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Ann. N.Y. Acad. Sci. 1060: 6–16 (2005). © 2005 New York Academy of Sciences. doi: 10.1196/annals.1360.002

Probing the Evolutionary Origins of Music Perception

JOSH MCDERMOTTa AND MARC D. HAUSERb aPerceptual Science Group, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA bCognitive Evolution Laboratory, Department of Psychology, Harvard University, Cambridge, Massachusetts 02138

ABSTRACT: Empirical data have recently begun to inform debates on the evo- lutionary origins of music. In this paper we discuss some of our recent findings and related theoretical issues. We claim that theories of the origins of music will be usefully constrained if we can determine which aspects of music perception are innate, and, of those, which are uniquely human and specific to music. Comparative research in nonhuman animals, particularly nonhuman pri- mates, is thus critical to the debate. In this paper we focus on the preferences that characterize most humans’ experience of music, testing whether similar preferences exist in nonhuman primates. Our research suggests that many rudimentary acoustic preferences, such as those for consonant over dissonant intervals, may be unique to humans. If these preferences prove to be innate in humans, they may be candidates for music-specific adaptations. To establish whether such preferences are innate in humans, one important avenue for fu- ture research will be the collection of data from different cultures. This may be facilitated by studies conducted over the internet.

KEYWORDS: music; preferences; monkey; consonance; evolution; adaptation

INTRODUCTION

From the standpoint of evolutionary theory, music is among the most puzzling things that people do. As far as we know, music is universal, playing a significant role in every human culture that has ever been documented. People everywhere love music and expend valuable resources in order to produce and listen to it. Yet despite its central role in human culture, the evolutionary origins of music remain a great mystery. Unlike many other things that humans enjoy (e.g., food, sex, and sleep), music confers no obvious value to an organism, and for this reason music has puzzled evolutionary theorists since the time of Darwin.1

Although the adaptive function of music, if any, remains unknown, there is no shortage of proposals for how it might have evolved. Some have noted that music

Address for correspondence: Josh McDermott, Perceptual Science Group, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, NE20-444, 3 Cambridge Center, Cambridge, MA 02139. Voice: 617-258-9412; fax: 617-253-8335.

[email protected]

7MCDERMOTT & HAUSER: ORIGINS OF MUSIC PERCEPTION

might promote social cohesion in group activities like war or religion; others have proposed a sexually selected role in courtship.1–6 Developmental psychologists have drawn attention to the pacifying effect music has on infant listeners, which could constitute an adaptive function.7 Still others suggest that music was not a product of natural selection and, instead, is a side effect of mechanisms that evolved for other functions.8 Despite the longstanding interest in music’s origins, there has thusfar been little empirical data with which to decide between these and other theories (see McDermott and Hauser9 for a review).

Rather than continue to speculate on putative adaptive functions, we have focused on gathering further empirical constraints on music’s origins. Our approach is to ex- amine aspects of human music perception, and for each of them attempt to answer three questions: (1) Is the feature in question innate in humans? (2) Is it unique to humans? and (3) Is it specific to music?

Each of these questions plays an important role in thinking about the evolution of music. Capacities that are innate, that is, determined from properties present in an organism at birth, are potential targets for evolutionary explanations, unlike capaci- ties that are learned. The question of uniqueness plays an equally important role, par- ticularly for music, because music is something that only humans do (see recent reviews10,29 for a discussion of animal song). If some feature of human music per- ception is found to be shared by a nonhuman animal, and that feature is assumed to be homologous to the human feature, then the feature in question must not have evolved for the purpose of making music. Testing for aspects of human music per- ception (e.g., octave equivalence,11–13 or relative pitch perception11,12,14) can thus place useful constraints on music’s origins. The third question of music specificity is most relevant for features of music perception that have been found to be uniquely human. If some aspect of music perception in humans is found to be innate and uniquely human, the possibility remains that it evolved to serve some uniquely human function other than music, such as language or mathematics. In contrast, perceptual capacities that are innate, unique, and specific to music are strong candi- dates for adaptations for music. We thus suggest that evolutionary theories of music perception would be well served by posing these three questions about different aspects of music perception.

