Precis of 'The mating mind: How sexual choice shaped the evolution of human nature’ (published April 2000 by Doubleday in U.S., Heinemann in U.K.)

 

by Geoffrey F. Miller

 

 

Abstract

 

'The mating mind' revives and extends Darwin's suggestion that sexual selection through mate choice was important in human mental evolution – especially the more 'self-expressive' aspects of human behavior, such as art, morality, language, and creativity.  Their 'survival value' has proven elusive, but their adaptive design features suggest they evolved through mutual mate choice, in both sexes, to advertise intelligence, creativity, moral character, and heritable fitness.  The supporting evidence includes human mate preferences, courtship behavior, behavior genetics, psychometrics, and life history patterns.  The theory makes many testable predictions, and sheds new light on human cognition, motivation, communication, sexuality, and culture.

 

Keywords: sexual selection, human evolution, art, language, morality, creativity

 

 

I. Chapter 1: Central Park (US edition)/Exhibition Road (UK edition)

 

1.  The human mind evolved somehow.  Yet it remains unclear what selection pressures favored our large brains, our creative intelligence, and our unique capacities for language, art, music, humor, romantic love, and moral commitment.  Following Herbert Spencer's misleading phrase 'the survival of the fittest', most theorists throughout the last 140 years have tried and failed to identify the 'survival value' of these capacities.

 

2.  'The mating mind' suggests that if one meta-theory doesn't work, we should try another one.  Darwin (1871) argued that sexual selection (for reproduction) is distinct from natural selection (for survival), and that traits inexplicable in terms of  'survival value' can often be explained as ornaments for attracting sexual partners.  The revival of sexual selection theory since about 1980 has confirmed the utility of Darwin's distinction between natural and sexual selection.  Sexual selection often creates an evolutionary positive-feedback loop that is highly sensitive to initial conditions.  It therefore tends to produce extravagant traits that have high costs and complexity, yet these traits are often unique to one species, and absent in closely-related taxa.   By contrast, natural selection for ecological utility tends to produce convergent evolution, where many lineages independently evolve the same, efficient, low-cost solutions to the same environmental problems. 

 

3.  Viewed from a macro-evolutionary perspective, the human brain fits this profile of sexually-selected ornaments: it is unique among living primates, has high metabolic costs and enormous complexity, and its capacities are conspicuously displayed during courtship (especially verbal courtship).  The brain's low level of sexual dimorphism is congruent with evidence that human mate choice is mutual, with males and females almost equally choosy about the mental traits of long-term sexual partners.  Sexual dimorphism is a distinctive outcome of sexual selection, but sexual selection does not always produce dimorphism.

 

4.  Mate choice suffices to explain many distinctive aspects of the human mind, especially those that yield no apparent survival benefits.  Darwin (1871) made the same argument in suggesting that human language and music evolved, like bird song, for courtship.   This view of the mind as a set of courtship adaptations can now be extended and refined in the light of modern sexual selection theory, evolutionary psychology, and evolutionary anthropology.  Although evolutionary psychology recognizes the powerful role of sexual selection in shaping sex differences in motivation, emotion, and cognition, it has neglected the possibility that sexual selection could produce psychological adaptations (such as language and creativity) equally in both sexes. 

 

5.  Many researchers have suggested that human mental evolution was so fast and unusual that it must have been driven by some sort of positive-feedback process.  Other candidates for the positive-feedback process have included gene-culture co-evolution, inter-tribal warfare, and social selection for Machiavellian intelligence.  Sexual selection has been strangely neglected, though it is the best-established, most powerful form of positive-feedback evolution known to biologists.

 

6.  Evolutionary psychology has tended to overlook sexually-selected mental traits because its criteria for recognizing psychological adaptations (e.g. efficieincy, modularity, low heritability, universality across individuals) have been too restrictive (Miller, 2000). Sexual ornaments violate the efficiency criterion because they have high growth, maintenance, and display costs precisely so they can function as reliable fitness indicators according to Zahavi's (1975) handicap principle.  They are only somewhat modular at the genetic, developmental, and metabolic levels, because total modularity would render them totally useless as indicators of general health and fitness.  Ornament quality tends to have moderate to high heritability, because ornaments often evolve to advertise heritable genetic quality.  Likewise, the whole point of sexual ornaments is to amplify apparent individual differences, so they show extreme variability rather than uniformity across individuals.  Unfortunately, some evolutionary psychologists such as Pinker (1997) have dismissed music, art, humor, and general intelligence as non-adaptations because they did not fit the typical profile of survival-selected adaptations --  hardly surprising, if they were sexually selected. 

