Integrative Literature Review

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Research Cite this article: Kline MA, Shamsudheen R, Broesch T. 2018 Variation is the universal:

making cultural evolution work in

developmental psychology. Phil. Trans. R. Soc.

B 373: 20170059. http://dx.doi.org/10.1098/rstb.2017.0059

Accepted: 12 December 2017

One contribution of 16 to a theme issue

‘Bridging cultural gaps: interdisciplinary studies

in human cultural evolution’.

Subject Areas: cognition, evolution, developmental biology,

behaviour

Keywords: cultural evolution, developmental psychology,

cross-cultural psychology, ethnocentrism,

evolution and human behaviour

Author for correspondence: Michelle Ann Kline

e-mail: [email protected]

†The second and third authors contributed

equally to this manuscript.

& 2018 The Author(s) Published by the Royal Society. All rights reserved.

Variation is the universal: making cultural evolution work in developmental psychology

Michelle Ann Kline1,2, Rubeena Shamsudheen3,† and Tanya Broesch1,†

1Department of Psychology, Simon Fraser University, Burnaby, BC, Canada V5A 1S6 2Institute of Human Origins, Arizona State University, Tempe, AZ 85287-4101, USA 3Department of Cognitive Science, Central European University, Nador u. 9, 1051 Budapest, Hungary

MAK, 0000-0002-1998-6928

Culture is a human universal, yet it is a source of variation in human psy-

chology, behaviour and development. Developmental researchers are now

expanding the geographical scope of research to include populations beyond

relatively wealthy Western communities. However, culture and context still

play a secondary role in the theoretical grounding of developmental psychol-

ogy research, far too often. In this paper, we highlight four false assumptions

that are common in psychology, and that detract from the quality of both stan-

dard and cross-cultural research in development. These assumptions are: (i) the universality assumption, that empirical uniformity is evidence for universality, while any variation is evidence for culturally derived variation; (ii) the Western centrality assumption, that Western populations represent a normal and/or healthy standard against which development in all societies can be compared;

(iii) the deficit assumption, that population-level differences in developmental timing or outcomes are necessarily due to something lacking among non-Wes-

tern populations; and (iv) the equivalency assumption, that using identical research methods will necessarily produce equivalent and externally valid

data, across disparate cultural contexts. For each assumption, we draw on cul-

tural evolutionary theory to critique and replace the assumption with a

theoretically grounded approach to culture in development. We support

these suggestions with positive examples drawn from research in development.

Finally, we conclude with a call for researchers to take reasonable steps towards

more fully incorporating culture and context into studies of development, by

expanding their participant pools in strategic ways. This will lead to a more

inclusive and therefore more accurate description of human development.

This article is part of the theme issue ‘Bridging cultural gaps: interdisci-

plinary studies in human cultural evolution’.

1. Human development requires culture Humans stand out among other animals because we adapt to new environments

both by being clever innovators [1] and through the accumulation of cultural

knowledge across generations [2,3]. Social learning, including intensive forms

such as teaching [4 – 6], can facilitate cumulative cultural evolution. In fact, low-

cost social learning mechanisms, as well as sources of innovation, are prerequisites

for the evolution of cumulative culture. For this reason, social learning mechan-

isms are central to the understanding of cultural evolution—and cultural

evolution is key to explaining why and how human ontogeny is so very flexible.

Culture is a human universal: all societies have shared knowledge, practices,

beliefs and rituals that are transmitted socially. At the same time, culture is also a

source of psychological and behavioural variation both within and across popu-

lations. Developmental processes that are sensitive to socio-environmental

influences are one way that flexibility can evolve [7,8], and evolution can produce

developmental processes that vary in adaptive ways in terms of the degree and

nature of their flexibility [9]. Elaborating on the relationship between culture

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and development first requires recognizing that evolution and

development are not mutually exclusive, then building on that

insight to explore how evolved developmental mechanisms

that are sensitive to cultural influence can create psychological

and behavioural variation across and within societies [8].