PREFERENCES

In this paper we will discuss one particular aspect of music perception—prefer- ences—framed by the three questions about innateness, uniqueness, and specificity. Clearly, many preferences that humans have for music are culture specific, as humans tend to prefer the music of their own culture. Preferences for entire pieces or genres of music may, however, be built on more elementary preferences that could themselves be universal and innate in humans. One simple preference that has re- ceived great attention in music literature is that for consonance over dissonance. It has been widely appreciated since at least the time of the Greeks that some combi- nations of musical notes are more pleasing than others. Although the fact that con- sonant and dissonant intervals are perceptually distinct seems to follow from what is known about the peripheral auditory system,15–17 it remains unclear why conso- nance is preferable to dissonance. This preference is generally acknowledged to be

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widespread among Westerners, but there is surprisingly little data from other cul- tures to support a claim of universality.18,19 Recent work in developmental psycho- logy, however, suggests that the preference for consonance is either innate or acquired very early, as infants as young as two months seem to exhibit the prefer- ence.20–22 There is thus some evidence that the preference is present independent from musical experience, although a larger cross-cultural database would help to augment the existing case.

Given the possibility that this and perhaps other elementary preferences are in- nate, our research has focused on the question of whether such preferences are unique to humans by testing for them in nonhuman primates. A consonance prefer- ence in a nonhuman primate would provide evidence that the preference did not evolve for the purpose of making and/or appreciating music, as nonhuman primates do not naturally make music. Conversely, any feature of music found to be uniquely human becomes a candidate for part of an adaptation for music, particularly if there is evidence that it is specific to music. Nonhuman subjects have the additional ad- vantage of being reared in a laboratory setting, in which their exposure to music can be controlled to an extent not possible in humans for practical and ethical reasons. As a result of this high level of control, many of the concerns often voiced about the role of musical exposure in experimental results from human infants can be decisive- ly addressed. We thus tested for various acoustic preferences, including that for consonance over dissonance, in nonhuman primates.

Our subjects in the experiments to be described are two species of new world monkey—cotton-top tamarins and common marmosets. Both species are native to the South American rain forest; their lineage diverged from that of humans approx- imately 48 million years ago (FIG. 1). They are generally regarded as the most prim-

FIGURE 1. Divergence times of some of the relevant taxonomic groups used in studies of the origins and evolution of music. The cotton-top tamarins and common marmosets used in our studies are New World monkeys. (Reproduced with permission from Hauser and McDermott.10)

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itive species of monkey, but are small (weighing on the order of about one pound) and harmless, making them useful experimental subjects. Their hearing characteris- tics have not been well explored, but the audiograms that have been measured in marmosets are similar to those of humans,23 and recent auditory physiology work suggests there may be higher-level similarities as well.24 Recent behavioral work in Japanese monkeys suggests that nonhuman primates can readily discriminate between consonance and dissonance,25 as one would expect given Helmholtzian the- ory and the recent physiological results that support it. What is unknown is whether nonhuman primates would also prefer consonance over dissonance as many humans do. All the animals used in our experiments were reared in captivity, and none had ever heard human music prior to the onset of the experiments.

A METHOD TO MEASURE PREFERENCES

To measure preferences in animals, we used a behavioral method in which sub- jects were placed in a V-shaped maze26 (FIG. 2); related methods have been devel- oped to test for preferences in birds.27 Each branch of the maze had a speaker at its end, and a subject’s position in the apparatus controlled their auditory environ- ment—one sound was played out of the left speaker when they were in the left branch of the maze, and another out of the right speaker when they were in the right branch. The stimulus for a particular side played continuously as long as the animal was on that side, and switched as soon as they switched sides. If a subject preferred

FIGURE 2. Photo of the apparatus used in nonhuman primate experiments. (Repro- duced with permission from McDermott and Hauser.26)

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one of the two sounds over the other, one might expect them to spend more time in the corresponding side of the apparatus, so as to increase their exposure to the pre- ferred sound. We left animals in the apparatus for five-minute sessions and measured the proportion of time they spent on the left and right.

To verify that the method was appropriate for measuring preferences for sounds, we began by conducting two control experiments. In the first, we presented subjects with a choice between loud (90 dB) and soft (60 dB) white noise. We expected the animals to find the high amplitude noise aversive, and to thus spend more time on the side of the soft noise. The average results from six tamarins over four sessions are shown in FIGURE 3. The animals exhibited a pronounced bias toward the soft side as early as the first session, an effect that increased in the second session. Between the second and third sessions the side–sound pairings were reversed, to rule out ef- fects due to side biases. Following the reversal, the animals spent an average of 50% of the time on each side. Coupled with the increase in the effect from the first session to the second, this indicates that the animals had acquired a side–sound association that took time to be unlearned. By the fourth session (the second after the reversal), however, the effect had reversed, such that they again spent most of the time on the side with the soft noise. The results suggest that the animals learn to associate a side with a sound and modulate their position in the apparatus to reflect their preferences.