 

7.  In other respects, 'The mating mind' is standard biological adaptationism, focusing much more on 'what' and 'why' than on 'how', 'when', or 'where'.  It makes very few claims about the geographical location, antiquity, or genetic changes associated with the evolution of our sexually-selected mental traits.  The emphasis, as in most animal behavior research, is on characterizing various adaptations and seeing whether their features are consistent with particular selection pressures hypothesized to have produced them.  The book's theory of mental evolution is more testable than most, because the heritable sexual preferences of our ancestors are probably still manifest in modern mate choice, so they can be assessed as selection pressures independently of the courtship adaptations they are posited to have favored.

 

 

II.  Chapter 2: Darwin's prodigy

 

8.  This chapter reviews the peculiar history of Darwin's theory of sexual selection through mate choice, tracing its rejection by Victorian biologists, its neglect for over a century, and its dramatic revival in the last couple of decades (also see Cronin, 1991).

 

9.  Charles Darwin, like his grandfather Erasmus, recognized that sexual reproduction was central to evolution.  His theory of sexual selection was developed not so much to explain sex differences, but to account for complex ornaments that seem useless for survival, and therefore inexplicable through natural selection.  He suggested that if animals of a species came to prefer a particular trait when choosing sexual partners, that trait would tend to grow in size, complexity, and quality over evolutionary time, even if the trait had high costs in every other domain of evolutionary competition.

 

10.  Darwin (1871) had a sophisticated view of the psychology of mate choice.  He emphasized that even relatively simple nervous systems (e.g. insects, fish, frogs) suffice for mate choice -- but that the more complex an animal's brain, the more intelligent its mate choice could be.  As mental complexity increased, the discriminatory power of mate choice would increase, so sexual selection would command ever greater importance in evolution, reaching its zenith in human evolution.  Darwin noted "He who admits the principle of sexual selection will be led to the remarkable conclusion that the cerebral system not only regulates most of the existing functions of the body, but has indirectly influenced the progressive development [i.e. evolution] of various bodily structures and of certain mental qualities."  He did not attempt a one-way reduction of  psychology to biology, but saw psychology as a driving force in biological evolution.

 

11.  Whereas Darwin's natural selection theory was widely accepted, his idea of sexual selection through mate choice was almost universally rejected by Victorian biologists.  Alfred Wallace was a leading critic, suggesting that most male ornamentation was a developmental side-effect of greater male energy and physiological exuberance.  Wallace's objections led mate choice theory to be viewed for the next hundred years as Darwin's most embarrassing blunder.  This sceptical view of mate choice was reinforced by leading biologists of the early 20th century, including Thomas Hunt Morgan, Julian Huxley, J.B.S. Haldane, and Ernst Mayr.  They combined a group-selectionist, good-of-the-species abhorrence of survival-reducing ornamentation with a Modernist machine aesthetic (derived from the Bauhaus and other puritanical sects of socialism), which viewed ornamentation as morally decadent, economically oppressive, and tasteless.

 

12.  As a result, almost all of 20th century psychology, anthropology, neuroscience, and the humanities developed without recognizing any role for mate choice in human mental evolution.  Instead, Freud's paleolithic fantasies dominated views of prehistoric sexuality, and his theory of excess libido being sublimated into artistic creativity echoed Wallace's surplus-energy arguments for ornamentation.  This bias against mate choice theory began to erode only in the 1970s, leading to a runaway revival of mate choice theory in evolutionary biology, to the point that animal behavior journals are now dominated by experiments on mate choice and sexual competition.  Yet this revival has gone largely unnoticed in mainstream psychology, neuroscience, and the social sciences, which still view 'survival of the fittest' as evolution's bottom line, and which therefore have trouble seeing any evolutionary rationale for those aspects of human nature most concerned with self-ornamentation, display, status, ideology, fashion, and aesthetics.

 

 

III.  Chapter 3: The runaway brain

 

13.  This chapter considers the possibility that runaway sexual selection drove the rapid escalation of brain size characteristic of our lineage, but it ultimately rejects this 'runaway brain' theory as neglecting the role of mutual mate choice, and failing to explain the sexual equality in human brain size and intelligence.

 

14.  Ronald Fisher (1930) suggested that sexual selection through mate choice could lead to a positive-feedback process, in which ever-more-discriminating sexual preferences become genetically linked to ever-more-extravagant sexual displays.  The runaway hypothesis was neglected for fifty years, until biologists developed mathematical models in the early 1980s showing that Fisher was right.   Heritable sexual preferences will tend to become genetically correlated with the sexual ornaments that they favor, and this genetic correlation gives the runaway process its evolutionary momentum.  The direction of runaway is so sensitive to initial evolutionary conditions that it is very hard to predict which sexual ornaments will evolve in a lineage, but once underway, runaway will tend to increase the complexity and magnitude of the favored ornament to extremes.  Runaway's unpredictability can explain why closely related species can differ so dramatically in their ornamentation and courtship behavior.