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2. Developmental psychology requires a culturally diverse sample

Despite the importance of culture to development, develop-

mental psychology as a field retains a near-absolute focus on

development in relatively wealthy Western, English-speaking

populations. Henrich et al. [9] term general psychology’s par- ticipant pool ‘WEIRD:’ Western, educated, industrialized, rich

and democratic. A recent review provides evidence that this

is also the case in leading developmental psychology journals:

more than 90% of study populations represented there are from

the USA, Europe and/or are English-speaking [10]. The rest of

the world is vastly underrepresented, with only approximately

7% of participant populations coming from non-Western

human populations (the remainder are non-human animal

populations). In this context, developmental psychologists

who pursue cross-cultural research are wisely expanding

the scope of research to include participants beyond predomi-

nantly Western, upper middle class and often ethnically

white participants [9,11,12]. We applaud these efforts—

anything less would only perpetuate an incomplete and

inaccurate picture of human development.

Poor sampling, however, is not the only problem in the

field. Arnett [11], and Meadon & Spurrett [13] address a lack

of inclusivity in the broader practice of psychology: theories,

studies and publications in the American Psychological Associ-

ation journals are all overwhelmingly created, reviewed and

edited by this same subset of the world’s population. This is

one reason why the sampling problem in developmental

psychology is not likely to be solved by laboratory-based

researchers making the decision to take on cross-cultural

work unilaterally, in the short term. Dropping in on commu-

nities with unfamiliar cultures to run brief, one-off studies

without a long-term reciprocal relationship with the commu-

nity can be ethically dubious [14], especially where there is a

power differential. Further, interpreting results in isolation

from a population’s daily cultural context can produce more

confusion than answers [15]. And yet avoiding these pitfalls

requires investing what can be a prohibitive amount of time,

effort and funding to start and maintain a field site. A more

plausible way to ameliorate psychology’s WEIRD problem is

to recruit, support, include and collaborate with more scientists

from beyond the WEIRD populations that have created the bias

in the first place [11,13]. Alternatively, researchers can work

with non-university populations nearby, to explore variation

among people in their own local context [14]. More generally,

researchers who study WEIRD populations must also recog-

nize that their populations are also influenced by culture and

should consider carefully how to define the specific population

from which they recruit participants. Both these strategies fit

with a broader, theoretically motivated approach to expand

the inclusiveness of sampling in developmental psychology.

This paper aims to show why developmental psychology

needs this change, and establish some guidelines for how to

study culture’s role in development, no matter how near or

far from home the study site may be.

3. Cultural evolution can motivate a better science of developmental psychology

Cross-cultural data are expensive to get, but valuable to have.

Their rarity in developmental psychology is due to more

than a lack of interest in cross-cultural sampling, and we

cannot dissolve those very real barriers in this paper. Instead,

our goals in this paper are twofold. First, we aim to convince

researchers in the field of developmental psychology that con-

siderations of culture are relevant to their work, even if they

do not do far-flung fieldwork themselves. Second, for cross-

cultural developmental psychologists, we aim to leverage

cultural evolutionary theory to enrich the central role of

cross-cultural data to developmental psychology as a field.

To achieve these aims, we highlight four common but false

assumptions in present-day approaches to cultural variation

in developmental psychology, and critique each in turn by

drawing on cultural evolutionary theory and empirical find-

ings. This step of identifying and refuting these assumptions

will help to integrate the ‘cross-cultural’ niche within develop-

mental psychology, in general, by demonstrating how culture

and culture-based assumptions underlie some of the basic

ideas that motivate research in developmental psychology.

Those assumptions are that: (i) universality and uniformity

are equivalent: that what is universal must necessarily follow

a uniform pattern of development; (ii) Western populations

are central in human psychology; (iii) differences among

populations in development are always indicative of deficits;

(iv) methods can automatically be transported across cultural

contexts and yet maintain validity. We critique each assump-

tion in turn, by drawing both on cultural evolutionary theory

and on positive examples from the developmental psychology

literature. In our conclusion section (§8), we summarize a gen-

eral strategy for research that eschews these assumptions, and

argue that this approach can pave the way for an improved

science of developmental psychology by placing the cultural

nature of humans at its centre.

4. Problem no. 1: the universality as uniformity assumption

The universality assumption is the belief that observed uniformity is evidence for species-wide, biologically based universality. By

contrast, any variation is regarded as evidence for culturally

derived differences. By ‘universal’, we mean core mental or

behavioural attributes shared by humans everywhere [16].

