Module 6
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Module6Chapter9.docxBiological Foundations of Personality
Questions to be Addressed in this Chapter
Evolution, Evolutionary Psychology, and Personality
Plasticity: Biology as Both Cause and Effect
Neuroscientific Investigations of "Higher‐Level" Psychological Functions
Chapter Focus
Why are some people generally happy and others sad, some energetic and others lethargic, some impulsive and others cautious? Why do men's and women's behaviors differ; for example, why are women more likely to wear makeup and men more likely to pay for dinner on a first date? Why does everyone recognize that some acts (e.g., incest) are immoral or “taboo,” even if they do not directly harm anyone? Do we learn these feelings and behaviors, or might they be part of our biological makeup?
Scholars have contemplated such questions for ages. In the 1880s, the British scientist Sir Francis Galton contrasted “nature” (heredity) with “nurture” (environment), setting the stage for decades of theory, research, and debate about their relative importance. In the recent era, scientific advances have brought many of the issues into sharper focus. This chapter presents some of those advances. We explore six topics: biologically based individual differences evident early in life, or temperament; the shaping of personality by processes from our ancestral past, or evolution; how personality is influenced by genes; the neuroscience of mood and emotion; environmental influences on biological structures, or plasticity; and the neural bases of cognitively “higher‐level” functions, including those involving self.
Questions to be Addressed in this chapter
1. How, and why, do infants differ in temperament?
2. How can the study of human evolution inform our understanding of the personalities of contemporary humans?
3. What role do genes play in the formation of personality? How do they interact with the environment in the unfolding of personality?
4. What is the relation between brain processes and personality processes involving mood and self‐concept?
Scientists sometimes learn from accidents. The story of an apple falling on Newton's head—even if it is apocryphal—wisely instructs us that an accident can inspire scientific insight.
Insight into the biological foundations of personality, the topic of this chapter, benefited greatly from an accident that took place in 1848. The accident was suffered by Phineas Gage, a railroad construction foreman who one day in 1848 had a “very bad day on the job”: While executing a procedure to blast a path through hard rock—drill a hole in the ground, fill it with explosive powder, insert an iron rod, and light a fuse—Gage become distracted. The charge blew up in his face, and the explosion blew the iron rod up through his left cheek, the base of his skull, and the front of his brain. It destroyed a large section of Gage's frontal cortex before exiting the top of his head.
Gage was stunned but, miraculously, alive. He could walk and speak. Indeed, he could describe the accident in detail and communicate about it rationally. Yet, Gage had changed deeply. “Gage's disposition, his likes and dislikes, his dreams and aspirations are all to change. Gage's body may be alive and well, but there is a new spirit animating it. Gage was no longer Gage” (Damasio, 1994 , p. 7). Previously serious, industrious, energetic, and responsible, Gage now was irresponsible, thoughtless of others, lacking in planfulness, and indifferent to the consequences of his actions.
Gage's story suggests that there are deep interconnections between brain functioning and personality functioning. If the explosion had blown a hole in his leg instead of his brain, it would have been a bad accident, but Gage would have been the same basic person as before. The simultaneity of Gage's (1) loss of frontal brain material and (2) change in personality qualities was—well, one might say it “was no accident”.
Psychological science has systematically explored the body–personality connection suggested by Gage's accident. This chapter reviews some of their findings, and in doing so, it differs from our other chapters. Rather than focusing on a theory, the present chapter focuses on scientific findings. (The same is true for Chapter 14 , which reviews findings on the relation between personality and social context.) They constitute a body of knowledge that must be taken into account by all personality theorists. Many of the findings relate strongly to the trait theories reviewed in Chapters 7 and 8 . But others bear on distinct viewpoints, including a theoretical perspective called evolutionary psychology, which is reviewed later in this chapter.
This illustration shows the location through which an iron rod blasted through the frontal cortex of Phineas Gage—who survived the accident but experienced a profound change in his personality.
Temperament
Right from the start, we differ. Children, even in infancy, vary in their styles of emotion and behavior. Since their experiences with the world are so limited, these variations cannot be the product of social experience; they must have biological roots. Temperament refers to biologically based individual differences in emotional and motivational tendencies that are evident early in life (Kagan, 1994 ; Rothbart, 2011 ). Early‐life variations in the tendency to experience positive or negative moods, to become aroused in response to stimuli, or to calm oneself down after becoming upset are examples of temperament qualities.
Constitution and Temperament: Early Views
Scholars have long been interested in the possibility that psychological differences among people have a biological basis (reviewed in Kagan, 1994 ; Rothbart, 2011 ; Strelau, 1998 ). In ancient Greece, Hippocrates posited that variations in psychological characteristics reflect variations in bodily fluids (see Chapter 7 , Figure 7.2 ). His view reflected the Greeks' beliefs about the universe. The Greeks thought nature was composed of four elements: air, earth, fire, and water. Hippocrates and other ancient scholars analyzed temperament through a similar fourfold scheme. The four elements of nature were said to be represented in the human body by four humors (blood, black bile, yellow bile, and phlegm), each corresponding to a temperament: sanguine, melancholic, choleric, and phlegmatic, respectively. Individual differences in temperament resulted from variations in the bodily humors. The Greeks, then, provided both taxonomy of temperament qualities and a biological theory of their cause.
This conception was remarkably long lasting. More than two millennia after Hippocrates, the great German philosopher Immanuel Kant distinguished four types of temperament and suggested that their basis was found in bodily fluids—a conception that was remarkably similar to that of the ancient Greeks. Needless to say, all contemporary psychological scientists reject the details of these bodily fluid theories.
Another view of historical note came from the 19th‐century biologist Franz Joseph Gall. Gall founded the field of phrenology , which posited that specific areas of the brain are responsible for specific emotional and behavioral functions (Figure 9.1). Through postmortem inspections of brains, Gall attempted to relate differences in brain tissue to individuals' capacities, dispositions, and traits before death. Bumps on the head were examined, since they might be indicative of the development of underlying brain tissue. Phrenology gained great fame in the 19th century but subsequently was discredited. Contemporary research shows that the brain simply does not work in the way Gall assumed, with localized regions producing specific types of thought and behavior. Instead, most complex activities rest on the synchronized action of multiple, interconnected brain regions (Bressler, 2002 ; Edelman & Tononi, 2000 ; Sporns, 2011 ).