 

15.  In previous work (Miller, 1993), I suggested that human mental evolution was driven by runaway sexual selection.  If hominid females happened to develop a sexual preference for creative intelligence, then males with more creative intelligence would attract more sexual partners and would produce more offspring.  Those offspring would inherit both the taste for clever courtship and the capacity for producing it.  Over many generations, average creative intelligence in the lineage would increase rapidly, perhaps explaining why brain size tripled in just two million years. 

 

16.  This ‘runaway brain theory’ could explain the speed of encephalization, the rarity of creative intelligence across species, and people’s propensity to show off their creativity and intelligence in sexual courtship.  However, the theory predicts large sex differences in brain size and creative intelligence, and neuroanatomical and psychometric evidence shows these differences are very small.  Male human brains are only about 100 cubic centimeters larger than female brains (after allometrically correcting for body size differences), and there is no apparent sex difference in the g factor that underlies IQ test performance.   Males do account for a large proportion of public cultural displays in every known society, such as the invention and dissemination of art, music, ideologies, religions, philosophies, and science (Miller, 1999).  But this cultural dimorphism might be conflated with Holocene patriarchal cultural traditions.

 

17.  The lack of sex differences in human mental capacities is a strong argument against the runaway brain theory.  This does not imply that sexual selection is irrelevant, only that the choosy-female, displaying-male model assumed by Fisher (1930) is too simple for the human case, in which both sexes are choosy (at least in establishing long-term relationships), and both display their mental traits during courtship.  If sexual selection drove human mental evolution, it must have been a form of sexual selection that could work given mutual mate choice.

 

 

IV.  Chapter 4: A mind fit for mating

 

18.  This chapter introduces the idea of sexual ornaments as costly, reliable indicators of fitness, and argues that our most distinctive mental traits evolved through mutual mate choice as fitness-indicators.

 

19.  Many biologists believe sexual reproduction evolved as a way to minimize the disruption caused by the copying errors and mutations intrinsic to DNA-based reproduction.  By extension, mate choice can be viewed as a method for enhancing this anti-mutation effect, by favoring sexual partners who carry 'good genes' (i.e. genotypes with a low number of deleterious mutations).   Mate choice for good genes will tend to focus on the observable traits of potential mates that best reveal their mutation load.  Therefore, traits that happen to reveal mutation load more accurately than other traits (e.g. through more complex development that requires the interaction of more genes) will tend to be favored in mate choice.  Under sexual selection, such traits will tend to grow ever larger, costlier, and more complex, so they function as ever more reliable indicators of good genes.  Insofar as genetic quality implies expected evolutionary fitness, these sexually-selected traits could be called 'fitness indicators'.

 

20.  Human brains make particularly good fitness indicators because their growth depends on about half the genes in the genome, thereby summarizing a huge amount of information about mutation load.  Brains are also good indicators of nutritional state and general health, because they have such high energetic costs, constituting only 2% of body weight, but consuming over 25% of adult metabolic energy (60% in infancy).  Moreover, the more important brains became during primate evolution (for whatever reason), the more incentive mate choice would have had to focus on specific indicators of brain quality (i.e. mental fitness as distinct from physical fitness). 

 

21.  Fitness indicator theory raises evolutionary-genetic issues about the heritability of fitness.  Throughout the 1980s, many biologists were skeptical about 'good genes' models of sexual selection because such models assumed fitness would remain heritable across many generations.  Yet Fisher's (1930) ‘fundamental theorem of natural selection’ implied that selection should decrease genetic variance, driving disfavoured alleles to extinction, thereby decreasing the heritability of fitness to zero at evolutionary equilibrium.  The 'heritability of fitness' debate continued today, but evidence is accumulating that fitness remains heritable in many species much of the time, probably through a combination of ever-changing selection pressures (e.g. geographic diversity of habitats plus migration, and host-parasite co-evolution), and ever-recurring deleterious mutations.  The remainder of the book takes for granted the heritability of a general ‘fitness factor’ (analogous to, and superordinate to, the g factor) in humans, though much more research needs to be done to demonstrate such a fitness factor (see Miller, in press).  Recent genetic analyses suggest that at least 1.6 harmful new mutations per individual per generation have been arising in our lineage for the last several million years (Eyre-Walker & Keightley, 1999).  This probably exceeds the mutation rate that natural selection could contain in the absence of sexual selection for genetic quality.

 

22.  Fitness indicators also raise reliability issues: what keeps low-fitness individuals from cheating by displaying the high-quality ornament?  Economists working on 'signalling theory' (a branch of game theory) showed in the 1960s that when there are incentives for deception, signals of quality or intention must be costly in order to be reliable.  This point was independently applied to animal signalling, especially sexual ornaments, by Amotz Zahavi (1975), with his 'handicap principle'.   In his view, mate preferences should only favor sexual ornaments that have such high marginal costs that low-fitness pretenders could not afford to display a high-quality form of the ornament.  Sick, starving, inbred peacocks cannot afford to grow large tails, for example, and this makes a large peacock tail as reliable fitness-indicator.  If peacock tail size was uncorrelated with general fitness, peahens would soon lose the sexual preference for such an uninformative ornament.  The handicap principle was vigorously debated for twenty years in biology, in ignorance of the economic game theory work.  Only recently have biologists started to accept that prodigious, survival-reducing waste is a necessary feature of most sexual ornaments, in order to keep them reliable as fitness indicators. 