This assumption sometimes takes the form of an explicit

claim that uniformity implies genetic underpinnings (often mis-

categorized as ‘biological’ or ‘evolutionary’), while variation

necessarily indicates ‘cultural’ influences [17]. In all its forms,

this assumption rests on the false nature/nurture dichotomy,

that culture and biology are separate, opposite and competing

explanations. In reality, human cultural capacities are part of

our biology [18,19]. Equating psychological or behavioural vari-

ation with cultural influence precludes a deeper understanding

of human behaviour, because a universally shared develop-

mental process can function to produce behavioural or

psychological variation. Instead, developmental flexibility and

culture are both parts of the biology of human development,

not alternative explanations—culture is a part of human

biology and development [8].

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This false dichotomy between nature and nurture pro-

duces two versions of the universality as uniformity

assumption: (a) that variation is equivalent to a lack of uni-

versality, and that (b) psychological/behavioural similarity

is equivalent to universality. For the sake of clarity, we

address each in turn.

(a) Variation equals cultural influence and lack of universality

This assumption is often implicit in data analysis and study

interpretation. For example, researchers conduct cross-site com-

parisons and conclude that any between-site difference is

‘cultural’, without explaining how culture produces differences

in psychology and behaviour. In addition, researchers often

treat whole cultures as if they are a single experimental con-

dition, without considering the influence of environmental

factors, such as resource availability, wealth or differences in

the interpretation of the method (see §6 below). For example,

directly comparing norms for anonymous sharing among

wealthy Americans with those among poor, food-insecure

Polynesian populations may result in differences—but those

differences may be due to circumstances specific to resource

scarcity, rather than some underspecified aspect of culture.

This line of reasoning is not considered sufficient for studies

of culture in other animals, and leads to energetic debates

about sources of behavioural variation even in our closest

living relatives (e.g. [20 – 22]). However, the same logic is

rarely questioned in cross-cultural comparisons of human

psychology. While cross-cultural comparisons do contribute

to our knowledge of the range of variation in human behaviour,

most fall short of understanding the sources and the scale of

variation that can emerge via developmental processes—the

real question at hand.

(b) Uniformity equals genetic roots and lack of cultural influence

The other side of the universality assumption consists of a belief

that uniformity in behaviour and psychology is indicative of

universally ‘innate’ traits that develop without cultural inputs.

When developmental psychologists ask whether a feature

is innate, and then seek to show that it emerges early and

reliably across human populations, they rely upon assump-

tions that equate sameness, universality and innateness. By

contrast, biologists have recognized notions of innateness as

useless in ecology, biology and behaviour since the early

1990s [19]. This rests on a recognition, as Barrett [8, p.157]

writes, that ‘. . .[t]here are not two kinds of things, the innate

and the non-innate, but only one, the developmental process

itself.’ Put simply, genes rely upon the environment in order

to create an organism, and vice versa. In humans, culture is

part of that ever-present environment.

(c) Improvements The equation of sameness with universality, and the desire to

describe a general human psychology in these terms, have

long been a driving philosophy in American psychology

[11,16,23]. While valuable as a first pass, documenting simi-

larities across sociocultural contexts is a subpar strategy for

data collection when the goal is to understand culture’s role

in shaping development, or vice versa. Cultural evolutionary

theory offers an alternative perspective for shaping research

questions: that genes and culture have co-evolved in humans.

Because of this ‘dual-inheritance’ system, both genetic and cul-

tural information are essential ingredients in any explanation

of human biology. Most developmental psychologists would

not argue with this stance, but putting it into action in a

research programme is still a challenge. Cultural evolutionary

theory is useful in this practical sense, because it provides a

working definition of culture that can inform quantitative

work: ‘[c]ulture is information capable of affecting individuals’

behaviour, that they acquire from other members of their

species through teaching, imitation, and other forms of social

transmission’ [19, p. 5].