In a second control experiment, we presented tamarins with a choice between two classes of species-specific vocalizations: chirps that they emit in the presence of

FIGURE 3. Results of the first control experiment, in which animals were presented with a choice between loud and soft white noise. Each bar plots the average data from 6 sub- jects, as a proportion of the total time spent in the apparatus. Error bars here and elsewhere denote standard errors. The dashed line denotes the reversal of the side assignment that occurred after the second session. (Reproduced with permission from McDermott and Hauser.26)

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food, and screams that they make when being held by a veterinarian. We reasoned that they would be likely to have negative associations with the screams and positive associations with the chirps, and thus might spend more time on the side with the chirps than that with the screams. Recordings of the two types of vocalizations were equated in amplitude to minimize loudness differences. The same six tamarins were again run in several five-minute sessions. As shown in FIGURE 4, the tamarins spend more time on average with the chirps than with the screams, providing additional ev- idence that our method provides an appropriate behavioral assay for measuring pref- erences for sounds.

CONSONANCE AND DISSONANCE

We next proceeded to test for preferences for consonance over dissonance. Before testing our animal subjects with such stimuli, we ran an analogous experiment in humans to confirm that a behavioral method such as ours would demonstrate the consonance preference believed to be widespread in humans. Our human subjects were placed in a room divided in half with a strip of tape (FIG. 5). A concealed speak- er was situated on each side of the room, and as in the animal apparatus, each speaker was assigned a particular stimulus. Only one speaker was on at a time, triggered by a subject’s position in the room. Our human subjects were given no instructions and were merely told they would be left in the room for five minutes and videotaped. All subjects were naive as to the purpose of the experiment and were involved for a single session. As with the tamarins, we measured the proportion of time spent on each side.

The consonant stimulus in this experiment was a random sequence of two-note chords, the notes of which were separated by either an octave, and fifth, or a fourth.

FIGURE 4. Results from the second control experiment, comparing tamarin food chirps with distress screams. Data are averages across sessions. (Reproduced with permis- sion from McDermott and Hauser.26)

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The dissonant stimulus was a similar sequence of minor seconds, tritones, and minor ninths. The notes composing the intervals were synthesized complex tones with ten harmonics. The bass note was always middle C. Each interval was 1.5 s in duration.

FIGURE 6 (left) plots the average results for four human subjects, all of whom spent most of their time on the side with the consonant intervals. Typically a human subject would wander around the room until by chance they crossed over the divid- ing line, thus changing the sound. After moving back and forth across the line a few times, they quickly realized that their position controlled the sound, and thereafter typically spent most of their time on the side of the sound they preferred. These re- sults suggested that our method would be sufficient to demonstrate a consonance preference in nonhuman primates were they to share this with humans,

FIGURE 6 (right) plots the average results for five tamarins. In contrast to the humans, they showed no effect. Note that the animals used in these experiments were the same ones used in the two control experiments, both of which yielded significant effects. Moreover, all 5 animals again showed a preference for loud over soft noise when tested at the conclusion of the consonance experiment, confirming that they had not somehow habituated to the apparatus or method. Rather, it seems that tamarins do not exhibit the preference for consonance over dissonance found in humans, even when tested with analogous methods.

FIGURE 5. Schematic of setup for human control experiments.

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SCREECHING

For a second test of whether nonhuman primates might share timbral preferences with humans, we turned to a sound that many humans find highly aversive—the sound of fingernails on a blackboard. We made recordings of a very similar sound produced by scraping a metal garden tool down a glass window; many listeners in- formally reported the sounds to be very unpleasant. Spectrograms of the sounds we recorded revealed harmonic structure superimposed on broadband noise, similar to what has been previously described.28 Little is known about why such sounds are so unpleasant, or about the relationship between the perceptual effect they have and that of musical stimuli, but given the strength of the reaction evoked in humans, they seemed a promising stimulus with which to test for timbral preferences in nonhuman primates.