 

23.  In some cases, however, ornaments can function as direct 'indexes' of fitness, reliably revealing an individual's fitness without imposing high survival costs.  Indexes depend on the existence of some intrinsic genetic or developmental correlation between fitness and ornament quality.  Human intelligence is likely to be a fitness index, whereas art, music, and humor are more likely to be fitness-indicating handicaps (unfortunately, the book did not make this distinction very clear).

 

24.  The book argues that our most distinctive psychological adaptations evolved mostly through mutual sexual selection as fitness indicators.  Of course, these species-unique courtship adaptations are far out-numbered by the psychological adaptations for social intelligence, foraging, predator avoidance, etc. shared with other primates.  Yet we still need some explanation of why small, efficient, ape-sized brains evolved into huge, energy-hungry handicaps spewing out useless luxury behaviors such as flirtatious conversation, music, and art.

 

 

V. Chapter 5: Ornamental genius

 

25.  This chapter examines how psychological biases (including sensory, perceptual, cognitive, and emotional attunement to certain stimuli) influence mate choice and hence the design of sexual ornaments.  It then makes some analogies between courtship and marketing, and between sexual selection and venture capital, and considers the possible interactions during mental evolution between runaway sexual selection, sexual selection for fitness indicators, and psychological biases.

 

26.  Mate choice is mediated by the senses and the mind.  Both include a large number of 'biases' or selective sensitivities that are evolutionarily contingent outcomes of their design details, or of adaptation to certain environmental conditions.   Biologists can sometimes predict which stimuli will excite a perceptual system that evolved to detect certain biologically relevant objects and events, but (as ethologists realized in the 1950s) it is often hard to predict which artificial super-stimuli might excite the system even more.  Many sexual ornaments may have originated as accidental super-stimuli (arising from novel mutations) that just happened to attract the attention of the opposite sex given how their perceptual systems work.  Since the super-stimulus sensitivities are often evolutionarily contingent, the design of the sexual ornaments that they favor may be evolutionarily contingent as well.  This is yet another reason why sexual selection is an unpredictable, diversifying process that rarely happens the same way twice.

 

27.  As Darwin (1871) realized, the psychological biases affecting mate choice are not restricted to low-level perception, but can include propensitites for novelty-seeking and pleasure-seeking.  Given that primates are unusually neophilic and derive pleasure from social interaction, these cognitive and motivational biases may have led mate choice to favor more creative courtship with more social content during hominid evolution.

 

28.  Any given sexual ornament, including the human mind, probably evolved through some combination of runaway sexual selection, sexual selection for fitness indicators, and psychological biases that happened to favor certain details of ornament design. The interaction of these three sexual selection principles also helps to explain the mechanisms of speciation, the proliferation of biodiversity, and the origins of evolutionary innovations.  Sexual selection works like venture capital, extending a line of reproductive credit to potentially useful evolutionary innovations before they show any ecological profitability.  This may help to explain hominid encephalization before significant signs of any cultural or technological progress.  The obsession with survival selection is analogous to the early 20th century corporate obsession with production as opposed to marketing and advertising.  Business had its ‘marketing revolution’ in the last half century; it is time for evolutionary psychology to recognize that creative social behavior is to the opposite sex what products are to consumers.

 

 

VI.  Chapter 6: Courtship in the Pleistocene

 

29.  This chapter reconstructs Pleistocene hominid mating patterns using a variety of evidence, and identifies the opportunities for mate choice to have influenced reproductive success in small groups of hunter-gatherers.  

 

30.  Popular culture images of prehistory sustain the myth of rapacious, unchoosy cave-men and helpless, unchoosy cave-women.  Likewise, early primatology suggested that 'alpha males' attained the 'right to mate' with all females in a group.  More recent studies of human tribal societies and of other primates suggests these views are wrong, and that both sexes exercise mate choice among chimpanzees, bonobos, and tribal humans, which are all characterized by multi-male, multi-female social groups.  Other evidence (ranging from comparative morphology to evolutionary psychology) suggests that hominids were not lifelong monogamists, but followed a pattern of serial social monogamy (with relationships lasting a few days to a few years) augmented by extra-pair copulations and some polygyny.  Rapists would have been ostracized or killed, and these costs would have usually exceeded the reproductive benefits of a single copulation, given concealed ovulation.

