The Sexual Competition Hypothesis For Eating Disorders

Riadh T. Abed, MBChB, DPM, MRCPsych.

Psychiatrist, Department of Psychiatry, Rotherham District General Hospital, Moorgate Road, Rotherham S60 2UD, United Kingdom, and Honorary Clinical Lecturer, University of Sheffield. Fax: 114 2507651; E-mail: abed@globalnet.co.uk

Acknowledgements: I am grateful to Dr. R.L. Palmer and Dr. K. de Pauw for reading and commenting on earlier versions of this paper. I also wish to thank Professor P. Gilbert Associate Editor and the two anonymous referees for their critical review of the paper and for offering valuable advice.

The Sexual Competition Hypothesis For Eating Disorders

A hypothesis is presented for eating disorders, based on Darwinian theory, that contends that these syndromes together with the phenomenon of the pursuit of thinness are manifestations of female intrasexual competition. It is suggested that eating disorders originate in the human female�s psychological adaptation of concern about physical attractiveness which is an important component of female �mate attraction� and �mate retention� strategies. It is argued that present day environment of Western countries presents a range of conditions which have led to the over-activation or the disruption of the archaic female sexual strategy of maximising �mate value�. The present hypothesis deals with the ultimate level of causation and is therefore compatible with a range of theories of proximate causation. Although the present hypothesis is not directly testable, it makes predictions that are testable and refutable. Finally it is suggested that the sexual competition hypothesis has more explanatory power than existing evolutionary theories of eating disorders.

Evolutionary science has made few inroads into psychiatry despite the fact that over 130 years have passed since Darwin�s Origin appeared. While a number of attempts have been made at the reformulation of the concept of mental illness using evolutionary theory (see Price, 1967; Demaret, 1991a&b; Wakefield, 1993; Nesse & Williams, 1995; Stevens& Price, 1996; Baron-Cohen, 1997), and some interesting hypotheses have been suggested in the area of affective disorders (see Gilbert,1992; Price, Sloman & Gardner et al 1994), schizophrenia (see Crow,1995) and anxiety disorders ( see Beck, Emery & Greenberg, 1985; Marks, 1987) these remain outside the mainstream of current psychiatric thought.

Evolutionary theory deals with questions of ultimate causation; that is the adaptive function of a certain trait or organ in the ancestral environment. However, evolutionary hypotheses are compatible and consistent with layers of proximate causation. Such proximate explanations will focus on biochemical, physiological, developmental, social and other immediate causes for the expression of a particular characteristic (Symons,1987; Thornhill & Thornhill, 1987).

The hypothesis on eating disorders presented here is derived from the evolutionary theory of human sexuality. The present hypothesis is based upon the assumption that, besides shaping anatomical systems, selection also designs psychological and behavioural adaptations that are just as important for the organisms survival and reproductive success (Lorenz, 1937; Dawkins,1982).

Hypothesis:

The present hypothesis is based upon the following assumptions:

1. In the ancestral environment, the female shape was a generally reliable indicator of the female�s reproductive history and hence her future reproductive potential.

2. The female nubile hour-glass shape is the product of sexual selection and represents the most desirable visual cue for males. In addition to the hour-glass appearance the hallmark of the nubile shape is its relative thinness compared to older females.

3. For the first time in human history mating behaviour has been decoupled from reproduction. This has been associated with a progressive decline in fertility in industrialised countries. Therefore progressively older females have been able to retain or recreate the nubile shape.

4. A range of factors within the environment of western developed countries present conditions that heighten female intrasexual competition for high quality long-term mates.

Therefore it is hypothesised that:

a) The pursuit of (relative) thinness among females in developed countries is a manifestation of the process of female intrasexual competition in an environment that contains a large number of pseudo-nubile females.

b) Late onset eating disorder (bulimia nervosa) represents a process of �runaway� female intrasexual competition. It is contended that the late onset disorder arises through the reactivation of the �nubile program� beyond the age of nubility as a strategy of increasing mate value.

c) Early-onset eating disorder (anorexia nervosa) is a developmental disorder that originates in female intrasexual competition whereby the nubile shape is set at an abnormally thin level in response to the novel stimulus of pseudo-nubile females within the young female�s environment at a critical stage in her development (in addition to other, presently, unknown factors).

The following sections of this article will present arguments and evidence in support of the above assumptions and hypotheses. Other evolutionary theories for eating disorders will be briefly reviewed and will be contrasted with the present hypothesis and a number of predictions based upon this hypothesis will be made. Figure 1 presents a brief outline of the reasoning behind the present hypothesis.

Evolutionary theory of human sexuality and human sexual strategy:

Natural selection and sexual selection are the two main processes that drive evolution (Darwin, 1871). The process of sexual selection occurs through both intra and inter-sexual competition.

Trivers (1972) has demonstrated that male and female sexual strategies can be predicted from the minimum investment each sex can make in the resulting offspring. He predicted that the higher investing sex will demonstrate caution and high selectivity in mating strategies while the lower investing sex will demonstrate sexual strategies which are less discriminating.

In mammals, including humans, the minimum male investment is trivial compared to that of the female who produces biologically expensive ova, undergoes a long period gestation and lactation and in the case of humans a prolonged period of care during infancy and childhood. Therefore, females are a limiting reproductive resource for which males will compete among themselves (i.e. a resource in constant short supply).

However, human male parental investment is high compared to other primates and to mammals generally. This takes the form of long term investment in a female and her offspring through the provision of resources and protection; therefore selective pressures have existed that favour a degree of discrimination in mating behaviour when long-term sexual strategies are adopted (Buss and Schmitt,1993).

