Evolution and the Social Sciences

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DOI: 10.1177/0952695107076197

2007 20: 29History of the Human SciencesRobin I.M. Dunbar

Evolution and the social sciences

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Evolution and the socialsciences

ROBIN I. M. DUNBAR

ABSTRACT

When the social sciences parted company from evolutionary biologyalmost exactly a century ago, they did so at a time when evolutionary

biology was still very much in its infancy and many key issues wereunresolved. As a result, the social sciences took away with them anunderstanding of evolution that was in fact based on 18th- rather than19th-century biology. I argue that contemporary evolutionary thinkinghas much more to offer the social sciences than most people haveassumed. Contemporary evolutionary research on human behaviourfocuses on two main issues at the micro-social scale: understandingthe trade-offs in individual decision-making and understanding thecognitive constraints that limit flexibility of decisions. I offer examples

of both of these approaches. Finally, I consider the broader question ofthe macro-social scale.

Key words behavioural decisions, cognitive constraints,Darwinism, evolution, naturenurture

It would probably be an understatement to say that evolution has not had aparticularly good press within the social sciences: it has not done so for thebetter part of a century. Yet, in some ways, this is surprising. The founding

fathers of modern sociology were much inspired by evolutionary ideas, andscenarios for the evolution of society were a major component of bothanthropology and sociology during the closing decades of the 19th century.

HISTORY OF THE HUMAN SCIENCES Vol. 20 No. 2 2007 SAGE Publications (Los Angeles, London, New Delhi and Singapore) pp. 2950[20:2; 2950; DOI: 10.1177/0952695107076197]

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Indeed, Herbert Spencer, historically one of the more influential founders ofmodern sociology, was a committed advocate of Darwinian ideas. However,during the early years of the 20th century, a radical move against biology

began within the social sciences. Origin stories were gradually banished astopics of serious discussion and, under the influence of Durkheim and Boas,the assumption that any aspect of human behaviour (and, in particular,socialbehaviour) could have a biological basis was dismissed as untenable.

In my view, this was unfortunate, albeit understandable when seen fromthe perspective of the times. It was unfortunate for two reasons. First, itpredisposed the social sciences to ignore potentially valuable ideas that werelater to emerge within evolutionary biology. Second, it was particularly un-fortunate because of its timing. In this respect, it is important to appreciate

that evolutionary ideas within biology were still in great flux at the end ofthe 19th century when the rift between the social and biological sciences firstopened up. The second half of the 19th century had witnessed the grand battlebetween the two major schools of evolutionary thinking, those associated withLamarck (largely reflecting an 18th-century perspective) and with Darwin(reflecting radically new ideas that emerged in the middle of the 19th century).While both agreed that evolution occurred, they differed fundamentally aboutthe mechanisms involved, and the implications that followed. Most of thishistory (which I summarize below) is well known and a detailed account withcitations would be inappropriate and out of place in the present context.Rather, my purpose here is simply to sketch in this background so as toprovide a better understanding of how and why the contemporary situationcame to be. For those requiring an authoritative account of the recent historyof biology, the best source is undoubtedly Mayr (1982).

The older school, followers of the great French biologist Jean-BaptisteLamarck, viewed the processes of evolution as the natural outcome of thecapacities inherent in the organism. They generally adhered to the venerableAristotelian concept of the Great Chain of Being whereby species graduallyand inevitably evolved from one form into another along a continuum of

increasing complexity. Lamarck famously added to this his concept of theevolution of acquired characters a form of biological tinkering wherebymodest but nonetheless striking changes in physical form might be intro-duced along the way through the unusually heavy use of some part of thebody (giraffes necks and blacksmiths arms being the conventional examples).Excess use leading to the overemphasis of some body-part would be trans-mitted down the generations because of resulting changes in the biologicalnature that underpinned inheritance. Nonetheless, acquired characters not-withstanding, the fundamental processes of evolution remained the natural

progression along the Great Chain. Humans, as the most complex beingsknown, were naturally higher up the chain (whose pinnacle traditionally laywith God), and that, by definition, meant they must be one of the oldest:

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implicitly or explicitly, the processes of evolution could not be speeded up,so a species age was necessarily correlated with its level of advance along theGreat Chain. Evolution was, by its nature, progressive.

Darwin upset this particular apple cart by arguing that evolution was arandom process with no fixed direction. Novel characters emerged by chance,and were promoted or removed by natural selection acting as a kind ofenvironmental filter. He had come to these views on two major grounds. Onewas the discovery that life forms on different continents did seem not tofollow the same Lamarckian pattern up the Great Chain; indeed, the lifeforms on different continents could be radically different. The Beagle voyageplayed a seminal role in this, of course, but Darwin was by no means theonly biologist to notice this as a consequence of the great age of Victorian

exploration: as is well known, Alfred Russel Wallace had come to similarconclusions, and he was far from being alone in this. The second piece offormative evidence underpinning Darwins view was pigeon-breeding.Darwins interactions with professional breeders, and his own experiments,taught him that careful selective breeding could result in dramatic changes inan animals form in just a few generations. The concept of natural selectionwas conceived as an analogy to the breeders art. Once this was understood,two important consequences followed. First, evolution could not be direc-tional: there was no one-directional progressive tramline on which everyspecies was forced to run. Second, as a direct consequence, an organismscomplexity tells us nothing about its age, merely about the intensity of theselection pressures that it has been under.