 

31.  It remains unclear how much paternal investment there was, as opposed to step-parental investment functioning as a kind of altruistic courtship display.  Data suggesting that women favor self-confident, high-status men may reveal a preference for high heritable fitness, rather than an expectation of long-term paternal investment.  Given that most individuals would have had their first child by their early 20s, and sought additional mates thereafter, there must have been considerable overlap between parenting and courtship -- stories told to children while within earshot of a potential mate may have functioned more as courtship displays than as parental investment.  Conversely, the mate choices of adult females (i.e. mothers) may have been influenced by the preferences of their children regarding which prospective step-father appeared kinder and more entertaining.  Given frequent interaction with kin, mate choice may have also worked as a distributed decision-making process, taking into account the collective wisdom of parents, siblings, and offspring.  Equally, if individuals were choosing for heritable fitness, kin of all ages would have had incentives to advertise their own fitness on behalf of their fertile relatives, giving rise to various collective courtship rituals in which kin groups show off to other kin groups.  Even 'arranged marriages' impose sexual selection, if the parents are using consistent mate choice criteria on behalf of their children (who may inherit the same preferences).

 

32.  A key section discusses a 'fitness matching' model of mutual choice for fitness affordances, based on game theory models of ‘two-sided matching’.  The model posits a mating market in which individuals assortatively mate for indicators of heritable fitness, and those indicators evolve through an interaction between parental mate choice and survival selection on offspring.  Surprisingly, fitness matching can drive the evolution of elaborate courtship behavior even under the limiting case of strict lifelong monogamy.  Fitness matching also maximizes the genetic variance in fitness in the next generation, giving natural selection more raw material on which to work.  The fitness matching process automatically generates sexual equality in courtship adaptations.  Such sexually-selected adaptations with no sexual dimorphism have usually been dismissed as 'species recognition markers' by biologists, though they are displayed in the courtship of many species.  Fitness matching based on creative courtship behavior may have been the key sexual selection process in human mental evolution.

 

 

VII.  Chapter 7: Bodies of evidence

 

33.  This chapter examines human morphological features that evolved under mate choice, especially features that reveal mutual choice.  These physical traits can work as metaphors for sexually-selected mental traits, as both fitness indicators and aesthetic ornaments resulting from runaway and perceptual biases

 

34.  Male human beards, penises, and upper-body muscles, and female breasts, buttocks, and orgasmic capacities, show evidence of evolving through sxual selection.  There are also some monomorphic traits such as long head hair, hairless skin, highly expressive faces, and full lips that function as sexual attractants.  These fleshy parts don't fossilize, so have been somewhat neglected in human evolution theorizing.  However, all these traits are valued in modern mate choice, are displayed during courtship, and are mostly unique to humans.  Also, the dimorphic traits develop fully only during puberty, in time for sexual attraction.   These criteria also apply to many mental traits.

 

35.  The chapter concludes by discussing how sexual selection shaped the evolution of physical and mental capacities for playing sports, which function as non-lethal domains of sexual competition. 

 

 

VIII.  Chapter 8: Arts of seduction

 

36.  The last four chapters offer four case studies of uniquely human mental traits that appear to have evolved under mate choice: art, morality, language, and creativity.  This chapter considers body ornamentation, art, and aesthetics.

 

37.  Art is an easy example because no one has posited a credible survival function for art, whereas it has obvious analogues to the sexually-selected visual displays of other species such as peacocks and bowerbirds.  Male bowerbirds construct large ornamental bowers to attract females, decorating them with brightly colored flowers, pebbles, shells, and feathers.  Females inspect multiple bowers, choose the one they find most attractive, and mate with its creator, raising her brood in a separate, simple nest of her own construction.  The bower is part of the bowerbird's 'extended phenotype' -- a genetically evolved display constructed outside the body.  Human aesthetic behavior also functions as an extended phenotype, ranging from ochre pigments applied on as skin decoration, to cave paintings and Venus figurines.

 

38.  Human aesthetic preferences could have arisen (as mechanisms of mate choice for good artisans) through runaway sexual selection, through sensory biases, and/or through selection as fitness indicators.  However, runaway can't make any predictions about which particular preferences will evolve, and sensory bias theory doesn't work very well here because other primates do not share the same aesthetic preferences as humans, despite sharing almost identical visual systems.  The idea that beautiful artefacts carry information about the fitness of their makers makes more sense.  Thorstein Veblen (1899) and Franz Boas (1955) insisted that an artist's manifest virtuosity (manual skill, access to rare resources, creativity, conscientiousness, intelligence) is the major criterion of beauty in most cultures.   Their view was eclipsed in aesthetic theory by 20th century Modernism (which rejected the concepts of beauty and virtuosity), but remains relevant to most popular culture, interior design, folk art, craft, and fashion.  This beauty-as-virtuosity theory also helps make sense of the handaxe, which can be viewed as a sexually-selected feature of hominid extended phenotypes for over 1.5 million years.