Cultural evolution’s distinction of culture as socially

learned information is useful as a research tool because it

means developmental psychologists need not ask whether

any particular trait is universal, biological and innate, versus

cultural. When biology and culture are not opposites, this

either/or is a meaningless, and therefore unanswerable,

question. Instead, developmental psychology can embrace a

transformed question: what is the relative influence of environ-

mental, cultural and other contextual factors on shaping

development of specific traits, in particular population? In

other words, how variable and flexible is the development of

this trait? Answering this context-rich question through studies

that theorize about the functional role of variation will produce

a body of evidence on how human psychological development

varies. From this, researchers can build a more complete map

of human psychological development.

This view, rooted in cultural evolutionary theory, places

flexibility at the centre of understanding what is universal

about human psychological development. This provides a

theoretically motivated way to predict when and how culture

ought to impact development, rather than simply checking

Western-based work against non-Western populations and

lumping traits that are the ‘same’ as universal, and those that

are ‘different’ as cultural.

(d) Developmental research case study Studies of human language acquisition and socialization pro-

vide evidence for both variation in a cultural context, and

shared developmental processes. Geographically and culturally

disparate populations typically speak different languages, and

in some cases even show variation in the neurological under-

pinnings necessary to master and use different languages [24].

The cultural expectations for children as language learners

are shaped by their cultural contexts, and in some ways are inse-

parable from socialization more generally [25]. Language

acquisition processes illustrate that developmental processes

themselves—such as statistical learning [26]—can constitute

universal learning mechanisms, which in turn generate behav-

ioural and psychological variation. The same can be said for

children’s early learning environments: there are both shared

and variable features, cross-culturally. For example, Broesch &

Bryant [27,28] find that mothers and fathers across disparate

societies routinely modify the properties of their speech when

addressing young infants compared to when they address

adults, yet they do so in different ways [28]. Despite identifying

the existence of infant-directed speech by caregivers in North

America, Kenya, Fiji and Vanuatu, they also find that parents

vary cross-culturally in the form their infant-directed speech

takes. Mothers across diverse societies and rural Vanuatu

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fathers modified their speech by adjusting features of the per-

ceived pitch of their speech to infants. However, fathers in

North America only slowed down the rate of their speech, with-

out adjusting the perceived pitch [27]. The results of this study

demonstrate why researchers cannot simply search for univers-

ality by equating it with similarity: it is too broad a question,

and would lead us to ignore key details about the flexible

nature of developmental processes.

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5. Problem no. 2: the Western centrality assumption

The Western centrality assumption is the belief that Western populations represent a normal and/or healthy standard

against which development in all societies can and should be

compared. This assumption literally fits the original definition

of ethnocentric [29], in that it divides global populations into two rough categories, ‘the West’ and ‘the Rest,’ with Western

societies at the centre of everything. This assumption is rarely

if ever made explicit in print, but it is worked into the foun-

dation of much developmental research, including the

cognitive and medical milestones that serve as guidelines for

both Western parents and international health agencies.

(a) Improvements From a cultural evolutionary perspective, lumping Western and

non-Western societies into two broad categories of analysis is

simply throwing data away. The study of cultural evolution is

necessarily built on the study of the cultural history of societies

all over the world, because explaining cultural variation

requires a breadth of data across socioecological environments

([19]; see e.g. the range of sites included in Mace et al.’s edited volume [30]). From this perspective, every cultural context is

an equally valid study site, and the importance of a particular

site is down to its specific cultural features and their relevance

to the research question. For example, Polynesia’s history of

step-wise settlement by ocean-faring canoe and its estimable

rates of contact among societies make its cultural history an

excellent case study on how population interconnectedness

can influence the accumulation of complex material culture

[31,32]. The key message from cultural evolutionary theory

here is that these studies stand alone, and do not require a

Western comparison sample to lend them value.