Unfortunately for the social sciences, the battlelines between these twoopposing conceptions of how evolution worked remained firmly drawn untilwell into the early decades of the 20th century. Only with the developmentof the so-called synthetic theory of evolution (sometimes also known as Neo-Darwinism) in the 1930s was the Lamarckian ghost finally laid to rest. At thetime that the social sciences parted company from biology, the Lamarckianview still held wide sway, not only within biology but especially outside the

discipline. Indeed, many people were unable to distinguish between the two.One of those was Spencer himself, whose enthusiastic promotion of Darwin-ian ideas (Spencer was in fact responsible for coining the Darwinian catch-phrase survival of the fittest) involved an amalgam of Darwins and Lamarckstheories. Social Darwinism (as Spencers main contribution came to be known)was not, in actual fact, Darwinian at all, but rather Lamarckian (which islargely why Darwin himself distanced himself from it, much to Spencerseternal disappointment). This was no great fault on Spencers part, becausemany people Darwin included successfully managed to hold ideas from

both traditions comfortably at the same time. In Darwins case, this wasthe idea of acquired characters: he never managed to provide a convincingmechanism of inheritance for his theory, and was, in the later editions of the

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Origin, forced back onto Lamarckian mechanisms1 though not onto Lamarckstheories of evolutionary progress. For Spencer, it was the idea of progressiveevolution: his view was very much one of a natural unfolding of social

complexity as a part of a natural developmental sequence and shared muchin common with the ideas of the German biologist Ernst Haekel who inter-preted Darwin in exactly this way. Unfortunately, by the time the biologistshad finally sorted out what was happening to their own satisfaction, the riftwith the social sciences was well entrenched, and the message never gotacross. Progressive evolution continued to be a major foundation for under-standing sociality within the social sciences for the better part of half acentury after it had been discredited within biology. More importantly, whenevolutionary ideas were eventually firmly ejected from the social sciences

once and for all, it continued to colour sociologists perceptions and under-standing of evolutionary biology.There was, of course, a second issue separating the social and biological

sciences during the latter half of the 20th century that derived in part fromthese earlier discussions, and this was the issue of genetic determinism. Forbiologists, the discovery of genes (and later DNA) provided a fundamentalplatform for understanding evolutionary processes. Evolution occurred onlybecause genes were inherited by offspring from their parents, thereby main-taining the integrity of the so-called germ line that was necessary to allowevolution to occur in the Darwinian manner. Rare mutations provided thegrist for the mill of natural selection, but what later became known asWeizmanns Doctrine or biologys Fundamental Theorem (that genes influ-ence the body, but the body [i.e. use in the Lamarckian sense] cannot changethe genes themselves2) remained the cornerstone that made evolution possible.

It is easy enough to understand why genes came to play such an import-ant role in biological thinking. Most biologists were, and to a large extent stillare, concerned with the physical attributes of the organism: how did its body-parts come to be, how did species (defined mainly by differences in thesebody-parts) come to be? And for these, genes play an important and substan-

tive role (even if, as has been recognized in the so-called interactionist stancesince the 1960s, the genes are not independent of the environment in whichthe organism develops). The discovery of genes, and later DNA, inevitablycame to play an important role as the focus of biologists interest. However,it is easy to forget that genes as such played no role at all in either Darwinstheory of evolution or, indeed, Mendels theory of inheritance. To be sure,both argued for a mechanism that allowed traits to be passed down the gener-ations in a reasonably (that is to say, statistically) reliable way, but bothremained wholly ignorant of the biochemical properties of genes as we know

them today: these came to be understood, at first rather dimly, only with theopening decades of the 20th century getting on for a quarter of a centuryafter these two particular giants had died.




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In fact, Mendels theory of inheritance (and hence Neo-Darwinism itself)rests only on what Mendel referred to as the fidelity of copying betweenparents and offspring. Mendels entire theory breathed not a word about

genes, but merely asserted that something (which he called a factor) allowedoffspring to resemble their parents in respect of individual traits or charac-ters. It was Mendels genius to figure out a mathematical model of how thismight work at the genetic level that has more than stood the test of time. Inthe excitement of the later discovery of DNA, the significance of all this wasoften lost sight of. It was not until the 1980s that evolutionary biologistsbegan to point out that, in actual fact, learning itself is a perfectly goodDarwinian process, and whatever mechanisms might underpin this (the wayanimals learn and what they may be predisposed to learn) would make just

as good a basis for Darwinian evolution as anything else. Thus was born thenotion of cultural evolution as a topic of study for evolutionary biologists,and, more notoriously perhaps, Dawkinss concept of a meme.3

Having set the broad historical context, let me now briefly summarize thethree main evolutionary approaches to the study of human behaviour, andthen provide some examples of these approaches.

EVOLUTIONARY APPROACHES

It is important to be clear at the outset that evolutionary approaches to thestudy of behaviour do not normally concern themselves with evolutionaryhistory that is to say, the evolutionary origins of behaviour and the sequenceof changes by which behaviour evolves sequentially from one species toanother. Rather, the focus lies mainly on the micro-evolutionary processesunderpinning the mechanisms of selection and, in the case of behaviour, thecognitive mechanisms involved. By the same token, there has been verylimited interest in the historical aspects of the evolution of societies after the1960s (although this may now be changing).

By convention, there are three broad approaches to the evolutionary studyof behaviour. The oldest of these is that of the behavioural ecologists; thisapproach has its origins in the study of animal behaviour (usually nowreferred to as behavioural ecology) and is largely the remit of evolutionarilyminded social anthropologists whose main focus of study is traditionalsocieties (who typically refer to themselves as evolutionary anthropologists).The second approach is that associated with what was previously referred toas Darwinian Psychology, but is now more commonly referred to as Evolu-tionary Psychology in the narrow sense;4 its focus of interest is principally

on cognitive mechanisms (the design of the mind) and is largely the provinceof scientists whose intellectual background lies within psychology (and inparticular cognitive and social psychology). The third, and much the smallest,




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of the difference between observed and expected patterns will cue us in towhat we have overlooked.