 

 

IX.  Chapter 9: Virtues of good breeding

 

39.  This chapter examines our sexual abhorrence of selfishness, cheating, and lying, and suggests that mate choice shaped our distinctive human capacities for sympathy, kindness, sexual fidelity, moral leadership, magnaminity, romantic gift-giving, and sportsmanship.  It suggests that sexual selection explains much of human altruism that cannot be explained through kin selection or reciprocal altruism, and tries to take seriously David Buss's (1989) finding that 'kindness' was the top-ranked most-desired trait in a potential mate across all 37 cultures he studied.   

 

40.  All evolutionary theories of morality have to find a hidden genetic benefit to apparently altruistic acts.  Kin selection does it by pointing out that genetic benefits can be spread across relatives by helping them, and reciprocal altruism theory does it by pointing out that benefits to oneself can be spread across time, through repeated interactions with trusted trading partners.  These theories are good at explaining parental solicitude, nepotism, economic prudence, and instincts for cheater-detection.  However, they leave most of human morality unexplained.  Sexual selection provides a complementary way of explaining how selfish genes can give rise to altruistic individuals. 

 

41.  Basically, the hidden genetic benefits of altruism could have been reproductive: conspicuous magnaminity and other moral behaviors became sexually attractive because they were good fitness indicators.  Their reliability was guaranteed by the costs of altruism, under the handicap principle.  Only the fit could afford to be generous.   Sexual selection can favor almost any degree of generosity or heroism, despite their survival costs, just as it can favor almost any length of peacock tail.  Mate choice can work as a moral filter from each generation to the next.

 

42.  The evolution of big-game hunting provides one example of sexual selection for magnaminity.  Kristen Hawkes (1991) has argued that male hunting of large, dangerous prey evolved not to 'feed one's family' (monogamous nuclear families being rare in the Pleistocene), but to attract multiple female partners, who appreciated hunting ability as a fitness indicator, and as a direct nutritional benefit to themselves and their offspring.  Anthropological data show that good hunters have more extra-pair copulations than poor hunters. 

 

43.  The most complicated argument in the book concerns the evolution of moral displays through an interaction between sexual selection for costly signals, and group selection among alternative signalling equilibria.  This relies on the recent evolutionary game theory research on games with very large numbers of Nash equilibria.  (At each equilibrium, by definition, all individuals are acting rationally selfish -- the issue is which equilibrium will be favored by evolution given competition between groups.)  The argument is too intricate to outline here, but it reaches the surprising conclusion that evolution can favor precisely those sexual display equilibria that bring the highest group benefits, without any conflict between individual-level selection and group-level selection (see Boyd & Richerson, 1990).  The relevance to human morality is that, given competition between groups, sexual preferences will evolve to favor apparently altruistic capacities for sympathy, magnaminity, group leadership, and punishment of cheaters -- rather than purely wasteful, non-altruistic displays such as the peacock’s tail.

 

44.  The chapter applies this logic to understand charitable behavior, male gift-giving during courtship, the emotional capacities for sympathy and sexual fidelity, the sexual aversion to psychopaths, the female preference for generous fathers and step-fathers, and the capacity for sportsmanship (where cheating implies any behavior that lowers the meritocratic correlation between a competitor's fitness and their success in the sport).   It concludes by urging evolutionary psychology to broaden its concept of human morality from what Nietzsche called the Christian 'morality of the herd' (fairness, conscience, equality, fidelity, altruism), to what he called the pagan virtues (e.g. bravery, beauty, skill, leadership, stoicism, sacrifice, good manners).

 

 

X.  Chapter 10: Cyrano and Scheherezade

 

45.  This chapter addresses the evolution of language, emphasizing that if talking gives away useful information to others, it raises the same evolutionary dilemma as altruism in general -- a dilemma that, as with morality, can be resolved by reference to sexual selection.

 

46.  The debates over language's innateness and uniqueness have been resolved in favor of Steven Pinker's (1994) view that language is a uniquely human, genetically based psychological adaptation that evolved for some set of biological functions.  The question remains: what functions?  Most theories of language evolution do not identify any specific selection pressures in favor of communication ability, and those that stress survival benefits cannot explain why no other species evolved such a supposedly useful ability.   Moreover, most such theories fail to explain the selfish genetic benefits from the apparently altruistic (information-giving) act of speaking, and fail to explain why people compete to say things rather than to listen in group conversations.

 

47.  Almost all complex acoustic signals in other species evolved as courtship displays through sexual selection: frog croaks, bird song, whale song, etc.  Human verbal courtship (i.e. conversation between lovers) serves an analogous function, with mutual advertisement of capacities for speaking, listening, thinking, remembering, story-telling, and joke-making.  Assuming they spoke three words a second for two hours a day during courtship, with an average of three months of sex before a viable pregnancy, the average hominid would have uttered a million words of verbal courtship before reproducing.  This million-word hurdle would have given prospective mates ample opportunity to assess verbal ability, creativity, and intelligence, and to reject dumb, boring mumblers.  The importance of verbal courtship is exemplified by the legends of Cyrano de Bergerac and Scheherezade, who provoked love through their poetic and story-telling abilities.