Nevertheless it remains true that copulating with many females can increase a male�s reproductive fitness whereas the reverse is not necessarily true for females. This means that a dual mating strategy of faithfulness and philandering can maximise a human male�s fitness (see Dawkins, 1989).

The sperm competition theory suggests that the human female can also enhance her fitness through pursuing a carefully balanced combination of short and long-term mating strategies (see Baker & Bellis, 1995; Baker, 1996). Such a mixed strategy can serve the dual aims of obtaining �good genes� while at the same time ensuring continuing parental investment of a long-term mate. However, as the female makes an asymmetrically high level of investment in any possible conception, it remains in her interest to show greater selectivity in mate choice compared to males.

It follows that males will need to expend much energy competing with other males for sexual access to reproductively active females. Human females on the other hand do not need to expend much time or energy competing for sexual access to males due to the trivial cost a single copulation represents to any given male. However, females do compete actively and vigorously with other females for male commitment (i.e. for long-term mateships) which, by its nature, is in limited supply. Therefore, selection has favoured female strategies designed to test and elicit long term commitment, reliability and parenting skills in men in addition to a sensitivity to characteristics related to high socio-economic status (Buss, 1989).

Such distinct selection pressures that males and females have undergone over countless generations explains why evolutionary theory predicts that males and females will have developed distinct sexual strategies. Furthermore, the prediction of sex differences in the domains closely linked to sex and mating have been consistently supported by empirical findings e.g. sex differences in jealousy (see Archer,1996; Buss, 1995; Buss, Larson, Westen & Semmelroth, 1992).

Our understanding of the processes of mate attraction and mate selection has been significantly advanced by the introduction of the concept of �mate value�(see Symons,1987; Ellis,1992). This concept is based upon the assumption that individual members of a given species are not equally valuable to members of the opposite sex as mates. It follows that as males and females differ in the traits that increase their mate value and therefore render them attractive to the other sex, the sexes should diverge in their mate selection and mate attraction strategies.

According to this view female mate value is highly correlated with her reproductive potential and hence males will be sensitive to visual evidence of youth and good health both of which are highly relevant to female fertility (Buss, 1987).

Male mate value, however, is dependent more upon ability and willingness to provide protection and material resources. However, attributes such as economic status, dominance and willingness to invest in offspring, which increase male �mate value� do not lend themselves to quick and reliable assessment through visual cues and therefore selection has favoured different mate selection and mate attraction strategies for males and females who are in the �mate market�.

Thus, it is assumed that males and females not only show distinct strategies for mate attraction but also show a gender specific pattern of response to sexually meaningful cues. Gender differences in patterns of sexual arousal have been demonstrated in experiments where both men and women were exposed to erotic visual stimuli. For example when both men and women were shown films of males or females masturbating, the men were aroused by the females but not by the males. The female subjects were less aroused than the male subjects but showed a similar arousal by the female film as they were by the film of the male (Mosher & Abramson,1977; Abramson & Mosher,1979). Steinman, Wincze, Sakheim, Barlow & Mavissakalian (1981) conducted a more elaborate experiment involving films depicting hetrosexual activity, male homosexual activity, lesbianism and group sex. They obtained both psychological and physiological data from all their male and female subjects. The male subjects were aroused in decreasing order by group sex, lesbian and heterosexual films. The female subjects were aroused in decreasing order by the heterosexual, lesbian and group films but there was less agreement in females between the physiological and self-report data. Neither the males nor the females were aroused by the male homosexual film. Thus while the males were consistently aroused by female nudity and sexual activity of any kind females did not show a similar response to male nudity and sexual activity (detailed discussion of this and other related data can be found in Symons, 1987).

Further data on gender differences in sexual strategy can be found in surveys on mate preferences in males and females. In a review by Buss (1987) of 7 major studies that had been conducted between 1945 and 1986 in the United States, it was consistently found that physical attractiveness remained the most valued attribute in a mate by males while a combination of social status, earning capacity, material wealth and ambition were the most desired attributes in mates by females. Such consistency in mate selection criteria over the 40 years in question is in stark contrast to the considerable change in social norms in many other areas of life in most Western countries.

The concept of Environment of Evolutionary Adaptiveness:

Natural selection designs adaptations that are suited to particular environments. Therefore, the anatomical, behavioural and psychological structures that evolved to solve the particular set of problems posed by a particular environment will be adaptive to that environment but not necessarily to any other.

Of course the ancestral environment was never a single entity and constituted a multitude of environments each with its own distinctive characteristics. Furthermore, the selective pressures that formed the human psyche may have been influenced as much by cultural factors in the Pleistocene as by the problems of hunting and gathering (see Turke,1990).

Nevertheless, it is widely accepted that hominids and humans evolved during the Pleistocene in an environment that has long since disappeared.

Most hunter gatherer societies have certain characteristics in common. Humans lived in small bands of largely related individuals. They subsisted through hunting and gathering and human mateships were based upon a clear division of labour by sex. Hunting and fighting were recognised male roles while gathering and child care were the preserve of females. Although it is likely that most mateships were monogamous, studies of hunter-gatherers in recent times show that polygyny was permitted in most such societies and indeed in most agricultural societies. The ethnographic record, for example, shows that 708 out of 849 human societies permitted polygynous marriages while only 4 permitted polyandry (Murdock,1967). However, polygyny was practised by relatively few men of high standing who made a disproportionate contribution to the gene pool through having several times more children than the average male (see Chagnon,1969).

