J. Social Biol . Siruc t . 1988 11, 3 3 7-352

Reproductive and sociocultural success in adual evolutionary model

J. Hill18 Garden Street, Lewes, East Sussex, BN7 1 TJ, UK

A dual model of sociocultural evolution, as distinct from organic evolutionary andco-evolutionary models, is described . The dual model incorporates concept pools,innovation, sociocultural fitness, environmental selection pressures and social isolationas essential evolutionary factors . Unlike other models, it lays great emphasis on socio-cultural success, or prestige, as the driving force behind sociocultural evolution . Itssalient feature is that it treats socioculture as an evolutionary system in its own right withthe potential to break away from organic evolution . This potential may be realized inthe presence of surplus wealth, for it is argued that it would make genetic sense undercertain conditions for the wealthiest kin group in a community to invest in novel, purelyprestigious activities, rather than to increase their reproductive success still further,because these prestigious activities would in effect raise the cost of reproductive successfor their competitors . Once such a step had been taken it would introduce a real conflict,for all but the wealthiest, between the goals of prestige and reproductive success, and itwould be virtually impossible to return to the original system . The relevance of sexualselection is considered, and it is concluded that the course of sociocultural evolution isalways correlated with female mate choice .

Introduction

The main problem this article will explore is the apparent lack of correlation in somehuman societies, including modern western society, between reproductive success (RS)and sociocultural success . This is not a problem for those who consider socioculture tobe always independent of biology, but that is not a viewpoint I share . Nevertheless, Ishall argue that it is independent in the presence of surplus wealth . This follows theaccount of a dual evolutionary model which shows how socioculture may be linked tobiology in the absence of surplus wealth .

First, however, a few terms must be defined . The word 'socioculture' is used inpreference to the terms `society' and `culture' because the one cannot exist without theother, and their separation except for the analysis of specific points or aspects is artificial .Certainly (pace White, 1949), no satisfactory explanation of the evolution of culture canbe arrived at unless society is also taken into account .

'Sociocultural success', or prestige, is a measure of the respect and esteem in which anindividual is held by the other members of his group and of the attention they pay to him .Definitions of `group' are discussed by Landecker (1964). All group members will havesome measure of prestige unless they are virtually rejected by the group . In hierarchically

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structured groups, prestige may be broken down into the components of status (depend-ing upon social position) and performance, the latter being largely a matter of goalachievement, whereas in unstructured groups performance will be the only component .Prestige exists only within a group context; an individual's prestige may differ fromgroup to group within his society .

The specific qualities which engender prestige vary from society to society, but someof the attributes that are likely to attract it may be listed under eight general headings .

(1) Physical attractiveness (including clothes and cosmetics) (Ford & Beach, 1952 ;Landy & Sigall, 1974) .(2) Toughness . For examples, see Omark & Edelman (1976) in children and Chagnon

(1977) in adults .(3) Technical and intellectual ability .(4) Generosity in many small-scale societies (Piddington, 1952) .(5) An even temper and the ability to persuade others to settle their quarrels . For

examples, see Eggan (1950) for the Pueblos and Marshall (1976) for the !Kung San .(6) Special relationship with an elite, human or spiritual, and the possession of esoteric

knowledge .(7) The invention, discovery or possession of something new ; new, that is, to the

relevant group . See Rogers (1983) for innovation by opinion leaders .(8) Command over human and/or material resources by virtue of wealth or status .For the sake of the arguments to be given below, it is better to discuss straightaway

to what extent variation in these attributes from individual to individual is likely to havebeen affected by genotype in primitive human societies . Physical attractiveness clearlywould have been, and it is surely reasonable to suggest that variation in personality isalso likely to have been, which covers attributes (1), (2), (4) and (5) . Variation in technicalor intellectual ability is very unlikely to have been free of genetic influence . Specialrelationship with an elite and the possession of esoteric knowledge in primitive humansocieties would be largely confined to shamans and other religious figures and would tothat extent be intimately connected with personality . Innovation and discovery would bedue partly to technical ability and partly to personality . In primitive societies, therewould probably have been little difference in wealth between the members of a band andwhat there was would have been due to the personal efforts of the individual basedupon technical ability and personality . Status in such societies would be achieved, notascribed, and would depend largely upon personality . Altogether, I should not want toargue that variation in any of these attributes was unaffected by genotype in primitivehuman societies .

Finally, I will attempt a minimal definition of culture by listing three essential distinc-tions. First, culture is what an organism learns from conspecifics as distinct from whatit learns from its own experience . Second, the importance of distinguishing betweencultural concepts and overt culture must be stressed . The former exist within the mindof each individual composing a group, whereas the latter consists of actions taken byother members of the group and exists in the conspecific (sociocultural) environment ofeach member. The two are not the same and actions are not determined solely byconcepts, although they are certainly influenced by them . There is a parallel here with`structure of ideals' and `structure of action' (Firth, 1959 ; p . 341). Unless this distinctionis made, the processes of sociocultural evolution will be incomprehensible . Third, theverbalization of concepts permits such a vast increase in the complexity of socialbehaviour that `culture' should be reserved for the human case and some other term, like'proto-culture' or 'quasi-culture', used for similar phenomena in other species .