(b) Developmental research case study The Western centrality assumption directly damages the accu-

racy and usefulness of developmental research. For example,

Karasik et al. [33] review how developmental textbooks and medical guidelines employ standards for motor development

that are built exclusively on American middle-class samples

as proscriptive milestones. Karasik et al.’s data, drawn from six different societies, document within- and between-

population variation in both the timing of the motor develop-

ment of sitting, as well as the social and material contexts that

contribute to those differences. This establishes a causal link

between context and developmental trajectories. Karasik et al. conclude that using American-centric guidelines as if they

are universal has ‘led to a gross misrepresentation of motor

development’ ( p. 1033). Treating Western samples as a univer-

sal measuring stick for development is, unfortunately, a

pervasive practice. Greenfield et al. [34] review evidence that

developmental trajectories derived from the study of Western

populations, with their focus on independence, are unlikely

to match how children learn and grow in sociocultural contexts

where interdependence is prioritized. This is particularly true

for social development. For example, while adolescence may

be a transition to autonomy in independence-focused societies,

in an interdependent society it is instead a relational shift that

makes sense only in the context of kinship and community [34].

Likewise, classic theories of attachment [35] presuppose

that the end goal of child development is independence and

autonomy, rather than locally appropriate integration into

kinship- and community-based interdependent relationships.

In a review, Keller [36] questions whether these theories hold

up when used to explain behaviour in cultural contexts

beyond Western societies, and argues that incorporating data

from additional populations requires revising existing theory

along lines suggested by cultural and evolutionary theories

of development.

6. Problem no. 3: the deficit assumption The deficit assumption is that population-level differences in developmental timing or outcomes are necessarily caused by

something lacking, typically in parenting or educational

systems. This line of reasoning allows for no flexibility, and

assumes a single, inflexible developmental outcome. The

assumption rides the coattails of the Western centrality assump-

tion, in that the timeline that establishes ‘normal’ development

from ‘delayed’ development is typically anchored on data from

Western populations. However, the deficit assumption can also

apply to Western populations or subpopulations therein. For

example, Lancy [37] argues that excessive levels of teaching in

Western societies may impinge on the development of a

child’s autonomy, The deficit assumption is also sometimes

applied to subpopulations within Western societies, and so

has recently become an important domain for self-critique in

the field of developmental psychology (see [38]). However,

the deficit assumption differs from the Western centrality

assumption in two important ways. First, the deficit assump-

tion carries an extra layer of interpretation in comparison

to the Western centrality assumption. By this we mean that

researchers simultaneously judge a given pattern in deve-

lopment as deviant and also attribute that difference to

something that is lacking or missing from a family’s or a popu-

lation’s way of raising children. This carries with it a value

judgement that goes beyond a scientific approach to describing

and explaining variation, and in doing so obscures the science

itself. Second, the Western centrality assumption functions

only in one direction. By contrast, the deficit assumption can

lead researchers to claim that Western children are somehow

worse off than non-Western ones. Often this takes the form

of arguing that Western children are coddled, spoiled or

excessively dependent on direct parent intervention.

In assuming that group-level developmental differences

are due to what is lacking in schooling or parenting, research-

ers frequently fail to (a) give any evidence for this mechanism

beyond handwaving that ‘culture’ is the cause, and (b) in

doing so, fail to consider the many specific axes of varia-

tion that comprise between-population differences. When

researchers fail to give a specific cultural mechanism yet attri-

bute differences to ‘culture,’ some of the variation may be due

to situation (e.g. resource insecurity) rather than culturally

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inherited differences (e.g. collective ownership norms). Where

this is the case, it is a serious challenge to the validity of cross-

cultural comparisons, in that it fails to account for potential

confounding variables. Recognizing and controlling for poten-

tial confounds are accepted as a crucial components of high-

quality research in developmental psychology, with particular

attention to detail in experimental studies. The same standard

should be applied at the level of cross-cultural comparisons.

The risk of neglecting to recognize a confounding variable

decreases with a research team’s expertise in the local context

at their study site. Finally, the deficit assumption reinforces a

deeper-seated assumption, (c) that there is one shared, correct

outcome for various stages of development, and that this does

not vary across populations or across societies.

(a) Improvements Cultural evolutionary theory instead presents a functionalist

perspective. This means that the focus is on how different

domains of development fit into both physical maturity and

context-dependent social, emotional and relational factors.