It is important to appreciate that costing out the fitness consequences of

behaviour involves assessing the net benefit (i.e. the benefits minus the costs)since all actions incur costs. Biological and behaviour phenomena oftenexhibit an inverted-U-shaped pattern of net benefits: doing something thatis generally speaking good for you too often incurs costs that increase expo-nentially such that the costs rapidly wipe out whatever benefits are gained.A widely confirmed principle in behavioural ecology known as Lacks Prin-ciple states that the optimal number of offspring (or, in its original formu-lation for birds, eggs laid) will invariably be well below the physiologicalmaximum: if the costs of rearing additional offspring increase with their

number, large broods will suffer disproportionately high mortality and thenet number of surviving offspring will often be lower than those obtained bymore restrained parents (Lack, 1968). More is not always better in evolution.This is because, to borrow a phrase from the eminent evolutionary biologistJohn Maynard Smith, evolution is not interested in babies as such, but ratherin (great-)grandchildren (i.e. fitness). A second important lesson is that every-thing in evolutionary biology is context-dependent: there are no absolutes inbiology, no traits or behavioural strategies that are absolutely better in allcircumstances (which we might therefore expect to occur universally). Indeed,behaviour in particular is so context-dependent that high- and low-rankingindividuals in the same social group may actually have radically differentoptimal strategies because their positions in the social hierarchy within thegroup influence the costs or benefits, or both, in different ways (for thisspecific example, see Cheney, 1983). In short, behavioural ecologists typicallyassume that behaviour is currently adaptive (i.e. that organisms are sufficientlyflexible to fine-tune their behaviour quite subtly to even small changes in thecosts and benefits that they encounter) and thus interest themselves in thedifferences between individuals (or, more generally, between species).

The Evolutionary Psychology approach, in contrast, tends to focus more

explicitly on human universals. Its underlying assumption is that there hasnot been significant evolution in the human brain within the last 100,000years or so, but cultural evolution since the Agricultural Revolution 10,000years ago has radically changed the environment within which humans maketheir behavioural choices. Since it is assumed that the human mind (i.e. brain)has been adapted to conditions that existed at some time in the past, this maylead to a significant disjunct between actual behaviour and what is nowoptimal in the novel circumstances in which we find ourselves. Our contem-porary behaviour need not, and often may not, be adaptive. In the limit, they

would argue that the behavioural ecologists attempt to determine whetheror not behaviour is adaptive is misguided: contemporary behaviour can tellus nothing about how or why behaviour (or the cognition that underpins it)




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is adaptive. Although such a hardline conception of the issue is probably anincreasingly minority view, it nonetheless reminds us that the principal focusof the Evolutionary Psychologists lies in understanding the extent to which

human behaviour (in particular) is guided by the particular design of themind that we have inherited. Underlying this is, however, a view of humanbehaviour that is rather more deterministic than the behavioural ecologistswould allow: the human mind is designed to facilitate certain kinds of behav-iour, because exactly those kinds of behaviour (or attitudes) were adaptive inthe environment in which humans evolved (some time in the late Pleistoceneprior to the Agricultural Revolution). In contrast to the fitness-matchingmethodology of the behavioural ecologists, the Evolutionary Psychologistsmethodology relies rather more heavily in demonstrating fitness for purpose

based on the design characteristics of either specific cognitive mechanisms orthe behaviour produced by those mechanisms.In this respect, the two sub-disciplines focus on alternative definitions that

evolutionary biologists conventionally use to determine the adaptiveness ofbiological phenomena. Biologists have always insisted that evidence foradaptation can be provided either by demonstrating fitness consequences (arelativistic measure) or by demonstrating engineering design for purpose (anabsolute measure) (Williams, 1966). The eye, for example, provides a classicexample of design for purpose (an adaptation): we can examine its propertiesand show that they do indeed produce the supposed effect (a procedure some-times known in evolutionary biology as reverse engineering). Antler size indeer would provide an example of the alternative approach: we would corre-late antler size with number of matings achieved during a breeding season,and infer a selective advantage (and hence adaptedness) from the presence ofa correlation.

Having sketched out the basics of the philosophical assumptions in the twomain evolutionary approaches to the study of (human) behaviour, I want toemphasize that, despite the acrimony that has sometimes existed betweenthem (for a recent summary, see Laland and Brown, 2002), these are in fact

two ends of a continuum (or, strictly speaking, maybe it should be threecorners of a triangle). They both represent perfectly good ways of doingevolutionary biology. Their difference lies simply in the questions to whichthey give priority. The behavioural ecologists are interested primarily in theprocesses that drive evolutionary change and, once such change has occurred,keep it in place; they are interested in individual differences between andwithin populations of individuals. In contrast, the Evolutionary Psychologistssensu stricto focus on the nature of the adaptation itself; they are interested inthe universal features that characterize groups of organisms (usually species).

They are asking, and seeking to answer, questions at different levels of theexplanatory framework. However, the lines can become blurred, and manyof those who study human behaviour from an evolutionary point of view




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mix both perspectives, depending on the questions they happen to be asking.In the following section, I give some examples of these two approaches, butI choose these examples specifically to point out that the same researchers

can work at both levels.