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Organic, co- and dual evolutionary modelsIn their promotion of the organic model, Alexander (1980) and Flinn & Alexander(1982) argue that sociocultural diversity results `from the inclusive fitness maximizingefforts of all humans who have lived' (Flinn & Alexander, 1982 ; p . 397). They allow forthe effects of novel environments that may result in non-adaptive behaviour, but theydiscount the possible existence of evolutionary processes which are specific to sociocul-ture . Co-evolutionary models (Durham, 1976, 1979, 1982 ; Pulliam & Dunford, 1980 ;Lumsden & Wilson, 1981), on the other hand, do concern themselves with such specific-ally sociocultural processes, although they still envisage them as subordinate in the endto the biological criterion of fitness maximizing . I say `in the end' because they allow forconsiderable latitude in sociocultural development .

The most sophisticated of these co-evolutionary models, that devised by Lumsden &Wilson (1981), relates genes to culture through brain structure and epigenetic rules, thelatter being said to influence the choice of cultural units (culturgens) made by individualsand hence to affect the form which particular cultures take . They do this by introducinga bias so that individuals tend to make choices that accord with their biological interests,thus producing usage bias curves which show the probability

that an organism will use one or other of various culturgens within a culturgen category, giventhat it possesses a particular genotype and lives in a particular environment (Lumsden & Wilson,1981 ; p . 384) .

Epigenetic rules, they say, are based in genes but are influenced by all aspects of theenvironment beginning with transcription to RNA and ending with their modulation byculture. Epigenetic rules of some kind may exist, but it is certainly difficult to assess theiroverall significance for sociocultural evolution when they remain largely unidentified .

In the dual evolutionary model to be described below, socioculture has its owncriterion of selection based on prestige and sociocultural fitness, and it has the potentialto break away from organic evolution, although it is related to it through a link betweenprestige and reproductive success (RS) . It is called a dual model because it involves twocomplete, though connected, evolutionary systems with factors which interact at thehuman phenotypic level. The circumstances under which the link between them canbe broken will be explored below . The dual model presupposes the existence of genericevolutionary factors (e.g . information pool, fitness, environmental selection pressuresand isolation) which take specific forms in organic and sociocultural evolution (Hill,1978) .

It will be argued that the evolutionary significance of certain sociocultural phenomenamust be recognized, e .g. articulate speech which permits the emergence of concept pools,sociocultural success which is the driving force behind so much human activity, con-specific (sociocultural) selection pressures and social isolation which promotes extensiveand complex competition between human groups .t The existence of these phenomenawould seem to make some form of sociocultural evolution inevitable .

A word of warning is needed here . This dual evolutionary model must not be confusedwith the `dual inheritance model' of Boyd & Richerson (1985) . The latter differs from theformer in many ways, not least in the explicit rejection of the conspecific environmentas a factor deserving attention and in its failure to distinguish cultural concepts fromovert culture .

t Social isolation (Hill, 1978) falls outside the scope of the present article .

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Concepts in sociocultural evolutionWhat people choose are mainly ideas, actions, people and artefacts, but althoughmost of these are subsumed under the term 'culturgen' (Lumsden & Wilson, 1981)that is too broadly and vaguely defined to be a useful term (Hallpike, 1982) . The ideasthey choose which are relevant here are concepts and higher-level mental constructs likescripts (Schank & Abelson, 1977), mental models (Johnson Laird, 1983) or schemata(Rumelhart et al., 1986). Since a concept involves the recognition of a class, and sincethe existence of any class necessitates the existence of at least one other comparable class,the essence of a concept is less the recognition of X than its differentiation from Y . Inview of this, it was suggested (Hill, 1978) that a concept should be defined as a set ofopposed elements, rather than as a simple entity, a suggestion based on the phonemicanalysis of Troubetzkoy (1949) followed by Levi-Strauss (1963) in his development ofstructural anthropology . This definition has the drawback of being different from whatis normally understood by a concept (Smith & Medin, 1981), so I now propose to retainthe word 'concept' for a single element and to use 'conceptual opposition' to cover theset of opposed elements, as this is less likely to cause confusion . Conceptual oppositionsmay be given a paradigmatic structure as illustrated in Figure 1 . Oppositional structure

Physical

Poison

Virus

Filterable

Not culturable

Cell wall absent

Enzymes absent

Fig . 1 . Etiological concepts of disease. In the list of properties under virus/bacterium,filterable' refers to their passing through a standard bacterial filter and 'culturable' refersto culture in a non-living medium . There are also exceptions, like mycoplasmas, which do

not fit into this deliberately simplified scheme

Environment

Biotic

Bacterium I FungusNon-filterable

Culturable

Cell wall present

Enzymes present

Cause of illness

Genotype

Infection Allergen

Protozoon I Helminth

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was also given to concepts by Kelly (1955) who arranged 'constructs' (percepts plusconcepts) in dichotomies .