The present day large city in industrialised countries provides both a physical and social environment far removed from the ancestral environment of the Pleistocene and human psychological adaptations that evolved ancestrally may not be currently adaptive. For example, behavioural strategies that would have been highly adaptive in the ancestral environment, such as male aggressiveness and violence, can prove to be quite maladaptive and eventually self-destructive in present day state societies (Symons, 1990). We were certainly genetically adapted to the past but we are not necessarily adapted to either the present or to the future (Tooby & Cosmides,1990 ).

The evolution of the female shape:

Human males and females have distinctive body shapes. The adult male is typically 8% taller and 20% heavier than a similarly aged female and the sexes show different patterns of fat distribution (Brown & Konner, 1987).

The human female develops a distinctive fat distribution following puberty that results in the hour-glass or so called wasp-waist shape typical of nubile women.

It is argued that during the Pleistocene most women of reproductive age were either pregnant or lactating and therefore temporarily infertile. Therefore, it has been suggested that, to avoid rearing another man�s offspring men must have developed an aversion to even a slight thickening of the waist (Ridley,1993). Such aversion would be expected to be directed towards novel females (i.e. a female who is not already the male�s consort) and should be particularly relevant to the male�s choice of a long term mate.

Experiments using line drawings of female figures where the ratio of the hips to the waist was subtly altered showed an unerring preference for the thinnest waisted pictures. However, the same experiments showed that a heavy woman with a low waist to hip ratio was preferred to a thin woman with a high ratio (Singh, 1993 & 1994a). The most attractive female waist-to-hip ratio (WHR) was judged to be 0.7-0.8 both by males and females (Singh, 1994b). Similar values for the attractive WHR in females were recorded by young American black men and women thus contradicting claims that black youth find overweight female figures as desirable and attractive (Singh, 1994c). It is of interest that, although the slimmest figures with the WHR within the female range (0.7-0.8) were not considered the most attractive, the slim figures were consistently considered to be the most youthful.

The study of male physical attractiveness showed that males with a low WHR were considered undesirable both as short and long terms mates (Singh,1995). Thus the gender specificity and consistency of the ideally attractive WHR suggests that such a preference is a human species-specific trait that is fundamental to mate attraction.

Therefore I argue that the nubile female hour-glass shape was designed by sexual selection through the differential reproductive success of females who possessed this trait compared to those who did not.

The case for �runaway� female intrasexual competition:

The present hypothesis suggests that what we now call eating disorders are a manifestation of �runaway� female intrasexual competition. This process is not to be confused with the phenomenon of �runaway sexual selection� (Fisher, 1930) that entails the preferential survival of certain traits (genes) due to their attractiveness to the other sex.

The process of �runaway female intrasexual competition� on the other hand is a phenomenon that entails phenotypic change as a result of environmental factors.

Reproductive potential (RP) in females is primarily a function of age and RP falls progressively with advancing years down to zero at menopause. A female�s maximum RP occurs 3-4 years following puberty and this coincides with the time the nubile female body takes its full shape. Thus the newly nubile female will have a higher RP than at any other time during her lifetime and it would follow that males would be selected to be particularly attracted to females who display the nubile shape. It has indeed been suggested that across cultures the female shape found to be most sexually attractive is that of the nubile female (Brown & Konner, 1987).

It is of interest that anorectic girls by the time they reach the clinic construe their destiny totally in terms of their shape (Crisp,1980). Furthermore Crisp (1980) has noted that anorectics quite often become sensitive about the fat around their waist and that sexual adventures or remarks such as �You are beginning to look pregnant� are common precipitants.

I suggest that the hallmark of the nubile female figure in the ancestral environment was its relative �thinness� compared to that of older women who would have undergone a range of changes in their bodily appearance which include stretched abdominal muscles and skin and an accumulation of fatty tissue as a result of the inevitable cycles of child-bearing, childbirth and lactation. Hence, it is argued that, in the ancestral environment, the female�s shape provided a generally accurate indicator of her reproductive history.

Thus it is argued that in the ancestral environment the short-lived nubile relatively slender hour-glass shape was the most reliable visual cue for maximum female reproductive potential (i.e. the maximum possible number of future years of active reproductive life).

Hence the visual cue representing this relatively slender nubile shape became a significant and highly effective part of the human female�s strategy for physical attractiveness. Males who developed a �taste� for the visual cue of the nubile female in the ancestral environment (i.e. sought nubile females as long term mates) would have had a reproductive edge through the acquisition of �breeding rights� for the longest possible period of a given female�s reproductive life.

Furthermore, sexual selection would favour females who possessed not only the nubile shape but were also endowed with a psychological adaptation that signals anxiety or alarm if such phenotypic cues are disturbed or threatened e.g. through premature obesity. Such premature obesity would have had the potential of reducing the female�s mate value through a greater resemblance in shape to that of older (i.e. lower RP) females. Therefore, it is suggested that an adaptation involving a behavioural strategy that aims at the preservation or the restoration of the nubile shape (the drive towards thinness) would have given females who possessed it (who were attempting to attract long term mates) a reproductive edge in the ancestral environment.

While female mate value is crucially affected by physical appearance, this was not the only determinant of mate value in females in pre-industrial societies. Other female qualities such as fidelity would have been highly prized in potential long-term mates as this would have increased the male�s confidence in paternity. In addition, the family or kinship group through the investment of energy and resources often significantly increased a given female�s mate value in pre-industrial societies (see further discussion below).

Furthermore, older kinsmen and women would have had a major say in the arrangement of marriages in most pre-industrial societies (Daly & Wilson, 1992; Symons, 1990).