Efforts of more enduring value were seen in the mid‐19th century. Three publications were critical: Charles Darwin's The Origin of Species (1859) and The Expression of Emotions in Man and Animals (1872) and Gregor Mendel's Experiments on Plant Hybrids (1865). Darwin's Origin, of course, was foundational to the contemporary science of biology. His Expression of Emotions documented numerous close relations between emotional expression in humans and emotional expression in other complex mammals; in doing so, it contributed indirectly to the study of temperament and also foreshadowed the development of contemporary evolutionary psychology (discussed later in this chapter). Mendel's work reported eight years of research on the breeding of pea plant characteristics and served as the foundation for modern genetics. Two 20th‐century investigators attempted to link temperament to an analysis of body types: the German psychiatrist Ernst Kretschmer (1925) and the American psychologist William Sheldon (1940, 1942). Their efforts were systematic, with careful measures of body type being related to indices of psychological qualities. Yet, in both cases, methodological problems limited the conclusions one can draw from their work; subsequent research indicates that the relationship between body type and personality are weak (Strelau, 1998). Early 20th‐century work of more lasting value was done by Pavlov. In addition to his research on how reflexes are changed by experience (see Chapter 10), Pavlov developed a theory of stable individual differences in nervous system functioning that highlighted the possibility of variations in nervous system “strength”—that is, in the degree to which normal nervous system functioning could be maintained in the face of high levels of stimuli or stress (Strelau, 1998). Constitution and Temperament: Longitudinal Studies The historical efforts to study temperament that we have just reviewed lacked an element that is crucial to contemporary research: longitudinal methods—that is, research methods in which a group of persons is studied repeatedly over an extended period of time. Longitudinal methods enable researchers to determine whether psychological qualities are evident early in life and are enduring, as one would expect if they are biologically based. A pioneering longitudinal study, the New York Longitudinal Study (NYLS), was conducted by Alexander Thomas and Stella Chess (1977). They followed over 100 children from birth to adolescence, using parental reports of infants' reactions to a variety of situations to define variations in infant temperament. On the basis of ratings of infant characteristics such as activity level, general mood, attention span, and persistence, they defined three infant temperament types: easy babies who were playful and adaptable, difficult babies who were negative and unadaptable, and slow‐to‐warm‐up babies who were low in reactivity and mild in their responses. This study and subsequent studies found a link between such early differences in temperament and later personality characteristics (Rothbart & Bates, 1998; Shiner, 1998). For example, difficult babies were found to have the greatest difficulty in later adjustment, whereas easy babies were found to have the least likelihood of later difficulties. In addition, Thomas and Chess suggested that the parental environment best suited for babies of one temperament type might not be best for those of a different temperament type. That is, there is a goodness of fit between infant temperament and parental environment. Subsequently, Buss and Plomin (1975, 1984) used parental ratings of child behavior to identify dimensions of temperament that included emotionality (ease of arousal in upsetting situations; general distress), activity (tempo and vigor of motor movements; on the go all the time; fidgety), and sociability (responsiveness to other persons, makes friends easily versus shy). Individual differences in these temperament characteristics were found to be stable across time and substantially inherited, with identical twins being particularly similar on the temperament dimensions. Their strategy of relying on parental ratings is limited, for parents may be systematically biased when rating the personality of their own children; for example, they tend to overestimate the similarity of identical twins (Saudino, 1997). Nonetheless, Buss and Plomin's work was of enduring value. Many subsequent investigators adhered to their approach, searching for a small set of individual difference dimensions that characterize major variations in temperament characteristics in the population at large (Goldsmith & Campos, 1982; Gray, 1991; Strelau, 1998). These efforts partly informed the five‐factor model of personality discussed earlier (Chapter 8). These early longitudinal studies were limited, primarily because they did not identify the exact biological systems that underlie the observed temperament qualities. Doing so requires moving beyond parental self‐report measures to direct measures of behavior and indices of biological response. Let's turn to such research now. Biology, Temperament, and Personality Development: Contemporary Research Inhibited and Uninhibited Children: Research of Kagan and Colleagues Harvard psychologist Jerome Kagan has spearheaded a highly informative line of research on the biological bases of temperament (Kagan, 1994, 2003, 2011). A key to his research has been his use of direct, objective measures of behavior. Rather than merely asking parents to report about the characteristics of their children, Kagan observes the children directly, commonly in laboratory settings. Based on these observations, Kagan noticed two clearly defined behavioral profiles in temperament: inhibited and uninhibited profiles. Relative to the uninhibited child, the inhibited child reacts to unfamiliar persons or events with restraint, avoidance, and distress, takes a longer time to relax in new situations, and has more unusual fears and phobias. Such a child behaves timidly and cautiously, the initial reaction to novelty being to become quiet, seek parental comfort, or run and hide. By contrast, the uninhibited child seems to enjoy these very same situations that seem so stressful to the inhibited child. Rather than being timid and fearful, the uninhibited child responds with spontaneity in novel situations, laughing and smiling easily. Struck by such dramatic differences, Kagan set out to address the following questions: How early do such differences in temperament emerge? How stable are these differences in temperament over time? Can some biological bases for such differences in temperament be suggested? His central hypothesis was that infants inherit differences in biological functioning that lead them to be more or less reactive to novelty and that these inherited differences tend to be stable during development. According to the hypothesis, infants born highly reactive to novelty should become inhibited children, whereas those born with low reactivity should develop into uninhibited children. To test this hypothesis, Kagan brought four‐month‐old infants into the laboratory and videotaped their behavior while they were exposed to familiar and novel stimuli (e.g., mother's face, voice of a strange female, colorful mobiles moving back and forth, a balloon popping). The videotapes then were scored on measures of reactivity such as arching of the back, vigorous flexing of limbs, and crying. About 20% of the infants were designated as high‐reactive, characterized by arching of the back, intense crying, and unhappy facial expression in response to the novel stimuli. The behavioral profile suggested that they had been overaroused by the stimuli, particularly since the responses stopped when the stimuli were removed. In contrast, the low‐reactive infants, about 40% of the group, appeared to be calm and laid‐back in response to the novel stimuli. The remaining infants, about 40%, showed various mixtures of response. To determine whether, as predicted, the high‐reactive infants would become inhibited children and the low‐reactive infants uninhibited children, Kagan again studied the children when they were 14 months old, 21 months old, and 4½ years old. Again, the children were brought to the laboratory and exposed to novel, unfamiliar situations (e.g., flashing lights, a toy clown striking a drum, a stranger in an unfamiliar costume, the noise of plastic balls rotating in a wheel at the first two ages, and meeting with an unfamiliar adult and unfamiliar children at the later age). In addition to behavioral observations, physiological measures such as heart rate and blood pressure in response to the unfamiliar situations were obtained. Again, findings revealed continuity in temperament. High‐reactive infants showed greater fearful behavior, heart acceleration, and increased blood pressure in response to the unfamiliar at 14 and 21 months and smiled and talked less than low‐reactive children during social interactions at 4.5 years of age. Further testing at age 8 indicated continuing consistency, with a majority of the children assigned to each group at age 4 months retaining membership in that group. As you'll see later in the chapter, evidence of differences in biological functioning also was obtained. Scary Claus, to some. Research on temperament explains why children differ greatly from one another in their reaction to unfamiliar people and situations. Although there is consistency across time in temperament, there also is evidence of change (Fox et al., 2005). Many high‐reactive infants did not become consistently fearful. Change in these children seemed particularly tied to having mothers who were not overly protective and placed reasonable demands on them (Kagan, Arcus, & Snidman, 1993). And some of the low‐reactive infants lost their relaxed style. Despite an initial temperamental bias, environment played a role in the unfolding personality. “Any predisposition conferred by our genetic endowment is far from being a life sentence; there is no inevitable adult outcome of a particular infant temperament” (Kagan, 1999, p. 32). Yet, Kagan notes that not one of the high‐reactive infants became a consistently uninhibited child, and it was rare for a low‐reactive infant to become a consistently inhibited child. Thus, change was possible, but the temperamental bias did not vanish; it appeared to set constraints on the direction of development. As Kagan summarizes, “it is very difficult to change one's inherited predisposition completely” (1999, p. 41). Another question is whether temperament qualities vary dimensionally (e.g., height) or categorically (e.g., eye color or biological sex). Woodward, Lenzenweger, Kagan, Snidman, and Arcus. (2000) employed statistical techniques that are designed to answer this question. These statistical methods are designed to identify categories or “classes” that may explain patterns of variation in data obtained from a large group of persons. To illustrate, suppose you did not know that some people are men and others are women. If you asked people a large number of questions about their personal habits, you might find out that there are distinct groups. A statistical analysis could indicate that some responses go together so strongly (e.g., people who say that they wear skirts also tend to say that they wear lipstick and own high‐heel shoes) that they indicate a group of people that is a categorically distinct class (women). Woodward and colleagues (2000) found that the group of infants showing high reactivity (limb movements, crying) in response to novel situations is a distinct class. A distinct group of about 10% of a large population of children was found to be consistently more reactive than the population at large. This finding is important because it conflicts with the common assumption that individual differences in personality inevitably involve continuous dimensions. Research also illuminates the brain regions that contribute to inhibited and uninhibited tendencies (Schmidt & Fox, 2002). More than one region appears to be involved, with behavioral tendencies reflecting interactions among the different neural systems. One important region is the amygdala, a region of the brain that, as we note below, is centrally involved in fear response. A second region is the frontal cortex, which is involved in regulating emotional response, in part by influencing the functioning of the amygdala. Interestingly, the functioning of these brain regions is not entirely determined by inherited factors; social experiences appear to modify brain functioning and thus influence children's emotional tendencies (Schmidt & Fox, 2002). Neuroimaging methods provide particularly clear evidence of the role of the amygdala in inhibited versus uninhibited temperament (Schwartz, Wright, Shin, Kagan, & Rauch, 2003). Researchers studied a group of young adults who had been categorized as highly inhibited or uninhibited at age two. The adults participated in a laboratory study in which, while in an fMRI scanner, they viewed pictures of human faces of two sorts: (1) familiar faces (i.e., pictures of people that the participant had seen previously, in an earlier portion of the experiment) and (2) novel faces (faces that had not been seen previously). Brain imaging results supported the hypothesis that uninhibited versus inhibited persons differ in amygdala functioning. When they viewed the novel faces, adults who back when they were only two years old had been identified as inhibited children, showed higher levels of amygdala reactivity. Individual differences in a biological mechanism underlying inhibited behavior thus were stable across years of life. More recent evidence suggests a molecular basis for fear—at least in animals, whose neural systems of fear may sufficiently resemble that of humans that results can be generalized. In this work (Shumyatsky et al., 2005), researchers identified a gene that contributes to levels of a protein, called stathmin, that influences the functioning of the amygdala. Mice with and without the stathmin gene differed in behavioral measures of fear, such as “freezing” in the presence of a potentially fear‐provoking stimulus and exploring (or not) novel open spaces (Shumyatsky et al., 2005). A fascinating aspect of this work is that it was not only observational but truly experimental (see Chapter 2). The research included genetic “knockout” techniques in which genetic material is manipulated experimentally (Benson, 2004). Interpreting Data on Biology and Personality In summary, the evidence that genetically based biological processes contribute to individual differences in inhibition and fear in response to novelty is strong, as is the evidence that the amygdala is involved in fear responses. Nonetheless, it is important not to overinterpret this evidence. Some interpretations that may at first appear appealing are overinterpretations, that is, conclusions that go beyond the actual data. Considering them is important to thinking critically about the biological foundations of personality. One might conclude that the amygdala is a kind of fear‐production machine: the necessary and sufficient cause of fear. This interpretation of the scientific evidence would be unwarranted for a number of reasons. First, the amygdala can be involved in many psychological functions other than fear responses; it is not specifically dedicated to the emotion of fear. Second, the amygdala is the only biological mechanism in fear responses. Some evidence suggests that it is not even necessary for the experience of emotions such as fear, even if it typically is involved in the fear response. Anderson and Phelps (2002) compared the daily emotional experiences of people with amygdala damage (lesions to and/or removal of portions of the amygdala, done surgically as a medical procedure to alleviate seizures these individuals had experienced) to people with normal, intact amygdalas. If the amygdala was necessary to the experience of emotions, these people's emotional life should have differed dramatically. But it turns out that they did not differ at all! People with amygdala damage experienced the same range of emotions as did biologically normal persons. The authors conclude that “the complexity and richness of human emotional life do not appear to be supported by the amygdala alone” (Anderson & Phelps, 2002, p. 717). Furthermore, the amygdala may primarily be involved in the processing not of fear but of novelty. Kagan (2002) has reviewed evidence indicating that, in fact, “a state of surprise is a more reliable incentive for amygdalar activation than a state of fear” (p. 13). Finally, the fact that inherited differences in a biological system, the amygdala, contribute to fearful behavior may prompt the interpretation that environmental experiences are unimportant and that a person's fearful tendencies cannot change. This conclusion, too, would be a mistake. Research (Fox et al., 2005) indicates that genetic factors interact with environmental ones in predicting behavioral inhibition in childhood. The environmental factor these researchers investigated was social support, specifically, the degree to which mothers provided nurturing, intimate social support when children were 4 years old. They also measured molecular genetic factors already known to be linked to inhibited behavioral tendencies. The genetic and environmental factors were used together to predict inhibited behavior with peers when children were 7 years of age. The main finding was that the link from genetics to behavior depended on the environmental factor, social support. Genetics were less strongly linked to behavior among children who received a high level of social support (Fox et al., 2005); high levels of social support, in other words, lessened the genetic differences that one would observe among children who experience less supportive environments. In sum, “just because a person is born with a particular temperament … doesn't mean there is a simple set of instructions or blueprints. Nor … are [people] ‘stuck’ with their personalities from birth. On the contrary, one of the marvelous features of temperament is a built‐in flexibility that allows us to adapt to life's hurdles and challenges. Everyone has the ability to grow and to change at every stage of life” (Hamer & Copeland, 1998, p. 7). It is always genes and environment rather than genes versus environment. Evolution, Evolutionary Psychology, and Personality When explaining the biological causes of a behavior, two types of causes can be cited; they often are labeled “proximate” and “ultimate” causes. Proximate causes refer to biological processes operating in the organism at the time the behavior is observed. Suppose that you take a break from reading this textbook to sit outside to get a tan. A proximate explanation of the tanning process would refer to the biological mechanisms in the skin that respond to sunlight, giving you a golden glow. (If, as a result of reading this example, you now are motivated to work on your tan, we note that you could always take your textbook out in the sun with you.) Ultimate causes ask a different question: Why is a given biological mechanism a part of the organism, and why does it respond to the environment in a given way? An ultimate‐cause explanation of the tanning process would ask why it is that humans possess skin that tans in response to intense, prolonged sunlight. Ever since Darwin, ultimate‐cause explanations have invoked principles of natural selection. Scientists try to understand how and why a given biological mechanism evolved. These understandings are grounded in the basic principle that some biological features are better than others, at least for organisms living in a given environment. The organisms that possess those features are more likely to survive, to reproduce, and thus to be the ancestors of future generations. Organisms lacking the adaptive biological feature are less likely to pass on their genes to the next generation. Across a number of generations, the population as a whole is increasingly populated by beings who possess the adaptive biological mechanism. The biological mechanism, then, evolves. This historical view, grounded in Darwinian principles of evolution via natural selection, provides an “ultimate‐cause” explanation.
In this section, we introduce you to ultimate‐cause, evolutionary‐based interpretations of personality functioning. We do so by reviewing developments in the field of evolutionary psychology (Buss, 2005, 2008, 2012) and their applications to questions of personality and individual differences. Subsequent sections of this chapter review proximate‐cause explanations of personality functioning that involve the action of genes and neural systems.
Evolutionary Psychology
Many psychologists have tried to build such evolutionary explanations of psychological functioning. As a review by Linnda Caporael (2001) explains, these efforts have been of more than one type. Although all contemporary psychologists recognize the importance of analyzing evolutionary forces, their analyses differ. As a result, there exist “evolutionary psychologies” (Caporael, 2001)—that is, plural. The main points of difference involve the degree to which psychological tendency is seen as “hardwired” (i.e., as a biologically fixed, inevitable aspect of human nature) versus being a result of interactions between biology and culture. The latter perspective leaves open the possibility that different cultures will produce different psychological tendencies (Nisbett, 2003).
In recent decades, writers who highlight the evolutionarily “hardwired” aspects of human nature (Buss, 2012; Buss & Hawley, 2011) have gained much prominence in personality psychology. Their work represents a startling challenge to many ways of thinking in the field. In this approach, contemporary human functioning is understood in relation to evolved solutions to adaptive problems faced by the species over millions of years. The idea is that basic psychological mechanisms are the result of evolution by selection; that is, they exist and have endured because they have been adaptive to survival and reproductive success. The fundamental components of human nature, then, can be understood in terms of evolved psychological mechanisms that have adaptive value in terms of survival and reproductive success. Such aspects of human nature, as our fundamental motives and emotions, can thereby be understood in terms of their adaptive value.
Four points about evolution and the human mind are highlighted in this approach to evolutionary psychology (Pinker, 1997; Tooby & Cosmides, 1992). First, the features of mind that evolved are those that solve problems important to reproductive success. The critical feature in evolution is the passing on of genes. However, note that the reproduction‐related problems do not merely involve acts of sexual reproduction. They include a wide range of problems relevant to the survival and reproduction of the organism. Consider the following simple example. Organisms need to see objects at a distance and to judge how near or far they are from objects. An organism that could not make these judgments commonly would be at a disadvantage (e.g., when hunting or trying to protect itself from a predator). To solve this problem, our nervous systems have evolved a solution: a pair of eyes that enables us to see in depth. The psychological capacity, depth perception, reflects a specific neural system that has evolved because of its usefulness in solving a recurrent problem faced throughout evolution. The intriguing feature of contemporary evolutionary psychology is that it extends this type of analysis to include patterns of social behavior that solve significant social problems faced across the eons of evolutionary history.
A second point is that the evolved mental mechanisms are adaptive to the way of life of hundreds of centuries ago, when our ancestors were hunters and gatherers (Tooby & Cosmides, 1992). An implication is that we may have evolved psychological tendencies that no longer are good for us. For example, our taste preference for fat was “clearly adaptive in our evolutionary past because fat was a valuable source of calories but was very scarce. Now, however, with hamburger and pizza joints on every street corner, fat is no longer a scarce resource. Thus, our strong taste for fatty substances now causes us to over‐consume fat. This leads to clogged arteries and heart attacks and hinders our survival” (D. M. Buss, 1999, p. 38).
Third, evolved psychological mechanisms are domain specific. According to evolutionary psychologists, we do not evolve a general tendency “to survive”. Instead, the body and mind consist of evolved mechanisms that solve specific problems that occur in specific types of settings, or domains. Fundamental aspects of human nature, such as specific motives and emotions, apply to specific problems and contexts. For example, evolution does not give us a general tendency to be afraid, but instead selects for psychological mechanisms that cause us to fear specific stimuli that have been threats to humans across the course of evolution. Similarly, evolution gives us specific emotions, such as jealousy, because these emotional reactions have proven adaptive in solving specific problems of social living. These domain‐specific motives and emotions have remained as part of our human nature because they facilitated survival and reproductive success given the problems to be faced in our ancestral environment. Note that this makes evolutionary psychology quite different than the trait approaches we discussed in the previous two chapters. In trait theory, a context‐free variable such as “agreeableness” might be seen as responsible for actions such as being agreeable on a date and being agreeable toward a young niece or nephew. In evolutionary psychology, these acts would be seen as merely superficially similar. Even though they might both be described as “agreeable” behaviors, they would be caused by different psychological mechanisms, since, throughout the course of evolution, attracting opposite‐sex mates and caring for children were distinctly different problems of social life.