 

48.  In Turing’s (1950) original ‘imitation game’, a male interrogator tries to determine whether he is interacting via computer with a real woman, or a computer program that imitates a woman.  Turing assumed that verbal courtship (including capacities for making jokes and composing poems) was the most challenging test for artificial intelligence, but this sexual-selection aspect of the Turing test was stripped away by later AI researchers as an irrelevant distraction. 

 

49.  Once language evolved, much more of our mental life became subject to sexual selection.  Verbal courtship could reveal whole new areas of mental functioning -- personality, intelligence, beliefs, desires, past experiences, future plans -- that are hidden in the more physical courtship of other species.  As language gave a clearer window on the mind, the mind became more easily shaped by mate choice.  This sexual selection feedback loop between mate choice, language ability, and creative intelligence was probably the mainspring of human mental evolution.

 

50.  A detailed analysis of vocabulary size provides a quantitative case study of how modern human language exceeds any plausible demands of survival and social reciprocity.  Our average 60,000 word vocabularies far exceed the 850-word vocabulary of the artificial language 'Basic English', invented by I. A. Richards and C. K. Ogden in the 1920s.  Basic English, like most small-vocabulary pidgin languages, suffices for all ordinary aspects of trade, cooperative work, and survival, including the exchange of threats, promises, warnings, and news.  Biology and astronomy textbooks have been written in Basic English.  This suggests most of our words are pragmatically redundant, and must be serving some self-advertisement function.  Since vocabulary size is highly correlated with general intelligence, and is highly heritable, it appears to function as a reliable indicator of heritable mental fitness.  People are generally unaware of their sexual preferences for large vocabularies (rare is the personal ad asking for a partner who knows 50,000 useless synonyms), but assortative mating for vocabulary size is higher than for almost any other mental trait.

 

 

XI.  Chapter 11: The wit to woo

 

51.  The final chapter explores how evolution can favor benignly unpredictable behavior, suggests hominids developed sexual preferences for interesting, funny mates over boring mates, and proposes that these sexual preferences for unpredictable courtship behavior drove the evolution of human creativity.

 

52.  The earliest game theorists such as John von Neumann recognized that many games require 'mixed strategies' -- strategies that randomize moves from one play to the next.  Unpredictability often brings strategic advantages.  With the theory of 'protean behavior', biologists independently developed the same principle in considering anti-predator behavior: an unusual appearance and unpredictable evasion movements could protect prey from being noticed and captured.  The unpredictability of animal behavior may reflect capacities for adaptively protean behavior, more than some side-effect of neural noise or ‘random error’ (N. B. analysis of variance, psychology’s favorite statistical method, can’t distinguish proteanism from noise). 

 

53.  With the evolution of 'Machiavellian intelligence' (the capacity for predicting and manipulating the social behavior of conspecifics), primates would have been under selection to be socially unpredictable to their reproductive competitors (Miller, 1997).  The unpredictability of primate aggression noticed by field researchers probably reflects a mixed strategy.  This sort of 'social proteanism' may have provided some genetic and neurological foundations for human creativity, by endowing our brains with mechanisms for randomizing social behaviors in general, which could be applied to the special case of courtship, to produce amusingly unpredictable romantic behavior.  Creativity could have been favored initially as a reliable indicator of social proteanism ability, and of youthfulness, insofar as juvenile primates tend to be more playful and unpredictable than adults.  In modern humans, creativity is also correlated highly with general intelligence, and moderately with health and energy level; its heritability appears to be a side effect of its correlation with intelligence, which is highly heritable.  

 

54.  Creative courtship may have also played upon neophilia, a fundamental attentional and cognitive attraction to novelty.  In terms of the psychological bias theory of sexual selection, neophilia is just another bias that might influence mate choice.  Darwin (1871) argued that neophilia was an important factor in the diversification and rapid evolution of bird song.  Primate and human neophilia is especially strong, with boredom often cited as a reason for terminating sexual relationships.  Partners who offered more cognitive variety and creativity in their relationships may have had longer, more reproductively successful relationships -- as symbolized, again, by Scheherezade.  A good sense of humor is the most sexually attractive variety of creativity, and human mental evolution is better imagined as a romantic comedy than as a story of disaster, warfare, predation, and survival.