As the nubile shape was relatively short-lived due to the effects of child-bearing, the nubile female would have had only to pay a moderate amount of attention to her figure to differentiate herself from the surrounding older women. The adaptation of the �concern for signs of nubility� may therefore, be more accurately described as the drive towards relative thinness i.e. relative to the other females in the immediate vicinity. This strategy of �concern about signs of nubility� is a form of female intrasexual competition designed by selection to secure the best possible long-term mate through the signalling of high RP. Intrasexual competition may be in the form of mate attraction or mate retention.

Therefore, eating disorders may be described as a state of �runaway� female intrasexual competition whereby the originally adaptive strategy spirals out of control in response to a range of environmental factors to the extent of becoming self-destructive.

It is likely that the human characteristics of language and conscious cognitive processing has greatly facilitated the process of social comparison thus enhancing the destructive potential of this runaway �feedback loop�.

Hence studies have shown that the fear of becoming fat and the striving for thinness continues in anorectic patients even after they become clearly undernourished and emaciated (Muuss, 1985). Furthermore, bulimic patients show an equally powerful striving for thinness which is one of the strongest variables that showed a direct correlation with the severity of the disorder (Vanderheyden & Boland, 1987; Kerr, Skok & McLoughlin, 1991).

The suggestion that eating disorders are the consequence of intrasexual competition between females would be consistent with the clinical observation that the symptoms of the disorder can be highly reactive to the social context and may show complete remission by a change of the socio-cultural setting (Raphael & Lacey, 1992).

Furthermore, it is recognised that the observation that eating disorders are related to concern about body shape and physical attractiveness in not new and has been noted by a number of authors since the nineteenth century (see Parry-Jones & Parry-Jones, 1995).

Clearly, however, only a small minority of females in any population ever display the full features of eating disorders. This can be explained in either of two ways. The first would be through attributing the differences to genetic variability for the trait of �concern about signs of nubility� within the female population or alternatively the differences can be explained entirely on the basis of �phenotypic plasticity� (Wilson, 1994) that allows single genotypes to achieve multiple forms in response to environmental factors. There is some evidence to suggest that genetic factors may be significant in the case of anorexia nervosa as a 50% concordance rate was found among monozygotic twins compared to 10% in dizygotic twins (Garfinkel & Garner, 1982; Holland, Sicotte & Treasure, 1988).

The drive towards thinness: a manifestation of female intrasexual competition?

The novel environment of the modern city in the advanced industrialised countries has produced a set of social and cultural changes that have affected the sexual strategies adopted both by males and females.

The cultural shift in European societies towards a preference for a thinner, slimmer female body form has been noted by a number of authors but there is no agreement as to why this has occurred (Littlewood, 1995). The question is: is this a peculiarly Western phenomenon or is it a common potential trend in any society once a range of identifiable environmental conditions prevail ?

Any hypothesis advanced to explain this phenomenon will need to account for the following: why thinness? why females? and why now?

Both the trend towards thinness amongst females and eating disorders are relatively recent phenomena. Although it is possible that both anorexia and bulimia may have existed much before the nineteenth century (see Van Deth & Vandereycken, 1995), there is wide agreement that their rates have shown a marked increase in the last few decades (see Gordon, 1990). It is likely, therefore, that the causal factors for these conditions are to be found within the range of environmental changes that have occurred in recent times within the advanced industrialised societies (social, economic, cultural and material). However, it is contended that such environmental factors produce their effects through a complex interaction with pre-existing innate biological tendencies within humans living within these societies.

One major factor in the chain of causation of this phenomenon is the declining fertility in Western societies. Fertility has been in a state of decline in some European countries since the 18th century. By the beginning of the twentieth century the trend was evident in most major Western countries (Vining, 1986).

Low fertility appears to have started among high ranking females and gradually spread to females in the middle classes. The trend became particularly clear with the advent of reliable contraception.

Around the same time a number of other major social changes took place within Western societies. These include the increasing status and rising economic and political power of women, a progressive weakening of traditional kinship bonds (see van den Berghe, 1979) and the rise of the ideals of individual autonomy and self-reliance.

The decreased fertility was achieved by women through delayed reproduction and greater spacing of births. This has led to the preservation of the nubile shape in increasing numbers of post-nubile females who have been able to preserve or recreate the nubile shape beyond the age of nubility.

The rise of the phenomenon of the pseudo-nubile female is an ecological novelty that has become widely prevalent with the advent of contraception.

Therefore, in modern industrialised societies, perhaps for the first time in human history mating behaviour has become disconnected from reproduction. Furthermore, the human female has reached a position whereby she is able to regulate her own short and long term mating strategies with minimal legal or social impediments (i.e. minimal control by male kin).

It is of interest that much female intrasexual competition for high quality mates in pre-industrial societies was taken over by the female�s kin (Dickemann, 1981). Therefore such practices as dowries (Dickemann, 1979), female claustration (Weisfeld, 1990), daughter guarding (Flinn, 1988) and even female foot-binding were processes that could only be undertaken by the female�s kin group and some would have required a collective effort. This kin group effort has been shown in studies to enhance the female�s mate value and improve the quality of mate she eventually attains (Flinn, 1988). Such participation in the competitive mating game by the kinship group on behalf of the mating pair has significantly diminished in modern society mainly as a result of the breakdown of extended kinship ties and the rise of the culture of individualism.

Therefore, while females (and males) in modern industrial societies can exercise freer choice in mating decisions, they are also having to compete for long term mates largely through their own individual effort. Thus the burden of competition in the mating game within modern societies, unlike the majority of pre-industrial societies, has had to be increasingly shouldered by the principals with little or no help from their kin.