CASE STUDIES FROM THE BEHAVIOURALECOLOGY APPROACH

There have been studies in the behavioural ecology tradition of both traditionalsocieties and contemporary modern societies. A range of such studies can befound in Barrett et al. (2002). I will describe just two examples here from my

own work. One concerns mate choice strategies, based on analyses of personalads. The second concerns parental investment decisions, with a particular focuson a Gypsy (or Roma) population which has a more ethnographic flavour.

Choosing the right mate can have significant consequences for an individ-uals fitness in a number of different respects. As a general principle, organ-isms can choose between investing in matings (in order to maximize thenumber of conceptions achieved in a lifetime) or in parental care (in order tomaximize the number of offspring successfully reared to adulthood). Sinceevolution is ultimately interested in the number of grandchildren, or betterstill great-grandchildren (offspring that do not in their turn reproduce do notcontribute to ones genetic fitness), there is an inevitable trade-off betweenthese two options, and hence, in principle at least, a more or less infinitenumber of ways in which the two can be balanced against each other (this is,essentially, Lacks Principle again).

The extent to which this is possible for any given species will ultimatelydepend on the balance of the costs and benefits of these two strategic options.Because the costs of rearing offspring are both heavy and prolonged in humans(mainly as a consequence of our very large brains), the demands of parentalcare have to be met if children are to be reared successfully. However, within

that constraint, it is possible still to opt for a strategy that maximizes matingsat the expense of rearing, providing the mate is prepared to carry a dispro-portionate share of the rearing burden. Because of the way mammalian repro-ductive biology is organized (an unavoidable given for mammals), there is,in the general mammalian case, an inevitable asymmetry in the way the twosexes experience these effects. All else equal, male mammals can make no or only a very limited direct contribution to the business of rearing(through parental investment), at least in the early phases (i.e. up to the pointof weaning), and maximizing the number of matings provides them with the

most effective means of maximizing the number of offspring produced. Incontrast, because females are committed willy-nilly to a significant invest-ment, they will gain less by maximizing the number of matingsper se and




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proportionately more by ensuring that (1) the male(s) who father theiroffspring provide the best genetic contribution to their offspring and (2) thecircumstances in which they have to rear the subsequent offspring are

optimal (in terms of the provision of the resources required for successfulrearing and that does not necessarily involve any contributions from thefather). Thus, there is in principle a natural contrast in the two sexes repro-ductive strategies.

While this contrast is naturally moderated in the case of humans by thefact that the costs of rearing are very heavy (thanks mainly to the need togrow a very large brain), humans nonetheless remain exposed to the samedichotomy and we can expect this to be reflected in the mate choice strat-egies of the two sexes: males can be expected to attempt to maximize matings

whenever possible, and will be less discriminating as a result in their choiceof partners (other than having a preference for fertile partners), whereaswomen will be more discriminating (i.e. choosy) and will place greater weighteither on the genetic quality of males or the males abilities to contribute(directly or indirectly) to the business of rearing, or on both. The latterdichotomy itself reminds us that there is an opportunity to trade one dimen-sion off against another, and we may expect, again on general evolutionaryprinciples, that womens mate choice preferences will have created a selectionenvironment in which males might come in two general types (sometimesreferred to in the evolutionary literature as cads and dads: Draper andHarpending, 1982; Cashdan, 1996). However, since evolutionary processesboth are dynamic and depend on the contextual contingencies of the balancebetween costs and benefits, we can expect that, while these general principleshold good, individuals may nonetheless trade off their opportunities on thedifferent criteria. This will be especially true of males, because female matechoice is more complex and involves more dimensions, making it harder forwomen to identify an optimal (i.e. perfect) mate. That will inevitably leavemales the option of fine-tuning their behaviour to target one dimension atthe expense of another. All this is standard behavioural ecology theory, and

can be found in any appropriate textbook.Personal ads provide a very convenient way of studying mate choice strat-

egies in humans, not least because they provide a succinct summary both ofthe advertisers expectations and of what they have to offer. More impor-tantly, they do so in a form that can be easily quantified, either by countingthe number of advertisements that exhibit a particular trait or by countingthe number of words an advertiser uses within a particular category (therebygiving a simple way of quantifying the advertisers inherent interest oremphasis on that category). We have been able to show (Waynforth and

Dunbar, 1995; Pawowski and Dunbar, 1999a, 1999b, 2001) that (1) womenare much more choosy than men in what they demand in a prospectivepartner (a finding confirmed by a wide range of other psychological and




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sociological research on mate choice: for a summary, see Grammer, 1989), (2)both sexes target their demands on those traits that have most impact on theirfitness (fertility in the case of men, resources/status and parental care capaci-

ties in the case of women) and (3) both sexes adjust their demands in the lightof what they themselves have to offer (i.e. those with weaker bargaininghands, as defined by the interests of the other sex, tone down their owndemands in order to remain competitive). More particularly, Pawowski andDunbar (1999b) were able to demonstrate that advertisers perception ofwhere they stand in the mating market was, in general, extremely sensitive(by and large, they really did understand how strong a bargaining hand theyheld) and directly influenced how demanding they were: those with less tooffer demanded less. Men who lacked economic resources to bring into a

relationship, for example, were less demanding in what they expected of apartner or emphasized alternative attributes (e.g. willingness to tolerate andinvest in children from a previous relationship) that might help to offset theirinitial disadvantage.