The fourth point concerns the components and overall structure of the mind or its “architecture”. One view of mental architecture is that the mind is like a computer with a central processing mechanism. All information—words, images, videos, games, regardless of their content—is processed by this one mechanism. Evolutionary psychologists reject this view of mental architecture. A core idea of evolutionary psychology is that the mind contains multiple information‐processing devices, each of which processes information from one specific domain of life (Pinker, 1997). The concept of domain is critical. Different challenges that recurred throughout evolution—attracting mates, finding edible food, taking care of children, and so forth—each constitute a distinct problem domain. Some evolutionary psychologists suggest that we have evolved a distinct mental mechanism for solving problems in each domain. These mechanisms often are called mental “modules” (Fodor, 1983), a term implying that they are special‐purpose mechanisms that carry out a domain‐specific mental function.
What is important to recognize here, as with all human evolved psychological mechanisms, is that they are not rigid behaviors like instincts but rather provide for flexibility in meeting the demands of basic adaptive mechanisms.
Social Exchange and the Detection of Cheating
Which psychological mechanisms have evolved through selection, and which adaptive problems did they evolve to solve? Seminal work on this question was conducted by the evolutionary psychologist Leda Cosmides (1989). She explored a particular type of social setting and associated problem that, she reasoned, has been of significance throughout the course of evolution: “social exchange,” that is, the exchange of goods and services. Throughout evolution, part of people's social interaction has involved the mutual exchange of beneficial goods. For example, a person may agree to help another with child‐care tasks one day if that other person agrees to do the same on another day. People in a village that grows a large amount of a particular crop may agree to exchange some of their food with people from another village that produces a desired manufactured product. In any such exchange, it is important to avoid being cheated; the ability to detect cheating, in other words, has survival value. If you chronically fail to notice that a person needing change asked you for “two tens for a five” instead of “two fives for a ten,” you lose resources that are required for social living, survival, and reproduction. Cosmides reasoned that cheating detection has been of such importance that a distinct mechanism for detecting cheating has evolved. She tested this idea in a clever manner that illustrates the overall approach of evolutionary psychologists to questions of mental architecture. Her work involved a particular type of logical reasoning task. In the task, people are asked to solve an “if then” problem—that is, to test a problem of logical relations in which one has to determine if a rule of the sort “if P then Q” is accurate.
As you might guess from this description, such abstract logical problems generally are difficult. People in psychology experiments commonly fail to solve them. However, Cosmides herself reasoned that people would be good at solving the problem if its content related to the detection of cheating. Although people might be poor at solving the problem “if P then Q?,” they might be quite good at solving a problem such as “if person made a lot of money, did they pay taxes?” If the problem concerns potential cheating, then the particular subsystem of mind that processes information about social contracts and cheating should come into play, and people should be better at solving the problem. This is precisely what Cosmides (1989) found. Although a minority of people correctly solve abstract “P then Q” problems, a large majority correctly solve the same problem if the content of the problem involves the detection of cheating.
Evidence suggests that the ability to solve cheating problems is a human universal, precisely as evolutionary psychologists would expect. Both U.S. college students and nonliterate research participants in cultures isolated from the industrialized world solve such problems accurately (Sugiyama, Tooby, & Cosmides, 2002).
Other evidence has begun to identify brain regions that contribute to reasoning about social exchange. Researchers tested a neuropsychological patient who, in a bicycle accident, incurred a head injury that damaged portions of his brain's frontal cortex and amygdala. The patient performed normally (i.e., in a manner similar to persons without brain injury) on reasoning tasks other than social exchange but showed impaired performance when solving problems involving social contracts (Stone, Cosmides, Tooby, Kroll, & Knight, 2002).
Sex Differences: Evolutionary Origins?
Another domain to which evolutionary psychologists have turned their attention is sex differences. The evolutionary psychologist's reasoning is that, throughout evolution, male and female human beings have had different roles to play as a natural result of biological differences between the sexes. Differences, of course, are found in physical stature, as well as in child care (e.g., pregnancy, breast feeding). Since these differences have been consistent across the course of evolution, it is reasoned that the human mind has evolved sex‐specific psychological tendencies. In other words, men and women, as a result of facing somewhat different problems across the course of evolution, are predicted to have somewhat different brains that predispose them to different patterns of thinking, feeling, and action.
Before considering this research, we note that drawing conclusions about psychological differences between men and women is a very tricky matter. Even if one finds such differences, it is hard to interpret them. True, men and women differ biologically. So one interpretation is that biology causes sex differences. But men and women also differ socially; specifically, they often develop within societies that do not treat men and women equally. Men commonly earn more money than women and hold more positions of power in society. It may be that, regardless of biological differences, any group within society that makes more money and holds more positions of power will differ, psychologically, from a group that earns less money and holds fewer positions of power. Sex differences, then, could be socially constructed, rather than being biologically caused. A core idea of evolutionary psychology, however, is that biology determines sex differences. Evolved psychological differences between men and women are seen as being responsible for the gender differences we observe in society. This notion has been advanced most vigorously by the evolutionary psychologist David Buss (1989, 1999). He has considered sex differences in two aspects of male–female relationships: mate preferences and causes of jealousy.
Male–Female Mate Preferences
Do you like men who are rich and professionally successful? Do you like women who appear youthful and have “curvey” hips? If so, evolutionary psychologists think they know why. According to evolutionary theory, as introduced by Darwin, selection pressures across the course of human evolution have produced sex differences in preferences for mates. The particular features of men that are attractive to women, and the features of women that are attractive to men, are thought to be a product of evolution.
Two ideas underlie the contemporary evolutionary psychologist's analysis of sex differences. One is something called parental investment theory (Trivers, 1972). The theory is an analysis of the different costs, or investments, that men versus women have made in parenting throughout the ages. The core idea is that biological differences between the sexes cause women to invest more in parenting. Women can pass their genes on to fewer offspring than men potentially can. This is because of both the limited time periods during which they are fertile and, relative to men, the more limited age range during which they can produce offspring. In other words, parental investment is greater for females because of the greater “replacement costs” for them. Also, women of course carry the biological burden of pregnancy, which lasts for nine months. Men not only do not have to bear the physical costs of pregnancy, but, unlike women, in principle they can be involved in multiple pregnancies at the same time. It follows that females will have stronger preferences about mating partners than will males and that males and females will have different criteria for the selection of mates (Trivers, 1972). Women need men to help with the burdens of pregnancy and child care and thus should seek men who have the potential for providing resources and protection. Men, in contrast, should be less interested in protection; instead, they are expected to focus on the reproductive potential of a partner (the person's youth and other biological markers of reproductive fitness). Although these preferences evolved ages ago, they still are present in the human mind. Thus, they should be evident in current social patterns. For example, since women are more interested than are men in a partner who can provide resources, the evolutionary psychologist would expect that, when on a dinner date, men would be more likely to pay for the dinner. Paying for dinner is viewed as an evolved strategy through which men display financial resources and thus add to their attractiveness to women.
In addition to parental investment theory, a second line of reasoning concerns parenthood. Since women carry their fertilized eggs, they can always be sure that they are the mothers of the offspring. On the other hand, males cannot be so sure that the offspring is their own and therefore must take steps to ensure that their investment is directed toward their own offspring and not those of another male (D. Buss, 1989, p. 3). Thus follows the suggestion that males have greater concerns about sexual rivals and place greater value on chastity in a potential mate than do females.
The following are some of the specific hypotheses that have been derived from parental investment and parenthood probability theories (Buss, 1989; Buss, Larsen, Westen, & Semmelroth, 1992):
1. A woman's “mate value” for a man should be determined by her reproductive capacity as suggested by youth and physical attractiveness. Chastity should also be valued in terms of increased probability of paternity.
2. A man's “mate value” for a woman should be determined less by reproductive value and more by evidence of the resources he can supply, as evidenced by characteristics such as earning capacity, ambition, and industriousness.
3. Males and females should differ in the events that activate jealousy, males being more jealous about sexual infidelity and the threat to paternal probability, and females more concerned about emotional attachments and the threat of loss of resources.