The verbalization of concepts facilitated the evolution of multifarious social positionslike father, aunt, elder, priest, soldier, etc . Such a wealth and diversity of positionsis impossible unless they can be identified and referred to, which cannot be done untilthey are named. And the same is true of institutions, which may be defined as organizedsets of social positions oriented towards the achievement of some specific goal or goals .Since diversity of institutions and social positions is essential for sociocultural diversity,the verbalization of concepts is one way in which human language makes that diversitypossible .

Historically, the result of the ability for conceptual communication within groups wasthe emergence of concept pools consisting of the conceptual information held by all themembers of each group, just as a gene pool consists of the genetic information held byall the members of a deme . Concept pool and gene pool are specific manifestations ofa generic information pool (Hill, 1978) . The existence of concept pools means thathuman groups have an additional source of information which members can use to solveadaptational problems, additional, that is, to the information coded in their genes .Concept pools may be divided into active and latent areas, the former consisting of thoseconcepts which are used by individuals to influence their choice of action, and the latterconsisting of concepts which are held but are not used . Some concepts will be found inboth areas, since they will be active in some group members but latent in others . Changesin the proportion of members who use a concept to those in whom it is latent ornon-existent will affect the group's action pattern and hence the course of its evolution .

When a concept is 'fixed' in a group, that is, when its use is common to all adultmembers of the group, there is no question of choice within that group, but whendifferent members use different concepts or conceptual structures as, for example,

Smoking

Unhealthy

Not smoking

Smoking

Not smokingor

Healthy

Healthy

a choice can be made . This choice is not concerned with forming a concept or aconceptual structure, since no one can choose or reject a concept until they have formedit, but with activating it, that is, using it to influence their actions . When a choice is made,it may be cognitive and/or affective and without reference to any other group member,as in the 'direct bias' of Boyd & Richerson (1985) when choice is made by trial and error,but it is more likely that the influence of other group members will be relevant (Bandura,1986) . In this case, the choice will fall according to the dual model on the concepts ofthe potential donor who has most prestige in the opinion of the recipient, within thecontext of the particular choice, 'indirect bias' (Boyd & Richerson, 1985) . The prestigemight be confined to a particular field in which the donor was considered to havesuperior knowledge or ability, although the choice could be affected by his generalprestige . Choice may also be influenced by what most group members do, 'frequency-dependent bias' (Boyd & Richerson, 1985), in which case individuals in the minority maybe said to have less prestige in the eyes of the recipient than those in the majority, at leastwithin the context of the choice .

The flow of ideas from individuals with high prestige to others has been recorded inthe spheres of fashion (Janney, 1941) and language (Labov, 1972, 1980), and Rogers(1983) concludes that high status is a general characteristic of opinion leaders, particularly

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in cases of heterophily, although even with homophilyt I would argue that donors havesome prestige in the eyes of recipients at least in the relevant field .

Choice of action by an individual must not be confused with choice of concepts . Thelatter involves modification of the cultural information which an individual holds inreadiness to structure the situations within which he has to act . The former involves theexpression of this information in an overt form in a specific situation. If one wishes tomake the analogy, the choice of a concept or a conceptual structure corresponds to theinheritance of a gene and the choice of an action corresponds to the transcription andtranslation of that gene . Previously held concepts will often affect the choice of new ones,so that those held by an individual will tend to be consistent with one another, but thatwill not prevent the existence of conflicting concepts, or groups of concepts, which makechoice of action more difficult in some situations or by some individuals . The essentialdistinction between cultural concepts and overt culture, the one consisting of ideas andthe other of actions, has already been stressed and in the choice of action by individualsit becomes manifest .

In addition to actions, individuals may choose people (e .g . as husbands, wives, friends,fellow workers or subordinates) and artefacts . Both people and artefacts are presumedto be chosen because it is thought, or hoped, that they will promote the chooser's goalachievement .

In their choice of concepts, actions and artefacts individuals contribute to socio-cultural evolution by affecting the relative frequency of these entities in their group orsociety, and thus the prevailing behavioural pattern . Some people have a greater thanaverage effect because they influence the choices made by others .

It may be claimed that I have paid too much attention to the cognitive and none tothe affective aspects of human behaviour, but it is cognition that underlies socioculturalevolution and, as Bandura (1986) argues in his social cognitive theory, cognitive factorsare of great importance in all human actions .