This emphasis on function in context is shared with dynamic

systems theories [39], but an evolutionary approach is further

motivated by understanding how developmental processes

have emerged over an evolutionary timespan and in compari-

son to other species. From this perspective, developmental

flexibility, including social learning, is part of what allows

human culture to evolve faster than the human gene pool

[40], and this in turn makes humans adaptable over short time-

scales [2]. (In contrast with dynamic systems theory, the term

‘adapt’ is almost never used in cultural evolutionary theory

to refer to the timescale of a single individual behaving flexibly,

but rather it is a population-level concept.) As a result, psycho-

logical development is pluralistic by design, and this evolved

because flexibility is incredibly useful for a wide-ranging, inva-

sive species like Homo sapiens. Barrett [8] has coined the term ‘designed emergence’ to capture the idea that developmental

processes are flexible as a result of evolution by natural selection.

Simply put, this means there is a range of healthy, functional

outcomes that emerge from developmental processes. Outside

of that range, pathology is still possible, especially in cases of

extreme abuse or neglect that fall outside the breadth of typical

human experience. Specific outcomes are not predetermined by

genes, but are instead shaped by the interaction between genes

and environment in ways that have been manufactured by

natural selection. For developmental psychologists, the take-

home message here is that shared processes of human dev-

elopment have a variety of outcomes, and this flexibility in outcomes is a feature rather than a bug. Developmental

researchers can leverage this insight to create and evaluate

hypotheses about how the form and developmental timing of

psychological phenomena fit in functional ways with children’s

roles in varying sociocultural contexts.

(b) Developmental case study For example, psychologists have long assumed that direct,

active teaching (often characterized by the verbal communi-

cation of abstract ideas) is the most efficient way to scaffold

learning, and that therefore it must be present in all human

societies (for review see [6]). By contrast, some anthropologists

have often conflated direct instruction with involuntary, forced

transmission, which replaces more enjoyable and (by this

account) effective forms of learning by participation

([37,41,42]; see [6] for review). For both accounts, at least

some societies have got the wrong answer to how children

learn best—and children in those societies are at a deficit.

Kline [6,43] uses cultural evolutionary theory as a foun-

dation to argue that there are many functionally distinct

types of teaching, which can be mixed and matched with learn-

ing problems. From this perspective, no single type always

provides a ‘best’ outcome for the learner, because it depends

on the learning problem at hand. This approach treats develop-

ment as an integral working part of evolutionary processes,

and prioritizes functional and causal explanations of variation.

This is in contrast with other evolutionary accounts that

explain why humans, and only humans, teach by referring to

constraints in other animals. When successful, a cultural

evolutionary approach uses the rich and culturally specific

interpretations offered by ethnographic research as insights

that can inform broader claims about the evolution and

nature of human developmental psychology. Taking a func-

tionalist, cultural evolutionary perspective offers power for

generating and testing hypotheses in developmental psychol-

ogy by incorporating the full range of human variation into

what developmental psychologists term ‘typical’ development.

7. Problem no. 4: the equivalency assumption The equivalency assumption is that using identical research methods, scales or questions will automatically produce equiv-

alent and externally valid data, even across disparate cultural

contexts. Arnett [11] elaborates on this rationale as the predo-

minant philosophy of science in experimental American

psychology: that in the laboratory, it does not matter who the

participants are, or where or how they live—it matters only

that the procedures within the experiment itself are sufficiently

controlled. The equivalency assumption is demonstrably false

when taken to the extreme: written methods must be trans-

lated, and translation inevitably brings up questions of

whether or not there are shared concepts and meanings,

across sociolinguistic contexts. Non-linguistic methods may

avoid the problem of translation, but the question of whether

methods and stimuli map to shared concepts, social context

and expected behaviour across cultural groups is still an impor-

tant one. Such comparisons are only useful when the meaning

of the protocol is comparable across societies [44 – 46]. Further,

assuming equivalency also means that researchers may fail to

account for culturally specific environmental factors in devel-

opment that are either present in WEIRD contexts but not at

their study site, or that are absent in WEIRD contexts and there-

fore may be unrecognized as important factors at their study

sites. For example, while direct verbal instruction may be rare

in many non-Western societies, ethnographic studies of devel-

opment in these contexts reveal a rich, interactive social context

in which learning happens via participant observation and

inclusion of children in everyday activities [37,41,47]. The

social learning mechanisms vary but learning and develop-

mental change happen in all cultural contexts.