Buss (1989) obtained questionnaire responses from 37 samples, representing over 10,000 individuals, from 33 countries located on 6 continents and 5 islands. His samples reflected tremendous diversity in geographic locale, culture, ethnicity, and religion. What was found? First, in each of the 37 samples, males valued physical attractiveness and relative youth in potential mates more than did females, consistent with the hypothesis that males value mates with high reproductive capacity. The prediction that males would value chastity in potential mates more than would females was supported in 23 out of the 37 samples, providing moderate support for the hypothesis. Second, females were found to value the financial capacity of potential mates more than did males (36 of 37 samples) and valued the characteristics of ambition and industriousness in a potential mate to a greater extent than males (29 of 37 samples), consistent with the hypothesis that females value mates with high resource‐providing capacity.
Causes of Jealousy
In subsequent research, three studies were conducted to test the hypothesis of sex differences in jealousy (D. M. Buss et al., 1992). In the first study, undergraduate students were asked whether they would experience greater distress in response to sexual infidelity or emotional infidelity. Whereas 60% of the male sample reported greater distress over a partner's sexual infidelity, 83% of the female sample reported greater distress over a partner's emotional attachment to a rival.
In the second study, physiological measures of distress were taken on undergraduates who imagined two scenarios, one in which their partner became sexually involved with someone else and one in which their partner became emotionally involved with someone else. Once more, males and females were found to have contrasting results, with males showing greater physiological distress in relation to imagery of their partner's sexual involvement and women showing greater physiological distress in relation to imagery of their partner's emotional involvement.
The third study explored the hypothesis that males and females who had experienced committed sexual relationships would show the same results as in the previous study but to a greater extent than would males and females who had not been involved in such a relationship. In other words, actual experience in a committed relationship was important in bringing out the differential effect. This was found to be the case for males for whom sexual jealousy was increasingly activated by experience with a committed sexual relationship. However, there was no significant difference in response to emotional infidelity between women who had and had not experienced a committed sexual relationship.
In sum, the authors interpreted the results as supporting the hypothesis of sex differences in activators of jealousy. Although alternative explanations for the results were recognized, the authors suggested that only the evolutionary psychological framework led to the specific predictions.
Evolutionary Origins of Sex Differences: How Strong Are the Data?
Based on our coverage so far, evolutionary psychology appears to provide a quite convincing explanation of sex differences. Indeed, many contemporary psychologists find the theory convincing in this regard. However, in recent years, new research findings have begun to raise questions about the validity of the theory as it applies to sex differences in social behavior. In evaluating evolutionary psychology, a major question is whether patterns of sex differences are found universally, that is, across all cultures of the world. Evolutionary psychology expects that sex differences will be universal. People share the same brain and physical anatomy. Humans share a common evolutionary past; throughout most of our species' evolutionary history, all humans lived in the same region of the world, Africa. If evolved psychological mechanisms are the cause of sex differences in social behavior, then those sex differences should be similar in all regions of the world and all human cultures.
A contrasting idea is that sex differences are a product of features of the society in which people live. In societies that treat men and women very differently, for example, in which there are particularly large differences in the work opportunities available to men versus women and in the income that they earn, sex difference may be larger than in societies in which men and women share more equally in the goods of society. Such a result would contradict the predictions of evolutionary psychology. Eagly and Wood (1999) have provided evidence on this question. They reanalyzed data from a multinational study of men's and women's preferences in mates. The evolutionary psychology prediction is that the same pattern of sex differences would be found in all cultures, with women preferring men who have the capacity to earn money and men preferring young women with domestic skills. On the one hand, some of Eagly and Wood's findings were consistent with evolutionary psychology. For example, when looking for a mate, men did tend to value the quality of being a good cook to a greater degree than did women. However, other findings contradicted evolutionary psychology by demonstrating the existence of variations in the nature of sex differences. Specifically, sex differences were found to be smaller within societies in which men and women have more similar roles within the overall social structure. In societies in which there was greater gender equality, women were less concerned with men's earning capacity, men were less concerned with women's housekeeping skills, and sex differences on these measures were smaller (Eagly & Wood, 1999). A subsequent review of anthropological research on sex differences similarly was “not very supportive of evolutionary psychology” (Wood & Eagly, 2002, p. 718). Instead of pointing to universal patterns of sex differences that result from biology alone, the data were consistent with a biosocial view of sex differences. In a biosocial perspective, sex differences reflect interactions between biological qualities of men and women and social factors, particularly those involving economic conditions and the division of labor within society (Wood & Eagly, 2002). Additional data question the original evolutionary psychology conclusions about sex differences. Miller, Putcha‐Bhagavatula, and Pedersen (2002) noted that initial studies of sex differences in mate preferences by Buss and colleagues sometimes failed to compare men and women on all relevant psychological variables. When reanalyzing these mate‐preference data, the Miller group (2002) found that “across the data, what men desired most in a mate women desired most in a mate. [There were] extraordinarily high correlations between men's and women's ratings for both short‐term and long‐term sexual partners” (p. 90). The evolutionary psychologist's claim that men and women differ in the events that activate jealousy (Buss et al., 1992) is also contradicted by recent data (DeSteno, Bartlett, Braverman, & Salovey, 2002). These recent findings suggest that the original findings of evolutionary psychologists in this area may have resulted from a methodological artifact; an arbitrarily chosen feature of the research procedures may have artificially contributed to the results. Most of the evolutionary psychological original research on the topic involved a multiple‐choice or “forced‐choice” method. Participants in research are asked if they would be more distressed if they found that their romantic partner (a) had sexual relations with another person or (b) formed a close emotional bond with another person. Note, first, that this is an odd question, particularly from an evolutionary psychological perspective. Over the course of human evolution, it cannot possibly be the case that people frequently were faced with learning simultaneously about a partner's sexual and emotional relations and then having to decide which was worse. Recognizing the oddity of this forced‐choice procedure, DeSteno and colleagues (2002) also asked participants to consider the sexual and emotional scenarios one at a time and to indicate how upset they would be by each one. With this change in procedure, the sex differences in jealousy predicted by evolutionary psychology were no longer found. Instead, men and women were highly similar. Both were more distressed by sexual infidelity than by news of a partner's emotionally close nonsexual relationship. Related findings come from the analysis of men's and women's physiological responses to imagining sexual versus emotional infidelity (Harris, 2000). If men and women possess different evolved modules of the sort suggested by parental investment theory, then they should respond differently to these two scenarios: Men should react with stronger feelings of jealousy when imagining sexual infidelity, and women should react more when envisioning emotional infidelity. In Harris's careful research, women were not found to be more responsive to emotional (versus sexual) infidelity. Men did respond strongly to sexual infidelity, but, as Harris points out, that may not have resulted from the infidelity but merely from the idea that sex occurred; men simply may respond relatively strongly to any scenario involving sexual content. On her physiological measures, Harris (2000) indeed found that men responded strongly to imagined sexual encounters whether or not infidelity was involved. Subsequent work similarly failed to find the sex differences predicted by evolutionary psychology when research participants were asked to contemplate actual instances of infidelity they had experienced, rather than the hypothetical instances of infidelity that some previous researchers had studied (Harris, 2002). The overall findings, then, contradict the evolutionary psychological account of sex differences in jealousy—an account that, as Harris (2000) noted, had previously been seen as a “showcase example of evolutionary psychology” (p. 1082). In summary, then, data do not provide consistent support for evolutionary psychological hypotheses about sex differences in mate attraction and jealousy. The exact nature of gender differences that might exist, and the roles of evolutionary hardwiring versus social structure in bringing them about, thus remain to be defined. Genes and Personality Whatever we inherit, we inherit it thanks to our genes. We possess 23 pairs of chromosomes, one of each pair from each of our biological parents. The chromosomes contain thousands of genes. Genes are made up of a molecule called DNA and direct the synthesis of protein molecules. Genes may be thought of as sources of information, directing the synthesis of protein molecules along particular lines. Information in the genes, then, directs the biological development of the organism. In appreciating the relation of genes to behavior, it is important to understand that genes do not govern behavior directly. Thus, there is no “extraversion gene” or “introversion gene,” and there is no “neuroticism gene.” To the extent that genes influence the development of personality characteristics such as the Big Five, described in Chapter 8, they do so through the direction of the biological functioning of the body. In addition, a single gene does not determine a trait. Rather, a personality trait reflects the interaction of many genes with many environmental influences (Turkheimer, 2006). Behavioral Genetics The study of genetic contributions to behavior is called the field of behavioral genetics. Behavioral geneticists employ a variety of techniques to estimate the degree to which variation in psychological characteristics is due to genetic factors. As we shall see, the methods of behavioral genetics also can, and do, provide evidence of environmental effects on personality. Behavioral geneticists employ three primary research methods: selective breeding studies, twin studies, and adoption studies. Selective Breeding Studies In selective breeding studies, animals with a desired trait for study are selected and mated. This selection and reproduction process is used with successive generations of offspring to produce a strain of animals that is consistent within itself for the desired characteristic. Selective breeding is not only a research technique; it is used, for example, to breed race horses or breeds of dogs with desired characteristics. Once one has created different strains of animals through selective breeding, one not only can study their typical behavioral tendencies. It also is possible to subject the different strains to different experimentally controlled developmental experiences. Researchers then can sort out the effects of genetic differences and environmental differences on the observed later behavior. For example, the roles of genetic and environmental factors in later barking behavior or fearfulness can be studied by subjecting genetically different breeds of dogs to different environmental rearing conditions (Scott & Fuller, 1965). Selective breeding research has enhanced our understanding of how genes contribute to problems that often are blamed solely on the individual. Consider work on alcoholism (Ponomarev & Crabbe, 1999). The researchers bred various strains of mice that proved to exhibit qualitatively different responses to alcohol. This work illustrated that genes play a role in responsiveness to alcohol, addiction, and withdrawal. It contributed to a more complete understanding of the fact that genetic factors present some individuals with severe vulnerabilities to lifelong problems with alcohol (Hamer & Copeland, 1998). Twin Studies Even the most enthusiastic researcher realizes that selective breeding research cannot and should not be done with humans. Ethical factors force the researcher to consider alternatives. Fortunately for science, a ready alternative exists: human twins. Twins provide a naturally occurring experiment. What the scientist wants, ideally, is a circumstance in which there are known variations in degree of genetic similarity and/or environmental similarity. If two organisms are identical genetically, then any later observed differences can be attributed to differences in their environments. On the other hand, if two organisms are different genetically but experience the same environment, then any observed differences can be attributed to genetic factors. The existence of identical (monozygotic) twins and fraternal (dizygotic) twins offers a good approximation to this research ideal. Monozygotic (MZ) twins develop from the same fertilized egg and are genetically identical. Dizygotic (DZ) twins develop from two separately fertilized eggs and are as genetically similar as any pair of siblings, on the average sharing about 50% of their genes. Researchers capitalize on these systematic differences between MZ and DZ twins by conducting twin studies to gauge the degree to which genetic factors explain person‐to‐person variations in psychological characteristics. Two logical considerations underpin the twin method. The first is that, since MZ twins are genetically identical, any systematic difference between them must be due to environmental effects. Interestingly, then, the study of genetically identical persons is particularly valuable for revealing the effects of environmental experience. Second, it is the difference in similarity between MZ twin pairs and DZ twin pairs that is crucial to estimating the effects of genetics. Specifically, we know that MZ twins are more similar to one another genetically than DZ twins are similar to one another genetically. If genetics influence a given personality characteristic, then MZ twins, as a result of being more similar genetically, also should be more similar on the given personality characteristic than are DZ twins. If they are not, then there is no genetic effect. When studying both MZ and DZ twin pairs, then, the researcher can compare them (MZ similarity compared to DZ similarity on a trait of interest) to determine the magnitude of the influence of genetic factors. This genetic influence usually is expressed numerically in terms of a heritability coefficient (described below). The twin strategy usually is conducted with twins who grow up in the same household. However, circumstances sometimes force parents to give up children for adoption early in life. As a result, MZ and DZ twins sometimes are reared apart. This creates a circumstance of remarkable interest to the psychological scientist and the public at large, namely, biologically identical people who are raised in different environments. What happens? Does biology win out, with genetically identical twins being psychologically identical despite their different experiences? Or do social experiences win out, with people differing substantially despite their identical genes?
These questions can be answered thanks to an international data set that features large numbers of reared‐apart twins who have completed various psychological measures (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990). Results provide clear evidence that the effects of biology endure across different circumstances. On multiple personality trait measures, MZ twins raised apart were found to be similar to a significant degree; twin correlations indicating the degree of similarity between the twins were in the .45 to .50 range. Of particular interest is that MZ twins raised apart were about as similar to one another as were MZ twins raised together (Bouchard et al., 1990). Being raised in the same household did not make the twins more similar on broad personality trait measures. We return to this fascinating finding, and its interpretations and implications, after reviewing further research findings below. A simple strategy for studying the influence of genetics on personality is to conduct research on twins. On each of a wide variety of psychological characteristics, identical twins are found to be more similar than fraternal twins. Such findings indicate that genetic factors contribute significantly to personality. Fascinating research by the psychologist Segal (2014) has compared identical and fraternal twins. The former, as aunts and uncles, are found to invest more in caring for the children of their twins than are the latter. They also mourn more for the loss of the twin sibling. Of particular interest is her research on what happens when identical and fraternal twins are reunited after being raised separately. She finds that reunited identical twins quickly establish much stronger bonds than do fraternal twins. She attributes these stronger bonds to their greater genetic similarity. Adoption Studies Studies of children who grow up with caregivers other than their biological parents are called adoption studies. (Adoption studies sometimes involve identical twins, as in the research reviewed in the paragraph immediately above, but commonly may involve nontwin siblings.) Adoption studies offer another method for studying genetic and environmental effects. When adequate records are kept, it is possible to consider the similarity of adopted children to their natural (biological) parents, who have not influenced them environmentally, and to compare this with the similarity to their adoptive parents, who share no genes in common with them. The extent of similarity to their biological parents is indicative of genetic factors, while the extent of similarity to their adoptive parents is indicative of environmental factors. Finally, such comparisons can be extended to families that include both biological and adoptive children. Take, for example, a family of four children; two of the children are the biological offspring of the parents and two of the children have been adopted. The two biological offspring share a genetic similarity with one another and with the biological parents that is not true for the two adopted children. Assuming the two adopted children are unrelated, they share no genes in common but share a genetic similarity with their parents and any siblings who might exist in other environments. Thus, it is possible to compare different parent–offspring and biological sibling–adoptive sibling combinations in terms of similarity on personality characteristics. For example, one can ask whether the biological siblings are more similar to one another than are the adoptive siblings, whether they are more similar to the parents than the adoptive siblings, and whether the adoptive siblings are more similar to their biological parents than to their adoptive parents. A “yes” answer to such questions would be suggestive of the importance of genetic factors in the development of the particular personality characteristic. It should now be clear that in twin and adoption studies, we have individuals of varying degrees of genetic similarity being exposed to varying degrees of environmental similarity. By measuring these individuals on the characteristics of interest, we can determine the extent to which their genetic similarity accounts for the similarity of scores on each characteristic. For example, we can compare the IQ scores of MZ and DZ twins reared together and apart, biological (nontwin) siblings reared together and apart, adoptive sibling and biological siblings with parents, and adoptive siblings with their biological and adoptive parents. Some representative correlations are presented in Table 9.1. The data clearly suggest a relationship between greater genetic similarity and greater IQ similarity. TABLE 9.1 Average Familial IQ Correlations® Source: Adapted from “Familial Studies of Intelligence: A Review,” by T. J. Bouchard and M. Mcgue, 1981, Science, 250, p. 1056. Reprinted from McGue et al., 1993, p. 60. As genetic similarity increases, so does the magnitude of the correlations for IQ, suggesting a strong genetic contribution to intelligence. Relationship Average R Number of Pairs REARED‐TOGETHER BIOLOGICAL RELATIVES MZ twins 0.86 4,672 DZ twins 0.60 5,533 Siblings 0.47 26,473 Parent offspring 0.42 8,433 Half‐siblings 0.35 200 Cousins 0.15 1,176 REARED‐APART BIOLOGICAL RELATIVES MZ twins 0.72 65 Siblings 0.24 203 Parent offspring 0.24 720 REARED‐TOGETHER NONBIOLOGICAL RELATIVES Siblings 0.32 714 Parent offspring 0.24 720 Note: MZ, monozygotic; DZ, dizygotic. Heritability Coefficient How, exactly, does the behavioral geneticist determine the degree to which genetic variations determine variations among people in a personality characteristic? This usually is done by computing what is called a heritability coefficient, or h2 (it is h “squared” because numbers are squared when computing variations around an average score). The heritability coefficient represents the proportion of observed variance in scores that can be attributed to genetic factors. In a study involving both MZ and DZ twins, h2 is based on the difference between the MZ and DZ correlations. If MZ twins (who share all their genes) are no more similar to one another than are DZ twins (who share half their genes), then there is no genetic effect: h2 is zero. If MZ twins differ greatly from DZ twins, h2 is large; its upper limit is 1.0, or 100% of the total variance. To the extent that h2 is less than 1.0, there exists variance that is not accounted for by genetic factors; this remaining variance is explained by environmental variation. Note that the heritability coefficient refers to variation in the population examined in a given study. This point has two implications. First, different heritability coefficients, for the same psychological trait, may be observed in different populations; for example, if one is studying a population in which many people have been subjected to environmental effects that exert a particular large influence on them (e.g., stress from disease or war), then the environmental effects in this group will be relatively large and h2 will be relatively small (Grigorenko, 2002). Second, the heritability coefficient does not indicate the degree to which genetics accounts for the fact that a particular individual has a particular characteristic. It is a measure of variation in the population. For some attributes (e.g., a biological feature or psychological capacity possessed by all humans), there may be no person‐to‐person variation. The h2 would be zero even if genetics explains why all people have the attribute. For other attributes (e.g., your ability to read), the attribute may be explained by an interaction of genetic and social factors, and it may make little sense to say that genetics versus the environment each accounted for X percent of the attribute. The h2 is an estimate associated with a population and not a definitive measure of the action of genes. Heritability of Personality: Findings We now consider additional behavioral genetic findings and the conclusions about personality to which they lead. An interesting feature of this work is the consistency of findings across studies and across personality traits. As key investigators write, “It is difficult to find psychological traits that reliably show no genetic influence” (Plomin & Neiderhiser, 1992). “For almost every behavioral trait so far investigated, from reaction time to religiosity, an important fraction of the variation among people turns out to be associated with genetic variation. This fact need no longer be subject to debate” (Bouchard et al., 1990). These quotes reflect findings from numerous twin and adoption studies. These studies have been conducted on a wide variety of personality variables, often with large samples of research participants and with the work extending over significant periods of time. The evidence of genetic influence is sometimes startling, as when identical twins reared apart and brought together as adults are found not only to look and sound alike but to have the same attitudes and share the same hobbies and preferences for pets (Lykken, Bouchard, McGue, & Tellegen, 1993). But beyond such almost eerie observations is a pattern of results strongly suggesting an important role for heredity in almost all aspects of personality functioning (Plomin & Caspi, 1999). Recent estimates of the overall heritability of personality traits converge on roughly 40%. Table 9.2 presents heritability estimates for a wide variety of characteristics. For comparative purposes, heritability estimates for height and weight are included, as well as a few other characteristics that may be of interest.