Hence, the decoupling of mating activity and reproduction, the increased economic and social status of females in Western societies and the breakdown in extended kinship bonds has greatly increased the opportunity for both sexes to adopt short-term mating strategies. One consequence of this may have been a decline in confidence in paternity and a probable reduction in the availability of male parental investment.

Clark (1988), for example, in his study of men in West Africa, found that they equated the loss of economic control of women traders with loss of sexual control thus lending support to the contention by Smuts (1995) that the reduction of female economic dependence on males can be equated by men with loss of control over women�s sexual behaviour.

It has been shown that male parental investment is positively related to paternity confidence in human marriage systems (see van den Berghe, 1979). Therefore, studies of pre-industrial marriage systems have demonstrated that when paternity confidence drops below a certain threshold the male switches his investment to his sister�s children who will guarantee him greater genetic relatedness as is the case in matrilineal societies (van den Berghe, 1979).

These factors may have contributed to the decreased stability of long-term mateships as shown by the dramatic increase in divorce rates in most of the Western world over the past few decades (see Stone, 1990).

Although frequent marital breakdown was common in some pre-industrial societies (see Fisher, 1992) this appears to have been particularly the case in matrilineal societies where paternal investment was minimal or insignificant (see van den Berghe, 1979).

The loosening of marital ties in modern societies may have been accentuated as a result of greater numbers of higher ranking men engaging in serial monogamy (Wright, 1995); a strategy that tends to enhance male fitness as more men than women tend to marry again and they are more likely to sire children from a second wife than a woman is from a second husband (Daly & Wilson, 1983).

One important effect of the decreased stability of long-term mateships in Western societies, is that significant numbers of males and females have to return repeatedly back into the �mate market� and therefore need to retain or to redevelop the traits and characteristics that increase their mate value to maximise their mate attracting potential.

Hence the females who return to the mate market in the modern environment (unlike in the ancestral environment) will have to compete in an environment that contains large numbers of pseudo-nubile females. However, while youth and good health are female attributes that greatly affect female mate value (see Buss, 1987 &1989), the lengths females have to go to in pursuit of the outward signs of these attributes must depend on whom they are competing with. Thus, in an environment where there are large numbers of pseudo-nubile females, female intrasexual competition will involve the display of the ultimate sign of youth i.e. nubility itself. And as nubility carries the highest possible RP for a female, this represents the ultimate weapon in the process of female intrasexual competition.

Furthermore, the competition for long-term mates has to be undertaken largely through the female�s own efforts.

A further factor that is likely to increase female intrasexual competition is that of socially imposed monogamy associated with social stratification which characterises all Western societies. Polygyny tends to reduce female intrasexual competition in stratified societies through having �more room at the top� (see Weisfeld, 1990).

In addition urban living , unlike the ancestral environment, entails living in close proximity to genetic strangers. While the ancestral environment consisted mainly of extended kin where the intrasexual competition would have been tempered by affiliative and even altruistic behaviour, this is not likely to be the case in present day urban environment.

It is, therefore, suggested that the process of female intrasexual competition (mate attraction and mate retention) in such ecological conditions lies at the root of the phenomenon of the drive towards thinness evident in females in Western societies. The ever increasing intensity of the female intrasexual competition would explain the fact that the standard of the desirable female shape has become progressively thinner in recent decades (Polivy, Garner & Garfinkle, 1986).

Female intrasexual competition may also explain the finding in an American sample, of a relationship between thinness of females of similar educational status (measured through the thickness of subcutaneous fat) and being married to men of high socio-economic status (Garn, Sullivan & Hawthorne, 1989). This suggests that the females who pursued this strategy have had greater success in mate attraction/retention.

It is contended that the extreme variant of this process is the �runaway� female intrasexual competition that manifests itself in the form of eating disorders. In other words, the fact that large numbers of post-nubile females in the mate market are displaying the nubile shape (the ultimate sign of youth) increases the risk that certain females will go �one better� thus entering a maladaptive and self-destructive state of �super-nubility� we call eating disorders.

To summarise, therefore, it is contended that declining fertility in Western societies has led to the preservation of the relatively slender nubile shape among increasing numbers of post-nubile women leading to the rise of the novel phenomenon of the pseudo-nubile female. This process has been associated with a process of increasing status and power of females in these societies, an increase in short term mating strategies as well as a loosening of marital ties and disruption of extended kinship bonds. The above factors have led to increased female intrasexual competition for desirable long-term mates which has resulted in the drive towards thinness. The extreme variant of this is what we call eating disorders.

Thus, the present Darwinian formulation would claim to be able to explain the reasons for prevalence of the ideals of thinness within industrialised societies, provide an account of why the ideal relates almost exclusively to females and identifies the main factors in the present day environment that are considered to be instrumental in the rise of this phenomenon.

Early and Late Onset Disorders:

The factors discussed so far in connection to the drive towards thinness such as delayed reproduction and the instability of long-term mateships are likely to be significant factors in late onset eating disorders (mostly bulimia) but not in the early onset type (mostly anorexia).

It is suggested that the early onset eating disorders could be defined as those occurring either during or prior to the time of maximum female reproductive potential. This means, broadly speaking, that early onset eating disorders have their onset before the age of 20 while the late onset disorders would be those that have their onset after this age.

The onset of the �early onset disorders� according to the present definition also coincides roughly with the age of highest risk for anorexia nervosa , which, according to Alford & Boyle (1982) is between the ages of 12-18.

Anorexia nervosa has been considered by some to be a developmental disorder (Gordon, 1990). Such a view would be consistent with the present hypothesis which would contend that the early onset form of eating disorder is a specific disorder of female sexual development which results in the disruption of mate attraction strategy.