(a) Improvements Cultural evolutionary theory treats the human brain, mind

and behaviour as having evolved in the context of human

interaction with the world, rich with social and cultural

context. Ignoring that this cultural context affects how parti-

cipants understand and respond to methods is particularly

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problematic when transporting methodologies across sociocul-

tural contexts that differ in broad ways [16,44,48,49]. This is a

problem even for developmental psychologists who do not

venture to do cross-cultural work, because it means their

methods and their results may be culture-bound and therefore

limited in ways they have not explored.

The equivalency assumption raises a particularly difficult

challenge for cross-cultural comparisons in developmental

psychology. The standards for experimental control are strin-

gent and technically demanding. For example, effect sizes

and statistical significance for studies with infants can

depend on looking times that differ in terms of milliseconds.

These tasks often require electricity, delicate equipment,

trained personnel and quiet laboratory space to run effec-

tively. However, even a perfectly replicated and controlled

methodology cannot guarantee that participants from two

different sociocultural contexts are interpreting the situation

in similar ways and therefore the behaviours observed may

not be comparable.

As Heine and co-workers [44,50] conclude, there is no

straightforward solution for this broad problem of context-

specific methodological validity. Instead, establishing real

comparability across populations requires more context, not less—and this means bringing ethnography into the picture

as a standard resource to inform the design and interpretation

of studies in developmental psychology. Cultural evolutionary

research may seem an unlikely resource for addressing this

methodological challenge because the field has no signature

methodology of its own: for example, its studies of learning

biases draw upon established psychological methods, and its

studies of behaviour build on human behavioural ecology

and animal behaviour. The formal mathematical models that

established the field are themselves built on established

models in epidemiology and genetics. The field is so

thoroughly interdisciplinary that some cultural evolutionists

have even proposed a division of labour within cultural evol-

utionary studies that subsumes existing disciplines [51]. We

advocate instead for a mixed-methods approach, deploying

methods in combinations that strategically compensate for

the particular shortcomings of each method, and that are

suitable for the research problem at hand. This is standard

practice in some areas of social science, including the anthropo-

logical sciences, where both qualitative and quantitative data

and analyses are used as needed [52].

(b) Developmental case study For example, researchers often treat mutual eye gaze between

infant and caretaker as a reliable and stand-alone indicator of

joint attention in the study of infant cognition. However,

Akhtar & Gernsbacher [53] point out that the social role of

eye gaze is variable across cultural contexts, and hence is not

always a reliable indicator of joint attention. North Americans

typically privilege eye contact and verbal interaction as a key

part of parenting [54], but Gusii mothers in Kenya avert their

eyes in response to mutual eye gaze with an excited infant, in

part to keep their babies calm [55]. According to LeVine &

LeVine [55], gaze avoidance by mothers is consistent with

polite behaviour by Gusii adults, where excessive eye contact

is considered rude and sometimes even aggressive. Gaze

avoidance does not mean Gusii mothers are inattentive to

their infants, but rather that they do not use mutual gaze as a

means of establishing joint attention. Instead, they may use

more physical types of interaction—a typical Gusii mother

cosleeps with her infant, breastfeeds on demand and responds

quickly to her infant’s distress. Based on Lancy’s review of the

ethnographic literature on children and childhood [54], the

Gusii approach of using more tactile contact and gestural com-

munication may be more typical around the world than the

North American approach, which emphasizes eye contact

and verbal communication. An excessive focus on eye gaze

as the key element in joint attention (e.g. [56]) may twist the

scientific understanding of joint attention by underestimating

its prevalence in societies where eye gaze is less important

than in North American contexts.

Rather than the narrowly Western-centric cue of eye gaze,

vocal and postural behaviours may represent a more culturally

generalizable set of cues for the study of infant social cognition

[53]. In fact, gestural, postural and vocal cues may play an

important role in Western contexts, but one that is de-empha-

sized in developmental psychology as a reflection of North

American culture. However, the plurality of methodological

approaches suggested by cultural evolutionary theory means

there is another option besides searching for single (or a set

of ) cues that always indicate joint attention, across sociocul-

tural contexts. Instead, researchers should use an array of

cues, designed for particular sociocultural contexts, to compare

the prevalence and behavioural form of joint attention across

human populations. Using identical methods based on cultu-

rally specific cues will produce only superficially comparable

data, and will produce a misleading picture of the ways in

which populations vary.