Once a mate has been chosen, of course, the business of reproduction isthe core to the evolutionary process, although a caveat needs to be added asa reminder: it is not necessary to reproduce oneself in order to maximizeones fitness. What many consider to be the fundamental theorem of evolu-tionary biology (conventionally known as Hamiltons Rule: see Barrettet al.,2002) reminds us that, when the issue is the replication of genes in futuregenerations, there are in fact two routes by which this can be achieved: oneis by direct personal reproduction and the other is through the reproductionof relatives who share the same gene(s). Hamiltons Rule formally providesone explanation as to how altruistic behaviour6 could evolve in a Darwinianworld. However, it stands as a general reminder of two important points.First, everything we do has implications for our fitness, not least because anyaction may affect the interests of our relatives as well as our own individualinterests. Second, whenever we decide to do something, we are alwayschoosing between at least two alternative courses of action: even in the most

trivial case of my deciding to give money to a beggar in the street, my doingso means both (1) that money will not be available later for me to use for myown purposes and (2) it will not be available for me to invest in my relatives(which would allow me to benefit through kin selection). In short, there arealways consequences for every action, although in some cases those conse-quences may be quite trivial. It is the balance between the net benefits(benefits minus costs) of these alternatives that lies at the root of the evolu-tionary process.

In this particular context, the issue is the choices that parents make between

investing in their various offspring. One implication of Hamiltons Rule isthat different individuals (including different offspring) will inevitably varyin their fitness value (for a good if technically difficult summary of this issue




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and its implications for human social structure, see Hughes, 1988). Not onlydoes their relatedness to me vary, but their fitness value (to me) will also varyas a function of their age (because age affects the number offuture offspring

they can produce), their sex (because sex affects their reproductive riskiness),and any number of other variables. All these sum together to define whatbiologists refer to as reproductive value (Fisher, 1930). This led Trivers andWillard (1973), in a seminal paper, to point out that, in order to maximizetheir own fitness, parents may choose to invest differentially in their variousoffspring. This may include preferring one sex of offspring over another, ordiscriminating for and against offspring of different birth order. The key issuehere is how investment in an individual offspring influences that offspringsfuture reproductive opportunities. Trivers and Willard pointed out that,

when the two sexes differ in the variance in lifetime reproductive output, thenparents in good condition who can afford to invest heavily should prefer themore risky sex, whereas those in poor condition who cannot afford to investso heavily should prefer the less risky sex. In mammals, the nature of thereproductive machinery will inevitably mean that the variance in male repro-duction will usually exceed that of females, even though both must have thesame mean (because that is set by the rate at which the reproductively morelimited sex can reproduce). In mammals, females are limited in their repro-ductive turnover time, whereas males can compete to mate with many females(as a result of which some males will do very much better than average,whereas others may fail to reproduce at all). Translated into human terms,this leads to the expectation that, if investment in children directly influencestheir reproductive success and does so differentially for the two sexes, wealth-ier parents should favour sons, whereas poorer parents should favour daugh-ters. One consequence of this would be the expectation that inheritance ofland and other forms of wealth will most often be male-biased, but only whensons can put such resources to better use than daughters by using them toacquire more wives (by which I mean to include not only formal polygamy,but also the acquisition of mistresses in formally monogamous societies).

The Trivers-Willard Effect (as this is known) has been widely documentedin animals, and a number of studies have attempted to explore its relevance tohumans. Of these, the most complete7 has been that by Bereczkei and Dunbar(1997). They compared parental investment behaviour in two subgroupswithin contemporary Hungarian society, namely Gypsies (Roma) and ethnicHungarians living in the same locations. As members of an economic under-class, the Gypsies represent a group whose opportunities are severely limitedeven in comparison with the lower-class ethnic Hungarians among whomthey live. However, Gypsy women that married into the relatively wealthier

Hungarian community had significantly higher lifetime reproductive outputsthan those who married into their own ethnic community, mainly because ofthe economic resources available in the Hungarian community. This classic




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form of hypergamy provides exactly the conditions for a Trivers-WillardEffect. Bereczkei and Dunbar (1997) were able to show, from analyses ofdetailed interviews with a large sample of women from both communities,

that Gypsy parents invested more heavily in their daughters, whereas ethnicHungarian parents invested more in their sons. This included differences inthe length of the interbirth interval following a son versus a daughter (ameasure of direct biological investment), abortion rates after sons versusdaughters (higher after sons than after daughters among the Gypsies, butreversed in the ethnic Hungarians), the duration of lactation (it was up to 6months longer for Gypsy girls than Gypsy boys, but the reverse was the caseamong the Hungarians) and even the duration of secondary schooling (whichincurred a direct financial cost for the parents, even under the Communist

regime). More importantly, the strength of this bias varied across fourcommunities (two Gypsy and two ethnic Hungarian) as a direct function oftheir capacity to benefit from hypergamy. In other words, these investmentdecisions were not fixed in some simpleminded sense, but were being titratedvery carefully against the anticipated fitness pay-offs. That this was so wasdemonstrated by showing a linearly and exactly proportional relationshipbetween the ratio of investment in the two sexes across the four communities(rural and urban Gypsy8 communities, and rural and urban ethnic Hungariancommunities) and the ratio of fitness pay-offs indexed by the numbers ofgrandchildren produced through sons versus daughters.