The role of concepts in the choice of action may be illustrated with fairly broadstrokes in the treatment of illness, for example . We have a concept of pathogenicmicro-organisms (Fig . 1) which invade the body, whereas the Orokaiva of Papuatraditionally thought that many illnesses were caused through the invasion of thebody by foreign objects, like boar's teeth or pieces of wallaby bone, or by spirits ofthe dead (Williams, 1930) . These different concepts lead logically to very differentmethods of treatment ; among us to the administration of selective poisons intendedto kill the micro-organisms without harming the patient, and among the Orokaivato various methods of expelling the foreign body or spirit of the dead by fumigation ormassage (Williams, 1930) . Since the micro-organisms are alive, it is logical to attempt tokill them, but foreign bodies and spirits of the dead cannot be killed, they can only beexpelled . This is an extreme example, but it does illustrate the way in which actiondepends upon the concepts available to the actors . The logic is impeccable, but theconcepts are very different so the actions are worlds apart . In both cases individualsact, not randomly, but in accordance with particular concepts . More subtle differ-ences are produced by the arrangement of concepts in higher-level mental constructslike scripts (Schank & Abelson, 1977), models (Johnson-Laird, 1983) or schemata(Rumelhart et al ., 1986) .

t'Homophily is the degree to which pairs of individuals who interact are similar in certain attributes, suchas beliefs, education, social status, and the like' (Rogers, 1983 ; p . 18) .

'Heterophily, the mirror opposite of homophily, is defined as the degree to which pairs of individuals whointeract are different in certain attributes' (Rogers, 1983 ; p . 18) .

InnovationSo far, individuals have been treated as accepters or rejecters of the items presented tothem by their culture, but they may also innovate by modifying individual items or bybringing together in novel combinations items which were not previously connected . Itis doubtful if anything is ever created without some stimulus from what already exists .Ideas inevitably come first in innovation, since no change of artefact or action is possiblewithout prior alteration of ideas (cf . Stuart-Fox, 1986), just as no alien artefact or actioncan be adopted by a group until some, at least, of the members have modified their ideasto accommodate it . Serendipity may appear to be an exception, but mental assimilationis neccessary before novel stimuli can even be perceived (Neisser, 1976), and they cannotbe discovered culturally until even more radical mental adjustments have been made .

Although innovation is essential for sociocultural evolution, which could never havebegun without it, it is not enough on its own any more than mutation is enough on itsown to account for organic evolution . Innovations must be favoured by conspecificselection pressures leading to their acceptance by a group before they have any chanceof affecting the course of sociocultural evolution . The nature of these selection pressureswill be outlined below, but in the meantime it may be suggested that innovations willprobably be accepted in small unstructured groups if they promote the goal achievementof a majority of the members and in larger structured groups if they promote the goalachievement of the leaders.

Selection pressuresAn organism's environment may be divided into physical, biotic and conspecificbranches. In humans, selection pressures from all three affect goal achievement, andhence prestige, some reinforcing one another and others conflicting, the final effect onthe individual depending upon the resultant of the various forces . The human conspecificenvironment may be further divided into internal and external sections, where the formeris provided by other members of the group and the latter by other groups (Hill, 1978) .Internal pressures take the form of positive and negative social sanctions (Radcliffe-Brown, 1952) and external pressures are expressed through intergroup competition, forexample, in trade rivalry, raids, feuds and warfare .

Positive social sanctions cover the formal and informal (or organized and diffuse)expression of group approval for a particular individual and increase his prestige bothdirectly and indirectly, the latter by increasing his likelihood of gaining the co-operationof other people and hence of achieving his goals . Negative social sanctions have a reverseeffect on goal achievement and prestige through the expression of group aggressions byimprisonment, corporal punishment, the imposition of fines, excommunication or ban-ishment, for example, which damage physical, economic, social or religious well-being .Informal negative sanctions like derision and verbal disapproval imply a threat ofrejection if the offender does not mend his ways . A reduction of prestige through socialsanctions may be seen as a form of negative feedback warning the individual that if hedoes not modify his behaviour his RS will suffer .

Darwinian and sociocultural fitnessThe fitness of an individual is a measure of its non-random contribution to the futuregene pool, the fitness of a gene is a measure of its displacement of alleles from the

tCompare Gruter & Masters (1986) on ostracism .

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gene pool . Although an individual may increase its fitness either by increasing its ownRS or by reducing the RS of its competitors, RS is the only practicable measure of fitnesswe have. However, RS includes accidental success, whereas fitness is a measure of whatWilliams (1966) called 'design for survival', and it derives from the different effects ofalternative designs . Accidents which are random with respect to design have no bearingon fitness . It is not really possible with this definition to measure the fitness as distinctfrom the RS of individuals but similar difficulties, discussed at length by Lewontin(1974), occur if one wishes to measure the fitness of genes .