It is suggested that the function of the innate female psychological adaptation of the preservation of features of the nubile shape within the ancestral environment was twofold. It was firstly to ensure that the nubile female clearly differentiated herself from the older (lower RP) females and secondly to facilitate the effective competition with other nubile females in the vicinity for the best available long-term mates (through the prominent display of the signs of nubility).

Thus the adaptation would have been designed to work through scanning the other females in the immediate environment and setting the desired shape at a level which is thinner than the older, post-nubile females but broadly similar to the other nubile females. It is contended that at a critical time in the female�s development this desired shape is �set� into a �mental template� through the available visual evidence of the bodily shapes of other surrounding females. The likely critical period for such an event would probably be just before or during the early stages of puberty. Thus the development of mate attraction strategy in females in the modern environment is crucially influenced by a process of social comparison (see Festinger, 1954; Gilbert, Price & Allan, 1995) and therefore their chosen strategy, as in all competitive states, is dependent on what their rivals are doing (see Maynard Smith,1988).

In the present day environment of advanced industrialised countries the immediate environment of pubertal females may include a number of �pseudo-nubile� females i.e. post nubile females who have retained or recreated the nubile shape. This would have the potential of setting the desired shape in the pubertal female at a level which is far below that which would have been the case in the ancestral environment. Thus this adaptation is designed to ensure that the nubile female �goes one better� than the older females in order to mark herself out as having a higher RP. Also it is clear that such a process of �runaway� intrasexual competition has the potential of escalating or spiralling out of control over time as successively thinner post nubile females will be acting as �templates� for pubertal females leading eventually to an increase in the prevalence of the conditions we call eating disorders. This process of �runaway intrasexual competition� within females could also lead to a divergence between the ideals of female attractiveness in males and females. Indeed there is some evidence to suggest that current male and female perception of the ideal female body shape do not, in fact, coincide with males preferring heavier figures (Fallon & Rozin, 1984).

It may be worth adding here that the media are likely to have had a significant effect in the escalation of this phenomenon. In the modern environment, the existing visual stimuli include not only females who are seen in real life but also ones that appear on the television screens, movies and magazines who although are never known or seen in the flesh may well be mistaken for �competitors�. The finding that the drive towards thinness correlated positively with the amount of time young women spent watching soaps and music videos on television (Tiggemann & Pickering, 1996) would be consistent with the present formulation.

It is important to note here that if intrasexual competition is the ultimate cause of both early and late onset eating disorders as this hypothesis contends it would follow that in both conditions the female�s own subjective assessment of her mate value must be judged to be low in comparison with her peers and in need of �enhancement�, hence the drastic action taken to �improve� her shape.

Little is currently known of how individuals develop a subjective appraisal of their own mate value. Nevertheless, the process is likely to be affected by a range of positive and negative experiences at critical stages of a person�s development. Thus it may be that such traumatic experiences as sexual abuse during childhood could lead to, among other things, a permanent damage to the victim�s subjective assessment of mate value. This may offer a potentially testable hypothesis for the finding by some researchers that anorectic patients have a high incidence of being abused in childhood (e.g. Palmer, Oppenheimer, Dignon et al, 1990).

Therefore, according to this hypothesis, the early onset eating disorder is a disorder in the mental �template� the female acquires at a critical stage during her development that defines the desired shape her body should have while the late onset disorders represent a reactivation of the nubile strategy beyond the age of nubility.

It has been suggested that the symptoms of eating disorders exist on a continuum of severity (Nylander,1971; Button & Whitehouse, 1981). Such a view would be consistent with the present hypothesis that suggests that the phenomena of the drive towards thinness and the syndromes of eating disorders all have their origins in the process of female intrasexual competition in the special environment of advanced industrialised countries.

Explanatory power of the model:

The sexual competition hypothesis has considerable explanatory power.

The hypothesis can account for the facts that eating disorders affect mainly young females, and that it declines with advancing age. It also explains why these disorders have arisen within modern industrialised societies.

The present hypothesis can also satisfactorily account for eating disorders having the most lopsided sex ratio known to psychiatry (Gordon, 1990).

For example up to 95% of cases of anorexia nervosa are female (Furnham & Hume-Wright, 1992; Woodside & Kennedy, 1995). The female preponderance in bulimia is even more pronounced. Some studies, for example, have failed to find any male subjects (Cullberg & Engstrom-Lindberg, 1988) while in a recent ten year study in Minnesota females showed an annual incidence 33 times higher than their male counterparts (Soundy, Lucas, Suman & Melton, 1995).

Moreover, the present hypothesis will account for the finding that adolescent girls and young women are at the greatest risk (Alford & Boyle,1982) i.e. females who have yet to bear children.

The sexual competition hypothesis is based upon the contention that the changing norms for female physical attractiveness in modern industrialised have arisen from the interaction of an ancient biological strategy and a novel environment.

There are a range of existing theories that attempt to explain the phenomena of eating disorders. A detailed evaluation of the theories dealing with proximate causation is beyond the scope of the present article, however, some of the main evolutionary theories will be briefly discussed.

Theories of Eating Disorders:

The theories on eating disorders that deal with proximate causation can be classified broadly into biological, psychological and socio-cultural approaches (see Condit, 1990).

The biological approaches have focused on genetic heredity (Holland, Sicotte & Treasure, 1988), and neurochemical mechanisms (Copeland, 1985; Kaye, Gwirtsman, George & Ebert, 1991). The psychological theories include psychoanalytic formulations by Freud, Janet, Bruch and others (see Bruch, 1973), psychobiological formulations (Ploog & Pirke, 1987) and cognitive-behavioural approaches (e.g. Fairburn, 1985; Garner & Bemis, 1985). In addition there are at least five different socio-cultural theories that locate the causation of eating disorders at the familial or social level (see Furham & Hume-Wright, 1992; Orbach, 1979; Orbach, 1986; Hsu, 1989).