CASE STUDIES FROM THE EVOLUTIONARYPSYCHOLOGY APPROACH

My choice of examples to illustrate the Evolutionary Psychology approachis slightly unusual, in that most Evolutionary Psychologists would focustheir interests on the way cognition limits the kinds of strategic decisionsassociated with mate choice or other everyday behaviours. There is, for

example, now a very large literature on the way that evolution acting on thehuman mind influences behaviours as diverse as disgust, leadership, matechoice, friendship patterns and altruistic behaviour, among others (for recentoverviews, see Schaller et al., 2006; Forgas et al., in press). Instead, I will takea slightly larger perspective and use, as my core example of this generalapproach, the work we have done on the size and structure of human socialnetworks. I do so mainly to show that the Evolutionary Psychology andbehavioural ecology (evolutionary anthropology) approaches are not intel-lectually mutually exclusive, but rather reflect an underlying unity of concep-

tual structure.We have been able to demonstrate that neocortex volume (essentially the

thinking part of the brain) correlates directly with social group size across a




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wide range of primate species, and that this relationship predicts extremelyclosely the size of human social groups. The predicted size for human socialgroups is approximately 150 individuals, and this value turns out to be

extremely common in a wide range of human social contexts. These rangefrom the size of villages in traditional societies to the size of clans (or regionalgroups) among hunter-gatherers, the size at which formal hierarchical struc-tures (line management, police forces) become necessary in both contem-porary communities and business organization,9 the optimal size for churchcongregations and the size of personal social networks (as determined bothby soliciting lists of friends and from small world experiments) (Dunbar,1993, 2004).

In addition, we have been able to show that human social networks are

highly structured, such that they have the form of a series of concentriccircles (the circles of acquaintanceship): each successive circle of acquain-tanceship includes more individuals, but involves relationships of markedlyreduced intimacy and frequency of contact (Hill and Dunbar, 2003; Zhouet al., 2005). More surprisingly, the size of these successive circles has a verycharacteristic ratio: each grouping is exactly three times larger than thatcontained within it. We are not sure why this pattern should be so consistent,but there are grounds for believing that there may be a cognitive limit on thenumber of relationships that can be held at a particular level of intimacy.Stiller and Dunbar (2007), for example, showed that the size of the innermostcircle of acquaintanceship (on average 5) was a more or less linear functionof individuals capacities to handle mind-reading tasks (i.e. the kinds of socialcognition that is involved in theory of mind, or the ability to understandanothers mental states).

The fact that humans are embedded in complex multi-level social systemshas important implications for the more conventional Evolutionary Psychol-ogy approach, because it reminds us that forces other than simple self-interestmay be at play when humans choose how to behave. We are embedded insocial networks that exist for a purpose to enable us to achieve those goals

of biological survival and reproduction as efficiently as possible. Such social-ities are, in effect, implicit social contracts (which may, of course, sometimesbe made explicit with verbal agreements): they allow us to trade some loss ofself-interest (we have to give up some personal interests to enable the groupto survive as a cooperating community) in anticipation of much greater futuregains via community-level processes. This recognition of the importance ofthese multi-level processes represents a new and exciting development inevolutionary biology (Plotkin and Odling-Smee, 1981; Laland et al., 2000)precisely because it allows us to draw attention once again to the community

and societal levels at which individuals interact with each other.This, of course, is simply to remind ourselves of Hamiltons Rule. As social

creatures, we are dependent on the effectiveness with which our local social




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structures work. Indeed, the evolutionary perspective would argue thatsocieties exist precisely to facilitate our capacity to solve the everyday problemsof survival and reproduction cooperatively. This is a generic primate charac-

teristic: all primate societies are implicit social contracts of this form, and theyinvolve a fine-tuned balancing of the demands of system stability against ourown personal self-interests. Cheating on the social contract risks destabiliz-ing the social system, thereby losing us its benefits. This is the familiarfreerider (or freeloader) phenomenon (see Dunbar, 1999, 2004). The pressureto cheat is always there, because it will always be in our individual short-term interests to try to gain more from the social contract than we put in.This fine-tuned balancing of selfish versus group interests has turned uprepeatedly in a range of experimental studies inspired by the evolutionary

approach (Henrich et al., 2005; Madsen et al., in press). In all these studies,the social and economic context has a strong influence on exactly howsubjects respond to a particular challenge, just as the behavioural ecologyperspective embedded as it is within evolutionary biology insists shouldbe the case.

CONCLUDING REMARKS

These issues become of particular interest from the macro-sociological scalebecause they emphasize the fact that humans are embedded in complex, multi-level social structures. The evolutionary perspective differs only insofar as itreminds us that, no matter how much we are socialized (in the Durkheimiansense) into the particular society that we belong to, nonetheless we do takepersonal decisions about how to behave towards each other. As a result, thereis often a natural tension between what is in our short-term personal interestand what is in our longer-term interest via the communal life. The need tobalance those two essentially incompatible demands creates the imperfec-tions of social life with which we perennially have to grapple. Indeed, it might

be argued that the evolutionary perspective uniquely provides us with anexplanation as to why society should be such an imperfect phenomenon why, despite its obvious advantages, societies are always riven by conflictsand bedevilled by individuals who willfully insist on cheating the system.Note that, by the same token, the evolutionary approach does not therebyprovide a justification for freeriding and other more explicit forms of theftand exploitation; rather, it simply provides an explanation as to why weshould constantly have to struggle with them.