Carrying this over to sociocultural evolution, the sociocultural fitness of an individual(Hill, 1984a) is a measure of the extent to which his concepts or higher-level mentalconstructs invade the active area of the concept pool and the fitness of a concept is ameasure of its displacement of rival concepts from the active area [cf . Cavalli-Sforza &Feldman (1981) who speak of second-order organisms] . The fitness of individuals dependsupon the 'design for success' of concepts, the success referred to being the goal achievementof those who use the concepts to guide their actions . Since goal achievement tends tobring sociocultural success, or prestige, which in its turn helps an individual to spread hisconcepts, this completes the evolutionary cycle . There is a difference from organic evolu-tion, however, in that RS is synonymous with the spread of an individual's genes, whereassociocultural success is not synonymous with the spread of concepts . It makes it morelikely, particularly since individuals who are concerned to succeed will tend to adopt theconcepts of those who have succeeded (unless they are innovators), but the connectionis not inevitable as it is with genes . This means that the sociocultural evolutionary cycleis rather weaker than the organic cycle, but it does not invalidate the cycle itself.

The central tenet of this model is the statement that in a given environment conceptswhich promote goal achievement, and hence prestige, more effectively will spread at theexpense of those which do so less effectively, just as the central tenet of the neo-Darwinian theory of organic evolution is the statement that in a given environmentalleles which are more effective in promoting RS will spread at the expense of those whichare less effective .

It follows from this that the course of sociocultural evolution in any society willdepend upon the particular attributes which engender prestige in that society, since thesewill be the areas where innovation is most likely to be rewarded by an increase in prestigebecause these are the areas where the major goals of the members are situated . The widerthe range of attributes which bring prestige in a given society, the more growing pointsits culture will have .

Since socioculture could never have emerged if it did not promote the Darwinianfitness of the individuals bearing it, there must be, or have been, a connection betweenorganic and sociocultural evolution . It has been argued elsewhere (Hill, 1984a, h) thatthe link is prestige, or more precisely the particular attributes which engender it . It wasshown by Hill (1984b) that in several subsistence societies these attributes specificallypromoted RS. It is suggested here that this was originally true of all primitive humansocieties so that sociocultural and organic evolution would have been congruent, sincethe course of the former in a given society depends upon the attributes which engenderprestige and the course of the latter in a given population depends upon the attributeswhich promote RS . The circumstances under which this link may be broken will bediscussed in the next section .

Turning now to prestige as such, regardless of what attributes attract it, it hasbeen shown to be positively correlated with RS in many societies (e .g . Chagnon,Flinn & Melancon, 1979 ; Irons, 1979 ; Kaplan & Hill, 1985; Shaikh & Becker, 1985 ;

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Turke & Betzig, 1985 ; Betzig, 1986 ; Borgerhoff Mulder, 1987a, b) . The mechanismswhich may underlie this connection are of some interest . Clearly, when the sameattributes promote both RS and prestige the two will be linked, but this is not the onlyway they may be connected . Two of the major components of RS are access to theopposite sex and survival of self and offspring . High prestige, especially when due towealth, may facilitate early marriage for a man (Nadel, 1942 ; Warner, 1958 ; Marshall,1976; Irons, 1979; Turke & Betzig, 1985), and where polygyny is permitted it may makeit easier for him to acquire more than one wife (Warner, 1958 ; Marshall, 1976 ; Chagnonet al., 1979; Betzig, 1986; Borgehoff Mulder, 1987a, b) . Among women, those who havethe qualities which are highly regarded may obtain a more prestigious husband who willbe able to invest more in her offspring than a less prestigious one. From a study of 190ethnographies, Ford & Beach (1952, p . 86) concluded that in

most societies the physical beauty of the female receives more explicit consideration than does thehandsomeness of the male . The attractiveness of the man usually depends predominantly upon hisskills and prowess .

As regards survival, the wealthier individual may be able to command a greater propor-tion of the available human and material resources than a less prestigious one, thusincreasing the viability of his offspring (Lang, 1946 ; Hollingsworth, 1964 ; Irons, 1979 ;Whitehead, 1987). The robustness of the link between RS and prestige may vary fromsociety to society so that in some of them prestige may have to fall to quite low levelsbefore RS is affected, but in all of them a slight fall in an individual's prestige could beseen as a form of negative feedback which would prevent a real fall in RS if it wereresponded to .

All this suggests that prestige may be a proximate goal leading to the ultimate goal ofRS, in view of which I do not think it is unreasonable to postulate that the avidity withwhich humans seek to increase, or at least to maintain, their prestige will vary fromindividual to individual for genetic as well as environmental (sociocultural) reasons .This propensity to seek prestige, based perhaps on the need to receive attention(Barkow, 1976 ; Hill, 1984a), provides an explicit psychological mechanism mediatingbetween genes and culture which is required by any model claiming to relate them to oneanother (Barkow, 1984) .