In addition there are multifactorial formulations that considers factors from a number of different domains to be aetiologically significant in producing eating disorders (see Garner, 1993).

None of the above theories or mechanisms that deal with proximate causation should, in principle, be incompatible with the present formulation that proposes a hypothesis at the level of ultimate causation.

There are also a number of existing evolutionary theories that deal with ultimate causation of eating disorders.

The existing evolutionary theories for eating disorders are variants of the reproductive suppression theory (see Surbey, 1987; Voland & Voland, 1989). According to this view female mammals possess an adaptation that permit them to postpone reproduction during unfavourable times in the hope of better times to come (Wasser & Barash, 1983; Wasser, 1990). In humans it is suggested that a range of stresses affecting females such as depression or lack of social support are likely to have limited the female�s ability to reproduce and rear offspring within the ancestral environment (Wasser,1990). It is, therefore suggested that females would have developed a sensitivity to such stresses and developed adaptations that restrict reproduction when such conditions prevail.

Voland & Voland (1989) suggest that anorexia nervosa is an emergency strategy that has evolved through selection that comes into play to suppress reproduction when other controls fail. This is achieved through the reduction of the critical fat mass to below the threshold for ovulation. is suggested that certain features within present day environment have led to the increase in the use of this emergency strategy.

There is no reason to doubt that reproductive suppression occurs in the human female as it does in most mammalian species. Furthermore it is entirely plausible that mechanisms such as kin selection and parental manipulation would be implicated in the adaptation of reproductive suppression (Voland & Voland, 1989). The involvement of the mechanism of kin selection implies that reproductive suppression can occur as a form of altruism towards kin. This is similar to the view proposed by Demaret (1991b) who suggested that anorexia may be an analogue to �the helper at the nest� found in a number of species. However, while studies conducted in some pre-industrial societies, have found that daughters born early tend to increase their parents reproductive success (see Turke, 1988), no study to my knowledge has demonstrated increased fertility in parents of anorectic patients.

Moreover, while the reproductive suppression and the reproductive filtering hypotheses would be relevant to anorexia nervosa where ovulation is suppressed, it may not be relevant to other eating disorders or to the general phenomenon of the pursuit of thinness.

The present hypothesis, on the other hand, provides a parsimonious explanation for the pursuit of thinness and for eating disorders through suggesting that these are manifestations of female intrasexual competition. The sexual competition hypothesis would contend that the pursuit of thinness can be (up to a point) an adaptive strategy for mate attraction/mate retention given the ecological factors prevalent within Western societies. However, the present hypothesis would contend that as the sexual competition intensifies the female�s response becomes increasingly maladaptive.

According to the sexual competition model anorexia nervosa is considered to be a developmental disorder that leads to the setting of the nubile shape at an abnormally thin level primarily as a result of the prevalence of the novel stimulus of pseudo-nubile females within the environment. Other vulnerability factors are expected to play a significant role in the causation of this disorder but these factors have not yet been identified. However, one likely candidate is low assessment of subjective mate value which may be caused by a range of adverse experiences during childhood.

Therefore, according to the sexual competition hypothesis for eating disorders the reproductive suppression that occurs in anorexia nervosa is an incidental by-product of the maladaptive runaway intrasexual competition.

It is possible, of course, that anorexia nervosa is entirely separate from other eating disorders and from the phenomena of the pursuit of thinness as the current medical classifications suggest and as would be implicit from the reproductive suppression theory.

However, there are a number of features that most eating disorders have in common. These include the fact that they are most common in females, there is a similarity in their core psychopathology (disturbed body image and the striving for thinness), they are thought to have arisen relatively recently, have all increased in prevalence over recent years and they are thought to have an almost identical geographical distribution.

These common features point to the probability that these disorders share the same underlying aetiology. Therefore, it is suggested that plausible hypotheses that attempt to explain these disorders parsimoniously through a single general mechanism should be favoured.

However, as both theoretical formulations make testable predictions they should ultimately be accepted, rejected or altered depending on empirical findings.

Finally, it is worth noting that these two hypotheses may not be mutually exclusive as the process of reproductive suppression may itself involve female intrasexual competition (or vice versa) (see Wasser & Barash, 1983) and therefore it is possible that both mechanisms may be acting together in certain cases.

It is contended that the sexual competition hypothesis has identified the mechanism that ultimately gives rise to the syndromes of eating disorders.

It is further contended that such identification should facilitate our understanding of these phenomena and would open the way to a range of further hypotheses about proximate causal factors. It is suggested that it would be impossible to have a proper understanding of the dysfunction of any given biological system until the normal functioning of such a system (i.e. the purpose it serves for the organism) is correctly identified (Bolton & Hill, 1996).

Thus, by offering the insight that these disorders stem ultimately from female sexual competition would open the way for areas of research not previously considered relevant. For example, rather than limiting research to the areas of appetite, craving and body image disturbance, attention can also be directed towards the various environmental, developmental and neurobiological factors that play a part in the formation or disruption of the female subjective assessment of �mate value� and the factors involved in the development female mate attraction and mate retention strategies.

Predictions based on the present model:

Prediction number 1: Societies showing high fertility will have low incidence of eating disorders due to the low incidence of pseudo-nubile females.

Prediction number 2: Greater direct involvement of kinship groups in mateship decisions and in intrasexual competition on behalf of their kin within a society will be associated with lower prevalence of eating disorders.