TABLE 9.2 Heritability Estimates Sources: Bouchard et al., 1990; Dunn & Plomin, 1990; Loehlin, 1992; McGue et al., 1993; Pedersen et al., 1998; Pedersen et al., 1992; Plomin, 1990; Plomin et al., 1990; Plomin & Rende, 1991; Tellegen et al., 1998; Tesser, 1993; Zuckerman, 1991. The data indicate a strong genetic contribution to personality (overall estimate of 40% of the variance), a contribution not as large as that for height, weight, or IQ but larger than that for attitudes and behaviors such as TV viewing. Trait h2 estimate Weight 0.60 IQ 0.50 Specific cognitive ability 0.40 School achievement 0.40 BIG FIVE Extraversion 0.36 Neuroticism 0.31 Conscientiousness 0.28 Agreeableness 0.28 Openness to experience 0.46 EASI TEMPERAMENT Emotionality 0.40 Activity 0.25 Sociability 0.25 Impulsivity 0.45 ATTITUDES Conservatism 0.30 Religiosity 0.16 Racial integration 0.00 TV viewing 0.20 Note: EASI = Four dimensions of temperament identified by Buss and Plomin (1984). E, Emotionality; A, Activity; S, Sociability; I, Impulsivity. A criticism made of behavior‐genetic research on personality is that most studies are based on self‐report questionnaire methods. A recent study is important in this regard in that two independent peer reports as well as self‐reports on the NEO Five‐Factor Inventory were collected on a sample of 660 MZ twins and 304 DZ twins (200 same sex and 104 opposite sex). The investigators found good evidence of reliability of ratings in terms of peer–peer rating agreement, good evidence of the accuracy of self‐report in terms of self‐peer rating agreement, and general support for earlier findings concerning genetic influence on all of the Big Five personality factors (Table 9.3) (Riemann, Angleitner, & Strelau, 1997). TABLE 9.3 Peer–Peer, Self–Peer, MZ and DZ (Self‐Report), and MZ and DZ (Average Peer Report) Correlations on the NEO Five‐Factor Inventory Sources: Adapted from Riemann, Angleitner, & Strelau, 1997, pp. 460, 461, 462. Peer–Peer Self–Peer Self‐Report Averaged Peer Report MZ DZ MZ DZ N 0.63 0.55 0.53 0.13 0.40 0.01 E 0.65 0.60 0.56 0.28 0.38 0.22 O 0.59 0.57 0.54 0.34 0.49 0.30 A 0.59 0.49 0.42 0.19 0.32 0.21 C 0.61 0.54 0.54 0.18 0.41 0.17 Mean 0.61 0.55 0.52 0.23 0.40 0.18 Note: MZ, monozygotic; DZ, dizygotic. Some Caveats Before concluding this section, we warn against two inappropriate conclusions that might otherwise be drawn from the behavioral genetic data. One is that heritability coefficients indicate the extent to which a characteristic is determined by heredity for a given individual. A heritability estimate of, for example, 40% for a personality trait does not mean that 40% of your own, individual personality trait is inherited. The heritability estimate is a population statistic; it describes variation between people in the overall population. At the level of the individual person, psychological traits commonly involve such an interplay of biology and experience that it is not meaningful to say that “X%” of an individual's trait is due to one factor or another. (See discussions of gene–environment interaction and biological plasticity below.) A second inappropriate conclusion would be that, because a characteristic has an inherited component, it cannot change. In reality, environmental experiences can alter even highly heritable qualities. Height is significantly determined by genes but can be influenced by environmental nutrition in childhood. Individual differences in weight are influenced by genes, yet your weight can vary greatly depending on your diet. Molecular Genetic Paradigms Researchers have moved beyond the traditional behavior‐genetic paradigm. Instead of merely comparing different types of twins, they have turned to a direct examination of the underlying biology. This work employs molecular genetic techniques in an effort to identify specific genes that are linked with personality traits (Canli, 2008; Plomin & Caspi, 1999). By examining the genetic material of different individuals, researchers hope to show how genetic variations, or alleles, relate to individual differences in personality functioning. Ideally, one might be able to show how a genetic variation codes for alternative forms of a biological substance or system that, in turn, has psychological effects. Initial research reported the discovery of a gene linked to the trait of novelty seeking, similar to Eysenck's P factor, and to low C on the Big Five (Benjamin et al., 1996; Ebstein et al., 1996). However, this finding has not been replicated uniformly in follow‐up studies (Grigorenko, 2002). Perhaps more promising, researchers recently have identified an interaction between a specific genetic mechanism and the social environment. This research studied the effects of maltreatment in childhood on the development of antisocial behavior later in life (Caspi et al., 2003). Despite such unfortunate maltreatment, some children have good developmental outcomes; they seem to be resilient in the face of early life stress. The question, then, was whether there might be a genetic basis to this resilience. To answer this question, the researchers identified a subset of the study's population of participants who possessed a gene that has an important property: It codes for an enzyme that lowers the activity of certain neurotransmitters in the brain that are linked to aggressive behavior. Among those who had experienced maltreatment in childhood, people with this genetic variation were found to differ from others. Specifically, people who experienced severe maltreatment but who had the gene that produced high levels of the enzyme were less likely to display antisocial behavior in adulthood. The genetic variation, in other words, seemed to lower the negative impact of maltreatment. This exciting finding requires replication. However, it suggests a promising feature for molecular genetic research on personality. Subsequent work by this same research team has discovered molecular genetic factors that make individuals more or less vulnerable to becoming depressed (Caspi et al., 2003). The genetic factor that was studied is one that influences levels of serotonin in the brain. Specifically, the researchers studied a naturally occurring genetic variation that involves two different versions of a gene that affects serotonergic activity. The researchers' expectation was not that possessing a particular genetic background would lead inevitably to the experience of depression. Instead, they again expected an interaction: Genes should predict the onset of depression only in people who have certain types of environmental experiences. The environmental experiences they investigated were those that involve high levels of stress. Adults were surveyed to determine the degree to which they recently had experienced stressful life events involving factors such as finances, health, employment, and interpersonal relationships. The expectation of a gene X–environment interaction was confirmed. Individuals who were genetically predisposed to have lower levels of serotonergic activity and who experienced numerous stressful life events were much more likely to become depressed than were other individuals (Caspi et al., 2003). Again, then, molecular genetic research indicates that genes affect psychological outcomes in interaction with environmental experiences. Environments and Gene–Environment Interactions Genetic researchers realized early on that genetic and environmental influences are inextricably linked and interact in their influence on personality and behavior in adulthood. A classic study by Cooper and Zubek (1958) nicely illustrates such gene–environment interactions using the selective breeding research. In previous research, strains of maze‐bright and maze‐dull rats had been bred so that the strain of “bright” ones were much more likely to learn how to navigate a maze than were the “dull” ones. The researchers wanted to study how early environment experiences would influence the adult problem‐solving capacity of these genetically different rats. Thus, they raised one group of each strain in an enriched, stimulating environment and another group of each strain in an impoverished environment. What happened? Compared to the normal lab environment, the enriched environment improved later learning ability in the dull rats but did not help the bright ones. Conversely, the impoverished environment markedly handicapped the bright rats but did not impair the dull group. Thus, even though these rats were not “prisoners” of their genetic predispositions, the environment interacted with their genes in a crucial way, modifying the way these predispositions were expressed. For human personality, if the behavioral genetic data indicate that roughly 40 to 50% of the variance for single personality characteristics and personality overall are determined by genetic factors, then the rest of the population variance is made up of some combination of environmental effects and measurement error. Indeed, one interesting aspect of recent developments in behavioral genetics has been the effort to use twin and adoption data to determine environmental effects on personality variables. Thus, although Plomin (1990) suggests that “genetic influence is so ubiquitous and pervasive in behavior that a shift in emphasis is warranted: ask not what is heritable; ask instead what is not heritable” (p. 112), at the same time, he suggests that the “other message is that the same behavioral genetic data yield the strongest available evidence for the importance of environmental influence” (p. 115). Shared and Nonshared Environment Behavioral genetics has two messages: nature and nurture (Plomin, 1990). Research findings provide evidence of both genetic and environmental influence on personality. Behavioral geneticists estimate not only the proportion of variability in a characteristic that is due to heredity but also the proportion due to the environments.