The effect of surplus wealthIn many societies, a man may use his wealth to promote his RS either by increasing theviability of his offspring by providing better food and shelter, or by increasing thenumber of his children by obtaining more wives. Since this is a male decision, the latterwill be a frequent choice partly for sociocultural reasons because polygyny is prestigiousand partly for biological reasons because a man can out-reproduce his competitorsthrough the sheer number of his children . In at least two societies (Chagnon et al., 1979 ;Borgerhoff Mulder, 1987a), differential male RS depends entirely upon the number ofwives the men have . However, as more wealth becomes available some of it may be usedto acquire more or better food or more adequate shelter, so that differential survival hassome effect on RS (Irons, 1979) . But there comes a point beyond which further expen-diture on food, shelter or clothing will have no additional effect on survival, and indeedoverfeeding will have a deleterious effect on the survival of self and overprotection fromthe environment may have a deleterious effect on the survival of offspring (Hill, 1984b) .It is true that in our society an advanced medical technology means that much may be

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invested in recovery from some forms of ill health, but this does not apply to lesstechnologically advanced societies, and even in our own its relevance is confined to quitea small proportion of the population . For these reasons, it will be ignored in the presentcontext.

Relatively rich individuals in some societies may have more wealth than they need forthe adequate nurture of themselves and their immediate existing familes . I refer to thisas surplus wealth . The existence of surplus wealth is not confined to industrial societies,nor even to agricultural societies, for it may be found among hunter-gatherers ifthey inhabit a rich environment like that of the Pacific coast of Canada where theIndians were able under their traditional system to accumulate a considerable surplus(Drucker, 1965) . Indeed, some agricultural societies may be much worse off, living barelyabove the subsistence level like the Tallensi of West Africa (Fortes, 1945, 1949) or theBemba of Central Africa (Richards, 1939) .

Surplus wealth may be seen in terms of available time . In circumstances where it ispossible to accumulate a surplus, a man need work for only part of the time to supplythe biological needs of himself and his dependents . What, then, should he do with therest of his time? He could devote it to the accumulation of more traditional wealth, whichin societies where polgyny is allowed could be used to acquire additional wives and thusto increase RS . Or he could devote it to the acquisition of novel luxury goods or toactivities which have no bearing on survival .

If the richest and most prestigious individual or small kin group in a communitydevoted time to the acquisition of novel luxury goods or to activities unrelated tosurvival, these goods and activities would themselves come to be prestigious in the eyesof less wealthy members of the community, simply because they were introduced by ahighly prestigious family . This means that if the less wealthy were not to lose prestige,they would have to devote as much time as they could spare to engaging in these pursuitsor acquiring the novel luxury goods . Since prestige would remain essential for access tothe opposite sex, the ultimate result of the innovations would be to increase the cost ofRS without specifically increasing bride price so that the less wealthy would find it moredifficult to compete. It might pay the innovating kin group genetically, under certainconditions, to raise the cost of RS for their competitors in this way rather than toincrease their own RS still further. It would not pay individuals or kin groups who hadno surplus to adopt this strategy because they would have to divert resources away frominvestment in their own survival and that of their existing families . It would be sensibleonly when optimum investment was assured in existing dependents and their replace-ments, should they die . Also, it would pay genetically only when the mean number ofviable children per adult in the innovating kin group was greater than the mean per adultfor the competing kin groups together, and when the total number of adults in theaffected groups was considerably larger than the number in the innovating group .

The separation of access to the opposite sex from survival of self and offspring inthis way follows the line Darwin drew between sexual and natural selection, and therelevance of sexual selection will be discussed below .

Why should leading members of the less rich kin groups not ignore the new activitiesand carry on regardless? First, genetic and sociocultural interactants in their develop-mental systems (Oyama's 1985 phraseology) would reinforce one another to give astrong drive towards the proximate goal of prestige, at least in those who achievedleadership or were concerned to achieve it . Second, if some of them failed to engage inthe new activities, they would risk being left out of the community altogether, whichwould entail a disastrous fall in their inclusive fitness, if not in their personal fitness,

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since they would have sons, daughters or other near relatives who needed to marry . Onlyif they all formed an alliance against the richest kin group would it be possible to ignorethe new activities, and an alliance would be unlikely for such a reason when some of themat least would have or would wish to acquire affinal links with the richest kin group . Ihave assumed that the society in question would be classless, but if there were two ethnicclasses as, for example, in the old Ugandan kingdom of Ankole (Oberg, 1940), this wouldapply only to the dominant class .