It is also important to appreciate that the evolutionary approach is a

statistical one. It does not assume that every individual makes exactly theright decision, but, rather, only that on average a population of individualswill do so. The fact that some individuals get it wrong does not disprove the




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hypothesis. That is only true if a majority of individuals do so. The evolu-tionary perspective always requires some individuals to get it wrong, other-wise there would be no variation on which natural selection can act; indeed,

when variation ceases to exist, evolution grinds to a halt.Perhaps the most frequent example raised in this context is voluntary child-

lessness (or, in the more general case, the reduction in family size associatedwith the demographic transition). The evolutionary bottom line is thatvoluntary childlessness is a choice that we can make, and it is one that wemay take up as a result of some form of cultural transmission (i.e. the thirdarm of the evolutionary approach to behaviour). It may be that it genuinelyis maladaptive in a biological sense, but if so we might expect the enthusiasmfor it to wane in due course (and it is probablyfartoo early to make any

judgement on that yet). However, it is important not to take too nave a viewof complex biological processes: because individuals are embedded in societiesthat are essential to their success, it is sometimes necessary to act in ways thatare, in immediate terms, maladaptive (in the sense of not being in ones short-term self-interest). One of those may be to persuade people to adhere tonorms of behaviour that, in the long term, are in everyones interests.Moreover, we need to remember that personal reproduction is not the onlyway of contributing to ones own fitness: investing in the reproduction ofrelatives may be just as effective. We would need to show that the voluntarilychildless do not invest proportionately more often in relatives before we canconclude that they are behaving in a strictly maladaptive way. There is nowample evidence from a number of studies that voluntarily or involuntarilychildless couples invest more heavily in their nieces and nephews (Pollettet al., in press; see also Silk, 1980, 1990). Doing so allows individuals to gainfitness through the genes that they share with their close relatives, in line withHamiltons theory of kin selection.

One of the problems that social scientists often have when engaging withevolutionary biology is that they tend to drastically underestimate thecomplexity of the biological processes (and hence the arguments) involved.

One example is provided by Moore (1990)10 in an attempted rebuttal ofChagnons (1988) imputed claim that humans are naturally warlike. Moore,in what seems like a perfectly sensible analysis, used historical demographicdata from the Cheyenne to show that peace chiefs (hereditary chiefs whotook no part in active war) had larger family sizes than war chiefs (non-hereditary chiefs responsible for actively leading the tribe in war), not leastbecause many war chiefs died on the battlefield before even having the chanceto marry, let alone reproduce. Moore argued that if there was such a thing asa gene for warlike behaviour (presumably ultra-aggressiveness), then such a

gene would very rapidly be bred out of the gene pool by the intense selec-tion pressure acting against it from the high mortality rates incurred byaggressive war chiefs. Unfortunately, while such an analysis would suffice for




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many simple anatomical traits (eye colour, leg length), it fails to take note ofthe complexities that enter into the calculations where behaviour in multi-level societies is concerned. In effect, it attacks the problem at the wrong end.

The actual choices faced by Cheyenne were not between being a peace chiefand being a war chief as two equal options (whether these are genetic traits,socially inherited traits or personal choices), but rather between the costs andbenefits of being eithera peace chiefora war chiefversus being a humblebrave at the bottom (relatively speaking) of the social hierarchy for twocompletely separate groups of men (sons of peace chiefs versus the orphanedsons of braves,11 respectively) (Dunbar, 1991). Only the sons of peace chiefscould be peace chiefs (with the benefits that arise therefrom), whereas theorphaned sons of ordinary members of Cheyenne society faced an invidious

choice between taking the very risky (but, if they survived, ultimately verysuccessful12) strategy of trying to become a war chief or being outcasts onthe fringes of society (where their chances of marriage were all but zero). Infact, the demographic data show very nicely that the relative frequencies ofthese two types of chief exactly balanced the risks they ran and benefits theygained. In effect, being a war chief is, for orphans, what evolutionary biolo-gists would refer to as a best of a bad job strategy (Krebs and Davies, 1993):they do not have the option of being a peace chief, so they actually only havetwo options to choose between whereas the sons of peace chiefs have three and wisely decline to consider the war chief route.

Moores mistake is to have assumed that he is dealing with a situationinvolving two alleles for a gene for warlike behaviour (e.g. aggressivenessversus peacefulness), when in fact he is dealing with a single gene (the genefor aggressiveness possessed by every individual in the population) which canbe expressed to different extents in different contexts. Indeed, this is not evennecessarily a genetic issue, but rather one of how individuals respond (oftenin a conscious and calculating manner) to the circumstances in which theyfind themselves.

By the same token, the demographic transition becomes understandable

when reproduction is seen in the context of the need for increasingly expen-sive parental investment as societies shift from agricultural to knowledge-based economies, with the resulting need to invest in costly education in thelatter (Mace, 1998). As Mace points out, we have often tended to assume thatsocieties are homogenous, when in fact they are not: as in the Cheyenne case,each of us operates within a limited socio-economic context which not onlylimits whom we compete with but also dictates the options we have availableto us and the costs and benefits of pursuing those options. It seems that theRegistrar Generals socio-economic classes have an evolutionarily intrusive

reality because they affect what you can afford to do when the strategiesavailable to you are economically costly. In short, the evolutionary perspec-tive is invariably much more complex than it often appears at first sight




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especially if first sight equates the evolutionary approach with the geneticdeterminism of behaviour.

If I have a plea on which to end, it is that social scientists refrain from the

kinds of knee-jerk abhorrence of the evolutionary approach, and ask insteadhow this approach might benefit what they do. The key message is that theevolutionary approach is not the same thing as genetic determinism; rather,it is about individuals strategic decision-making under the constraints ofcircumstance. More importantly, perhaps, adopting an evolutionary approachdoes not ipso facto necessitate throwing all of sociology, anthropology, econ-omics, or even history, into the dustbin. These disciplines remain funda-mentally important to the evolutionary endeavour because they provide theessential details of both the context and the costs and benefits that are

required by the evolutionary approach. The evolutionary approach is not analternative to the various social science disciplines, but rather a complement.And, importantly, it is a complement that offers the opportunity of integrat-ing the disparate social sciences into a single intellectual framework.