Behavioral genetic research identifies environmental influences of two types: shared and nonshared. Shared environments are environmental influences that make siblings more alike (e.g., experiencing similar events while growing up in the same family). Nonshared environments are ones that create differences among siblings who grow up in the same family (e.g., siblings may be treated differently by parents or may develop different friendship patterns that affect their social development).
Behavioral geneticists compute numerical estimates of genetic, shared environmental, and nonshared environmental influences on individual differences. They most commonly do so by studying the similarity of identical and fraternal twins. Their studies yield a surprising finding about environments. Shared environmental effects on personality are negligible; nonshared effects are large. Put differently, the unique experiences siblings have inside and outside the family appear to be far more important for personality development than the shared experiences resulting from being in the same family. Literature reviews indicate that roughly 40% of the variability in personality trait is due to environmental factors that cause people—even people who grow up in the same household—to differ (Dunn & Plomin, 1990 ; Plomin & Daniels, 1987 ).
Loehlin, McCrae, Costa, and John ( 1998 ) examined genetic and environmental effects in three different measures of the Big Five, with results generally consistent with the above conclusions. Three findings stood out. First, all five of the Big Five dimensions showed substantial genetic influences of the same magnitude; that is, individual differences in A, C, and O were just as heritable as individual differences in E and N, which had been studied extensively in the context of Eysenck's model of these two superfactors (see Chapter 7 ). Second, these findings were independent of the effects of intellectual ability, which had also been measured and were controlled in the behavior‐genetic analyses; that is, openness was found to be a personality dimension independent of intelligence, with its own genetic basis. Third, from a methodological perspective, having available three measures for each Big Five factor made it possible to test generalizability across instruments and to estimate error separately, rather than including it with the estimate of nonshared environment as in some previous research.
Same family, different experiences. Research on genetic and environmental influences on personality shows that siblings who grow up together in the same family often differ from one another psychologically. One possible reason is “nonshared” environmental effects. Despite being in the same household, siblings may have different day‐to‐day experiences that contribute to differences in personality.
In an analysis of the data from the self‐peer ratings of MZ and DZ twins on the NEO scale (Riemann, Angleitner, & Strelau, 1997 ), Plomin calculated the percentage of the variance due to genetic factors, shared environments, and nonshared environments (including measurement error) for both self and peer ratings on the Big Five. The resulting percentages closely approximate those reported earlier, although the percentages for genetic factors tend to be lower for peer ratings than for self‐ratings (Plomin & Caspi, 1999 , p. 253).
Understanding Nonshared Environment Effects
These findings suggest that differences among families seem to matter less for the development of children than do differences within families. Recent research (Reiss, 1997 ; Reiss, Neiderhiser, Hetherington, & Plomin, 1999 ) has begun to focus on the particular processes linking genetic, family, and social influences on personality development during the important years of adolescence. This work focuses on the unique relationship between the parent and each adolescent sibling in terms of conflict and negativity, warmth and support, and so forth. In other words, the research seeks to separate out the effects of parenting common to siblings in a family from the effects of parenting unique to each sibling. The evidence to date shows substantial differences in the way siblings are treated by their parents. What is striking, however, is that much of the parenting unique to each child seems to be due to the genetic characteristics of that child. That is, differences in the way parents treat each child seem to be due to different behaviors evoked in the parent by that child, in line with earlier suggestions that children from the same family grow up to be different in part because of genetic differences that lead them to be treated differently by the parents. Most students with siblings can readily testify to such differences in parental treatment!
Does the finding that children from the same family differ due to nonshared environments mean that family experiences are unimportant? Not necessarily. Family influences may be very important but may also be unique to each child rather than shared by children in the same family. Rather than the family unit being the sole focus of study, researchers must focus on the potentially unique experiences of each child in the family.
Three Kinds of Nature–Nurture Interactions
Until now, we have considered the effects of genes and environment on personality separately. However, nature and nurture always are interacting with one another: “The critical point to remember in all of this is that in the dance of life, genes and environment are absolutely inextricable partners” (Hyman, 1999 , p. 27). Along with the continuous unfolding of the effects of genes and experience, three particular forms of gene–environment interactions have been distinguished (Plomin, 1990 ; Plomin & Neiderhiser, 1992 ). First, the same environmental experiences may have different effects on individuals with different genetic constitutions. For example, the same behavior on the part of an anxious parent may have different effects on an irritable, unresponsive child than on a calm, responsive child. Rather than a straightforward effect of parental anxiety that is the same for both kinds of children, there is an interaction between parental behavior and child characteristic. In this case, the individual is a passive recipient of environmental events. Genetic factors are interacting with environmental factors but only in a passive, reactive sense.
In a second kind of nature–nurture interaction, individuals with different genetic constitutions may evoke different responses from the environment. For example, the irritable, withdrawn child may evoke a different response from the parent than will a calm, responsive child. Within the same family, siblings can evoke different parental behaviors that then set in motion two completely different patterns of parent–child interaction. Such differences were indicated in the research considered earlier on differential parental treatment of siblings associated with genetic differences in the children. Beyond this, differences in inherited characteristics lead to different responses from peers and others in the environment outside the family. Attractive children call forth different peer responses than do less attractive children. Athletic children call forth different responses than do unathletic children. In each case, a genetically determined characteristic evokes a differential response from the environment.
Genes and Environments Interact. A child's inherited tendency to be irritable will create stress in parents. As a result, the child will experience a different environment ‐ one featuring higher levels of parental stress – than a child who inherits a lesser tendency to be irritable.
In the third form of gene–environment interaction, individuals with different constitutions select and create different environments. Once the individual is able to take an active form of interaction with the environment, which occurs at a fairly early age, genetic factors influence the selection and creation of environments. The extravert seeks out different environments than does the introvert, the athletic individual different environments than the unathletic individual, and the musically gifted individual different environments than the individual gifted in visual imagery. These effects increase over the course of time as individuals become increasingly able to select their own environments. By a certain point in time, it is impossible to determine the extent to which the individual has been the “recipient” as opposed to the “creator” of the environmental effect.
In sum, individuals can be relatively passive recipients of environments, they can play a role in environmental events through the responses they evoke, and they can play an active role in selecting and creating environments. In each case, there is a nature–nurture, gene–environment interaction. In considering the nature and nurture of personality, we must keep in mind that the development of personality is always a function of the interaction of genes with environments, that there is no nature without nurture and no nurture without nature (Manuck & McCasffrey 2014 ). We can separate the two for purposes of discussion and analysis, but the two never operate independently of one another. Indeed, genetic factors and environmental experiences are so intertwined that the usual formulation “nature versus nurture” may not even make sense any more. Instead, it may be better to think of “via nurture” (Ridley, 2003 ). The basic nature of genetic material, in other words, is that it “creates now possibilities for the organism” (Ridley, 2003 , p. 250) that are realized only if the organism encounters particular environments—that is, only if it is nurtured in a particular way. Instead of asking how heritable a trait is, we might better ask about the circumstances under which the genetic contributions to the trait are enhanced or suppressed (Krueger & Johnson, 2008 ).