Prediction number 3: The corollary of prediction number 2 is that the prevalence of eating disorders should be positively correlated with high female autonomy and high levels of female economic and political status within society and negatively correlated with societal norms characterised by male domination and institutionalised male control of female sexuality.

Prediction number 4: Factors that lower a female�s subjective assessment of her mate value (during reproductive years) should be associated with higher risk of eating disorders.

Prediction number 5: High stability of long term mateships (low divorce rates) should predict low prevalence of eating disorders.

Prediction number 6: As it is hypothesised that that eating disorders are a manifestation of intrasexual competition for long term mates, it should follow that patients with eating disorders should be less interested in short term mating compared to their peers.

Prediction number 7: It is predicted that women who are exclusive lesbians would have a significantly lower incidence of eating disorders compared to heterosexual females as their mate attraction strategies are quite distinct (Symons,1979).

Prediction number 8: It is predicted that exclusive homosexual men would have a higher incidence of eating disorders than heterosexual men as they are subject to similar mate selection strategies to heterosexual females (Symons,1979). Although the specific adaptation of �concern for signs of nubility� will not apply to males it has been suggested that reliance on visual evidence for mate attraction is similar to the strategy used in heterosexual females. This prediction is consistent with the finding in some of the published series of men with eating disorders reporting rates of homosexuality of about 25% compared with an estimated rate of 6-10% in the general population (see Woodside & Kennedy, 1995). It is also consistent with the suggestion that many male anorectics show explicit homosexual conflicts (Herzog, Norman,Gordon & Pepose, 1984) and that male homosexual students have higher scores on the Eating Disorder Inventory ( Yager, Kurtzman, Landsverk & Weismeier, 1988). In addition there are a number of anecdotal clinical accounts of male anorectics that suggest that they tend to have �severe gender identity problems� (Romeo, 1994).

Prediction number 9: Factors that maintain the female in the �mate searching� , �mate attracting� or perhaps �mate retaining� mode should be associated with a higher incidence of these disorders (e.g. marital and relationship problems, marital breakdown, continued single status etc.). This prediction is consistent with evidence that indicate that patients with eating disorders have a higher rate of problems with intimacy and experience low satisfaction within their marital relationships (Van den Broucke, Vandereycken & Vertommen, 1995). However, it may be argued that the eating disorders are causes and not effects of the relationship difficulties.

The weaknesses of the model:

1. It may be argued that the present hypothesis has attempted to explain the enigma of the pursuit of thinness among females in Western societies by shifting the causation to yet another social phenomenon of unknown cause, namely the declining fertility in Western societies. This is a legitimate challenge that cannot be answered satisfactorily at present .

The decline in fertility in the West has predated contraception (see Kaplan,1993). The lowered fertility in the rich countries of the West and the fact that wealthy people do not appear to be converting their wealth into reproductive success remains an evolutionary puzzle (see Kaplan, 1993; Pérusee, 1993). It is of interest that Burley (1979) had suggested somewhat speculatively that the concealment of ovulation in human females may have evolved to deceive the female herself about the timing of her own ovulation. Such concealment of the time of ovulation would counter the human female�s inclination to refrain from copulating during the fertile period thus avoiding the pain and the dangers (including the high maternal mortality) that were associated with childbirth in the ancestral environment (Daly & Wilson,1983). Thus it may follow that when females have the power to determine their own reproductive strategy they may well go all out for quality at the expense of quanitity.

2. The model does not predict in any detail the proximate causal factors that lead an individual female to develop an eating disorder.

3. It has been suggested that in non-western populations the characteristic fear of fatness and the distorted body image is absent (Lee, 1991; Khandelwal & Saxena, 1991) . However, other studies have argued that the syndrome is similar in different cultures (Mumford, Whitehouse & Choudry, 1992). As the present hypothesis argues that eating disorders arise from a universal female psychological trait of �concern about physical attractiveness�, the absence of concern about physical appearance would pose a serious challenge for the present hypothesis.

4. It may be argued that the existance of male anorectics and bulimics regardless of their low proportion at present is difficult to accommodate within the present hypothesis. Therefore. if it can be shown that the rate of eating disorders in males is similar to that in famales (in any society), this would represent a serious challenge to the sexual competition hypothesis.

Conclusion:

1. Physical attractiveness is a major component in the human female�s mate attraction strategy.

2. Concern and anxiety about physical attractiveness is a female psychological adaptation that has evolved through selection and is an important component of female intrasexual competiton for long-term mates.

3. Genetic and/or phenotypic variation exists within the female population for the psychological trait of �concern about the signs of nubility�.

4.It is argued that the reduced fertility within Western societies has resulted in the retention or the recreation of the nubile shape by an increasing proportion of women beyond the age of nubility thus giving rise to the novel phenomenon of the pseudo-nubile female characterised by the hour-glass shape and its relative thinness. In addition, there have been a range of ecological factors that have tended to intensify female intrasexual competition for long-term mates and has resulted in the progressive tendency towards thinness.

5. The extreme variant of this state is the outcome of runaway female intrasexual competition and is what we call eating disorders.

6. A distinction has been suggested between early and late onset disorder. The early onset disorder (anorexia) is considered a developmental disorder whereby the female nubile shape is set at too thin a level in response to the novel stimulus of the pseudo-nubile female. The late onset disorder (mainly bulimia) arises from the activation of the nubile strategy beyond the age of nubility.

7. This hypothesis gives an account of the ultimate causation of both eating disorders and the pursuit of thinness in females in Western societies. Although it cannot be directly tested and refuted, the predictions the hypothesis makes are testable and refutable.

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