NOTES

1 The problem was not resolved until the rediscovery of Mendels laws of inheri-tance in the first decade of the 20th century, long after both Lamarck and Darwinwere dead. It was the synthesizing of Mendels mechanism of inheritance withDarwins theory of natural selection that produced the grand synthesis of Neo-Darwinism during the 1920s and 1930s (Mayr, 1982).

2 The key distinction here is the role that use (or natural selection) plays.Weizmanns point was that use cannot alter the germline (i.e. the actual geneticmaterial itself); rather, it merely alters the frequencies with which differentversions (mutants, alleles) get passed into the next generation.

3 I should hasten to emphasize that memes per se are not an integral part of thebiological study of cultural evolution and its underlying processes. They aresimply a convenient metaphor for thinking about the processes of culturalevolution so as to avoid unnecessary cross-slippage from the biological conceptof the gene. Many who study cultural evolutionary processes do not even botherto use the term.

4 The term evolutionary psychology (often lower-case) is also used to refer to allthree of these sub-fields combined. This distinction is not widely appreciated,but it is important to understand that many of those who label themselves asevolutionary psychologists (in the broad sense) would not sign up to some ofthe more extreme views of the Evolutionary Psychologists (in the narrow sense)(for a lucid overview, see Laland and Brown, 2002).

5 The critical distinction lies in the choice of the utility function (that which is

maximized). Economists assume that this is monetary profit (or, at least morerecently, occasionally psychological variables such as happiness). For behav-ioural ecologists, these are merely proxy indices for the real utility function,




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namely fitness (which is usually better approximated by real biological outcomessuch as numbers of offspring). In the biologists view, the complexities of thereal world mean that proximate economic goals like happiness or profit are not

always well correlated with fitness: maximizing the numbers of survivingoffspring produced (i.e. fitness) does not always involve maximizing the numbersof offspring born (or conceived), because producing too many may dilute theparents ability to invest sufficiently in each to ensure their survival. Theimportant issue is that the mathematics of economics is neutral as to the exactnature of the utility function, so the same mathematics can be used in both cases.

6 Formally, Hamiltons Rule states that a gene for altruism could evolve within apopulation whenever the benefits to the recipient, when devalued by the co-efficient of relatedness between altruist and recipient, exceed the cost to thealtruist, when both are measured in terms of the numbers of additional futureoffspring gained or lost, respectively. This, in a nutshell, is Hamiltons Theoryof Kin Selection. Altruism is formally defined as behaviour that results in arecipient gaining additional future reproductive opportunities which result in theactor losing future reproductive opportunities as a direct result. In the limitingcase, the actor loses all future reproductive opportunities if he or she actuallygives up his or her life in the process. Altruism in the everyday sense (e.g. smalldonations to beggars) will rarely come under this rubric because the costs willnot usually be sufficient to affect the altruists fitness; evolutionary biologistssometimes refer to altruism of this kind as weak altruism and do not considerthat it needs an explanation. However, it is worth emphasizing that kin selectionis just one of at least four biological explanations for the evolution of altruisticbehaviour (for more details, see Barrett et al., 2002 or any other standard textbook).

7 Complete in that, uniquely among studies of the Trivers-Willard effect inhumans, they show that the assumptions of the Trivers-Willard Hypothesis holdin their study population, that the effects arise as a within-population phenom-enon, and that the fitness consequences of the alternative behavioural strategiesfollow.

8 It is important to appreciate that the Hungarian Gypsy community was settledin villages under imperial edict in the mid-19th century, and has not been nomadicfor a century and half. The urban Gypsy and ethnic Hungarian communities livedin the same towns, the rural ones in separate but nearby ethnically homogenousvillages.

9 The GoreTex company provides a particularly nice example of this because theyhave insisted on subdividing their factories into separate buildings whenever thenumber of employees in a factory reaches 150 precisely so as to avoid the needto have hierarchical management structures while at the same time ensuring theefficient smooth running of each factory unit (Gladwell, 2000). It may be noaccident that they have been a particularly successful company.

10 I cite this example without intending to be personally critical of Moore. My pointis simply that, given his disciplinary background, Moores analysis was a perfectlynatural one. Many other examples could be chosen: this is simply one that has

been worked out in detail.11 Moore (1990) reminds us that war chiefs were, almost without exception, the

orphaned sons of ordinary members of society.




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12 Although war chiefs could not marry and habitually pursued risky strategies liketaking oaths never to leave the field of battle alive unless they won, those thatdid survive did in fact revert to being normal members of society (i.e. they were

relieved of their battlefield oaths and allowed to marry). These individuals wereoften very attractive husbands (and, in all likelihood, illicit lovers) and oftenproduced more than the average number of children (even without taking intoaccount any illegitimate offspring they may have had).

BIBLIOGRAPHY

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BIOGRAPHICAL NOTE

ROBIN DUNBAR, FRAI, FBA, is British Academy Research Professor in theSchool of Biological Sciences, University of Liverpool, and co-director of theBritish Academys centenary research project Lucy to Language. His researchfocuses on the evolution of sociality, and combines fieldwork on animalsand humans in their natural setting, experimental studies of cognition andcomparative analysis of brain evolution.

Address: British Academy Centenary Research Project, School of BiologicalSciences, University of Liverpool, Crown Street, Liverpool L69 7ZB, UK.

[email: [email protected]]


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