Two important related questions must now be considered . What behavioural geneswould tend to spread as a result of this strategy and what proximate conscious goalwould be involved? Humans have a great propensity to innovate, presumably stemmingfrom their complex brain structure. This propensity would be adaptive in a changingenvironment, but only innovations that promoted the survival of the innovator or hisclose kin would be favoured by natural selection . With the strategy outlined above,however, this restriction would be greatly relaxed, because the innovators would beprotected from natural selection by the umbrella of surplus wealth, so that alleles whichpromoted innovation would tend to spread more rapidly than hitherto . Innovators neednot belong to a rich kin group provided their innovations are acceptable to and are paidfor by the rich . Such innovations will then promote the survival of the innovators andtheir offspring, although the same innovations might not have done so under the originalsystem .

The conscious goal of the innovating kin group, or of its head, would be the innova-tion itself as a consummatory act, although a direct attempt to increase prestige stillfurther might also be present, since competition for prestige, unlike that for physical andbiotic resources, is open ended . I am certainly not suggesting that there would have beenany deliberate attempt to raise the cost of RS for the innovator's competitors, but simplythat this would have been the result under the specified conditions .

This is one way in which a break between RS and the attributes engendering prestigecould be initiated . There may well be others, like that proposed by Irons (1983) depend-ing upon a human propensity to value the quality of life, but they will not be pursuedhere. I doubt whether runaway processes could affect this on their own, as suggested byBoyd & Richerson (1985) . Once a prestige system unrelated to survival has been set up,it is virtually impossible to escape from it . It may certainly be modified, or exaggerated,by further innovation but only with the agreement of those with the greatest surpluswealth, and it will not pay them genetically to return to the original system . The cost ofraising children may increase but this is irrelevant provided the richest members of thecommunity can still command surplus wealth . Genetically, such a system is evolutionarilystable because no mutation reducing interest in prestige could invade, since any reduc-tion in this would reduce the inclusive fitness of the bearer because prestige as such is stillnecessary for RS, although the form it takes is no longer germane .

The relevance of sexual selection must now be considered . Of the three main com-ponents of RS, survival and fecundity are subject to natural selection, but access to theopposite sex is subject to sexual selection, and there can be little doubt that under certainconditions female mate choice can promote the spread of male traits which have adetrimental effect on viability, without evolution being able to extricate a species fromthe consequent maladaptive tendencies (Lande, 1981 ; Kirkpatrick, 1982; Arnold, 1983 ;O'Donald, 1983) .

In many human societies, female mate choice is exercised by the female's close malerelatives, and whenever `female choice' is mentioned below this should be borne in mind,but that does not alter its evolutionary significance . There is no reason why the attributes

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which attract prestige among humans should not play the same part in this context asthe traits that females of other species find sexually attractive, but human males mustinvest much more than their gametes if the viability of their offspring is not to suffer, andthis prevents female choice in subsistence societies from forcing male behaviouralphenotypes away from the naturally selected optimum .

However, the accumulation of surplus wealth changes this by reducing the impact ofnatural selection on viability, and in the presence of a continuing surplus the attributesthat attract prestige may increase in number and variety regardless of any effect theyhave on the viability of individual males or their (future) offspring . For although femalemate choice should always favour wealth, in the presence of what I have called surpluswealth other attributes which are neutral with respect to survival may provide the reasonfor the choice . If the model proposed here is correct, the course of sociocultural evolutionmust always be correlated with female mate choice .

The human case is complicated by male choice because men are only prepared toinvest in the offspring of the women they choose . Male and female choice are notsymmetrical, however, because the disposal of wealth is usually a male prerogative sothat the attributes which bring prestige in women have continued to be mainly relatedto RS even in the presence of surplus wealth. Physical appearance is important (Ford &Beach, 1952), as is wealth, although in some societies the former may be overshadowedby the practical abilities needed for the successful upbringing of children, and in modernindustrial societies other attributes often play a part . Male-male competition is reallyonly a side-effect of striving for prestige and is not important in its own right since it doesnot automatically ensure female choice . This takes no account of marriage by capture(Lowie, 1950; Piddington, 1952), but that was often a `put up job', and elsewhere itwould be better regarded as an aspect of warfare .

Clearly, the system outlined above would affect the course of sociocultural evolution,and particularly the growth of sociocultural diversity, by opening avenues previouslyclosed by natural selection, but how has it affected the behaviour of individuals? Underthe original system, those who accumulated wealth could invest it only in RS, but underthe new system they can invest in prestige separately from RS, so that for all but therichest this produces a conflict .