We used a concatenation of several screech recordings as an experimental stimu- lus. For a control stimulus, we generated white noise with the same amplitude enve- lope as the screech stimulus. This control stimulus was as loud as the screech stimulus, but otherwise sounded quite different, and we intended it to be much less annoying to human listeners. We again began by running an experiment with human subjects, using the same method as for the consonance experiment. As expected, our method revealed a pronounced preference in humans for the white noise over the screech, shown in FIGURE 7a. In contrast, the tamarins showed no evidence of a pref- erence one way or the other, even when run over many sessions (FIG. 7b). Evidently the screeching sounds that are so annoying to most humans are not particularly aver- sive for tamarins, at least no more so than our amplitude-matched control stimulus.

FIGURE 6. Results from experiment comparing consonant and dissonant musical in- tervals. (Left) Results for human subjects. (Right) Results for tamarin subjects. (Reproduced with permission from McDermott and Hauser.26)

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ARE HUMAN ACOUSTIC PREFERENCES UNIQUELY HUMAN?

Two timbral preferences that are pronounced in humans thus appear to be absent in cotton-top tamarins. We have recently replicated the consonance result in com- mon marmosets (McDermott and Hauser, unpublished data), and although it would be ideal to test other species of primates as well, our results raise the possibility that nonhuman primates may lack the timbral preferences that appear to at least partly underlie human appreciation of music.

One key difference between our primate subjects and our human subjects, how- ever, is that the humans all had a lifetime of exposure to music, as do virtually all humans. The consonance preferences apparently present in young infants suggest that a lifetime of exposure is not necessary to develop the preference, but whatever exposure the infants inevitably had may nonetheless be important. Given this, our results suggest three main possibilities: (1) Simple acoustic preferences for conso- nance and other stimuli could be innate in humans, and unique to them, given the absence of such preferences in the nonhuman primates we have tested. (2) Such preferences might not be unique to humans and could primarily be the result of ex- posure to musical stimuli, which our nonhuman primate subjects lacked. (3) Such preferences could require exposure to music but might also involve specialized learning mechanisms that could be unique to humans, and perhaps specific to music.

A key issue, therefore, involves determining the role of exposure to music. One important avenue for future research will be to explore the effects of extended mu- sical exposure on nonhuman animals. If nonhuman animals can develop preferences given enough exposure to human music, domain-general learning mechanisms might then also be responsible for human preferences. Conversely, if animals tested after musical exposure still do not exhibit any of the preferences found in humans, the case for uniqueness would be bolstered, for even with similar auditory experience, humans and nonhumans would exhibit different behavior. Further explorations of the

FIGURE 7. Results from experiment comparing a screeching sound to amplitude- matched white noise. (Left) Results for human subjects. (Right) Results for tamarin subjects. (Reproduced with permission from McDermott and Hauser.26)

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effects of musical exposure on humans could help to determine whether exposure coupled with uniquely human learning mechanisms is involved, or whether the pref- erences in question are, in fact, innate.

MUSIC UNIVERSALS STUDY

In an attempt to assess the effect of the varying musical exposure that occurs in different cultures, one of us (J.M.) has set up an experiment on the internet to mea- sure aspects of music perception in people all over the world. Anyone can participate in the Music Universals Study by visiting <http://music.media.mit.edu>. Our goal is to collect large amounts of data from people with vastly different musical cultures, to examine whether any aspects of music perception are invariant across culture. Dif- ferences across cultures would suggest an important role for learning. Web-based ex- periments are not a replacement for conventional cross-cultural studies, as the subject pool is limited to those with internet access, but they are potentially a useful additional tool with which to ask many questions of interest in music perception.

CONCLUSIONS

The role of musical exposure could also be clarified with a richer cross-cultural database. We propose that evolutionary theories of music’s origins will be facilitated by investigating whether aspects of music perception are innate in humans, and, of those, whether any are unique to humans and specific to music. Our studies of pref- erences in nonhuman primates suggest that many simple acoustic preferences that are pronounced in humans are not shared by our primate relatives. Additional re- search is needed to investigate the role of musical exposure, but such preferences may thus be innate and unique to humans. Given that some of them appear to be spe- cific to music, they are candidates for part of an adaptation for music. We believe that future research investigating the innateness, uniqueness, and specificity of other aspects of music perception will place strong constraints on the evolutionary origins of music.

ACKNOWLEDGMENTS

We are grateful to Matt Kamen, Altay Guvench, Fernando Vera, Adam Pearson, Tory Wobber, Matthew Sussman, and Alex Rosati for their assistance in running the experiments.

[Competing interests: The authors declare that they have no competing financial interests.]

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