8. Conclusion For each assumption above, we offer a shift in perspective that

uses cultural evolutionary theory to pry those assumptions

loose from present-day developmental psychological research.

For standard developmental psychology, this means seeing the

culture-bound nature of the questions, methods and results,

and appropriately characterizing the generalizability of the

research given the limited samples. For cross-cultural develop-

mental psychology, this means guarding against some of the

assumptions that are common in psychology more generally,

and employing cultural evolutionary theory to improve how

cross-cultural research is designed, conducted and interpreted.

Using this approach, researchers can take some small steps to

remediate the sampling problem in developmental psychology.

Researchers working at institutions in WEIRD societies can step

off campus to create more inclusive study by sampling popu-

lations in their towns but beyond campus, and in doing so can

increase the inclusivity of their samples with a moderate level

of investment in community engagement. They can also collab-

orate with and learn from colleagues at institutions outside of

North America and Western Europe, to work with scholars

who are both highly trained academics as well as regional

experts in the societies in which they work and live. We do not

argue that researchers should avoid studying or drawing

comparisons between WEIRD populations and additional

populations around the world. Instead, we argue that carefully

specifying the meanings of cross-cultural studies, using cultural

evolutionary theory, may open up a rich avenue for compara-

tive research. This includes comparisons both within and

between populations, to look for robust relationships between

cultural variation and corresponding psychological, behavioural

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and developmental variation. This kind of data will allow

researchers to study just how flexible human psychological

development may be, because it allows us to ask whether the

same causal relationships hold for development across popu-

lations, or whether the relationships and processes themselves

are flexible. In essence, this approach ties the form of develop-

mental flexibility to the sociocultural and ecological contexts in

which human psychology functions over the lifespan.

Researchers before us have tackled the question of appro-

priate cross-cultural comparisons, with a similar emphasis on

the need for strategic selection of field sites and research

problems (see e.g. [9,16]). In addition to these existing rec-

ommendations, we caution against any approach that treats

entire ‘cultures’ or nations as indivisible wholes that are cultu-

rally, psychologically or behaviourally homogeneous. Rather

than comparing whole ‘cultures,’ researchers should aim to

map variation both within and across populations, along mea-

surable axes of variation. This is especially applicable to broad

cross-site surveys, which often include only coarse measures

of cultural variation (e.g. gross domestic product, Gini

coefficient or years of education), treat single sites as represen-

tative of entire countries, and further conflate those countries

with ‘cultures.’ However, it is equally applicable to studies

restricted to Western populations, where researchers can both

expand the inclusivity of their samples, and be more explicit

about the degrees of variation included in those samples.

Both these practices will lead to better science in developmen-

tal psychology. By placing cultural context—and the flexibility

that it entails—at the centre of this work, researchers will gain a

deeper understanding of the developmental processes that

build human cultural variation.

The overarching message from a cultural evolutionary

perspective is that developmental trajectories and endpoints

can vary due to the human ability to learn flexibly, acquire

information from others, and to recombine socially and

individually learned information in creative ways. Using

this as a springboard, developmental psychologists are well

positioned to explore the developmental mechanisms and

processes by which human children adapt to their local socio-

cultural and environmental contexts. Doing so will mean

shedding light on one of the broadest human universals of

all: variability.

Data accessibility. This article has no additional data. Authors’ contributions. M.A.K. conceived of and drafted the manuscript. R.S. and T.B. both made intellectual contributions prior to the manu- script’s first draft, and made edits and contributions to manuscript drafts. R.S. and T.B. contributed equally. All the authors approved the final version of this manuscript.

Competing interests. We declare we have no competing interests. Funding. This research was made possible through the support of a grant from the John Templeton Foundation to the Institute of Human Origins at Arizona State University (no. 14020515). The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation.

Acknowledgements. We would like to thank Central European Univer- sity’s Department of Cognitive Science, for inviting the authors to a Social Mind Institute Workshop, which led to the formation of some of the early ideas for this paper.

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