The resolution of this conflict will be affected by numerous factors, varying from societyto society, of which polygyny and efficient contraception are two important examples . Butjust as much to the point are the relative strengths of the drives to increase prestige andto increase fecundity. Since prestige originally promoted RS, there is no genetic reasonwhy the latter should be stronger than the former, but both will always be greatly affectedby sociocultural pressures which will differ between societies, and in a class society maydiffer between classes. Large families may be prestigious, as among the Chinese (Lang,1946), or they may be the reverse because they are considered to be characteristic of an out-group. An ambitious man who indulges in conspicuous consumption may be admired orhe may be denigrated as a social climber . Since the number of grandchildren is a measureof RS, the outcome may also depend upon the prevailing type of inheritance, whetheror not there is primogeniture or entail and whether status can be inherited, and upon thenature and abundance of inheritable wealth . But apart from these societal differences,individuals will exhibit considerable flexibility in their resolution of the conflict, so thatsome will prefer to increase their fecundity rather than their prestige . All these things aregoing to affect the statistical relationship between prestige and RS in a wealthy popula-tion, thus making it very unlikely that societies will all show the same pattern, except thatthe very richest may be expected to have a high reading on both parameters .

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It is worth noting, however, that in societies where, for one reason or another, thegreat majority of individuals have as many children as they can afford, sociocultural andreproductive success should tend to be positively correlated and because most people areattempting to increase their RS the population level will rise rapidly . On the other hand,in societies where most individuals keep their families below the level they can afford,there may be no real correlation between sociocultural success and RS, and also becauseof this restraint on fecundity the population level will rise more slowly . This predictionaccords with the findings of Vining (1986) who concludes that sociocultural and repro-ductive success are positively correlated when the rate of population increase rises butnot when it falls .

Cavalli-Sforza & Feldman (1981) and Boyd & Richerson (1985) have both demon-strated mathematically that under certain circumstances cultural selection can counter-act negative natural selection and maintain maladaptive cultural traits in a population,but these are special cases, whereas the hypothesis outlined here has a more generalapplication .

DiscussionAre there any tests or observations which would distinguish between the dual modeldescribed here and the co- and organic evolutionary models? The crucial area is the effectof surplus wealth, for if it can be shown on further study that it would never make geneticsense under realistic conditions for a group of prestigious kinsmen to divert their surpluswealth into non-reproductive activity in order to force their competitors to do likewise,the dual model as described here would be undermined . But if it can be shown byinvestigation in existing affluent societies that a real conflict can exist for all but thewealthiest members between the goals of prestige and RS, this would strengthen the dualmodel and weaken the others, since if RS were the sole ultimate goal for all individualsno other goal could conflict with it . The existence of a real conflict with RS means thatthere must be another equivalent goal, which the organic and co-evolutionary models donot admit. The dual model also explains why there should be a positive correlationbetween sociocultural and reproductive success in subsistence societies but not necess-arily in affluent societies . This is not explained by the other models .

In the case of societies with subsistence economies, the dual model is in substantialagreement with the others as far as the ultimate goal is concerned, but it differs from theorganic model in specifying the existence of special evolutionary factors and processeswhich are peculiar to socioculture and from co-evolutionary models in the precise natureof these processes and factors . The weakness of the organic model lies in its disregardof the evolutionary significance of certain sociocultural phenomena, articulate speech,sociocultural success and social isolation . Flinn & Alexander (1982) are not wrong tostress inclusive fitness, but to leave it at that is to shelve the problem not to solve it .

It is difficult to say what test or observation could falsify the co-evolutionary modelof Lumsden & Wilson (1981) because their basic terms `culturgen' and `epigenetic rule'are so broadly defined that it is difficult to conceive of any sociocultural entity or processwhich would not be covered by one or other of them . Also, they do not pay sufficientattention to prestige and they fail to appreciate the role of female mate choice .

With regard to the human propensity to seek, or at least to maintain, prestige, I donot claim that everyone is equally avid for it, some indeed hardly appear to be botheredat all, but such variation is what one expects in polygenic characters, especially when theenvironment is variable. However, it is those who seek it most actively who have the

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greatest effect on the course of sociocultural evolution and on the development ofsociocultural diversity and who are, therefore, of most interest here . If it can be demon-strated that this is not so, or that the quest for prestige is not an important source ofhuman motivation, my hypothesis would be falsified, as it would also if it could be shownthat there is no connection between prestige and the spread of concepts within therelevant field . Low prestige in other fields is not germane . A man's ignorance of musicor literature or his not belonging to an illustrious family will not prevent him gaininghigh prestige as a scientist or a politician, for example .

Seeking to increase one's prestige is only one possible human strategy for promotingRS; another, probably more common, is the maintenance of an existing level of prestige,with the exhibition of altruism as one of its tactics (Hill, 1984a). If these are indeedalternative strategies, group leaders should tend to be less altruistic than the rank andfile, at least where leadership is achieved rather than ascribed .

AcknowledgementIt is a pleasure to thank Monique Borgerhoff Mulder for her comments and discussionon an earlier draft .

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