 

55.  Sexual selection for creativity undermines some of the evolutionary epistemology claims about the reliability of human knowledge.  Whereas natural selection might tend to favor minds with accurate, survival-enhancing world-models, sexual selection might favor minds prone to inventing attractive, imaginative fantasies -- as long as fantasy-invention ability remains a reliable fitness-indicator.  (Brigham Young's religious visions revealed his imagination and intelligence, and attracted his 27 wives, but that does not guarantee the veracity of his belief that dead ancestors can be retroactively converted to Mormonism.)  Sexual selection rarely favors displays that include accurate representations of the world -- peacock tail eye-spots catch attention by resembling eyes, but they are not very good likenesses.  Likewise for human ideologies: they may be sexually attractive be revealing an individual's character, intelligence, and moral ideals, but they need not be good representations of reality.  Sexual selection can explain why most people prefer fiction to non-fiction, religious myth to scientific evidence, and political correctness to intellectual coherence.

 

 

XII.  Epilogue

 

56.  The epilogue lists the theory's limitations, emphasizing that it is not a complete theory of mental evolution, only a theory of the more distinctive, display-oriented human capacities that have proven hard to explain in 'survival value' terms.  It admits that my ideas may have been unduly influenced in some cases by my being male, and calls for further refinements by researchers of both sexes.  Without committing the 'naturalistic fallacy', it notes that debates about moral values and social ideals can and should be informed by our concepts of human nature and human evolution -- especially debates over educational policy, consumerism, political philosophy, and bioethics.  It calls for the conscious suppression of our instincts for discriminatory mate choice in social contexts where such discrimination is inappropriate, and concludes with a hint about how sexual selection may continue to shape human evolution in the future.

 

 

XIII.  The theory's limitations, weaknesses, and needs for further research

 

57.  This final section goes beyond the book’s contents somewhat to provide some clarifications for Psycholoquy readers.  First, the theory’s limitations.  'The mating mind' offers a shap-shot of a provisional theory under construction.  It does not pretend to be a complete account of human mental evolution, because it addresses only those psychological adaptations manifest in sexual courtship.  It accepts that natural selection and other forms of social selection account for all the other adaptations for perception, cognition, and movement, and all the other important domains of behavior such as foraging, parenting, making friends, trading goods and services, and avoiding predators and parasites.  The theory does not apply to all the psychological adaptations that we share with other vertebrates, mammals, primates, and apes.  It does not address the quandary of 'consciousness' or the dynamics of cultural change, though it might have some bearing on these issues.   It is best at explaining behaviors that young adults do more than children or older people, that men do more conspicuously than women, that high-fitness people do more than low-fitness people, that take lots of energy, time, and skill, that reveal genetic quality, and that people cite as sexually desirable traits.  I leave it to others to discuss the existential and theological implications of living in a lineage that directed its own evolution through its powers of mate choice.

 

58.  One weakness of any theory that relies on sexual selection is that sexual selection theory (like natural selection theory) is still very much under development, with debates continuing about the heritability of fitness, the relative importance of indexes versus handicaps, the dynamics of sexual selection given diverse preferences and multiple ornaments, and the game-theoretic complexities of mating markets given mutual mate choice.  As the higher-level meta-theory of sexual selection develops, some of my mid-level hypotheses about human mental evolution may turn out to be evolutionarily implausible.  Given the minimal sex differences in most human courtship abilities discussed in the book, a high priority should be given to the development of better models of sexual selection under mutual mate choice. 

 

59.  Another weakness concerns traits like language that probably evolved through some interplay of sexual selection and other forms of social selection (including group selection between different evolutionary equilibria).  Arguably, the theory over-emphasizes the courtship functions of language and creative intelligence, while neglecting their other fitness benefits in teaching children, making friends, helping kin, trading services, making commitments, sustaining social norms, fighting other tribes, and so forth.  However, my intention was not to dismiss these other benefits, only to stress a sexual-attraction benefit that had been neglected in previous theorizing. 

 

60.  A third weakness is the provisional nature of the reconstruction of hominid mating patterns.  This reconstruction is based on current data from archaeology, anthropology, psychology, primatology, and genetics.  However, today's data may be supplanted by tomorrow's.  Given the number of radical re-thinks that have occurred about prehistory over the last several decades, it would be foolish to pretend that the current reconstruction of Pleistocene mating is the last word on the subject.  Nevertheless, the requirements for sexual selection to influence the mind's evolution would remain valid under a fairly wide range of mating patterns, because only perfectly random mating would render mate choice irrelevant.

 

61.  The book also has some pseudo-weaknesses that aren't really specific to this theory.  Evolutionary accounts of the most cherished human abilities are often criticized as 'genetic determinism' by those who wish to retain the mystique of the humanities and the performing arts.  Such criticisms are generic to all adaptationist accounts of human creativity, and are especially inappropriate to this sexual selection account, which emphasizes the feedback loop between psychology and biology via mate choice, and sexual selection's ability to favor non-deterministic, unpredictable, creative forms of courtship. 

 

62.  Another pseudo-weakness is that the theory only has patchy evidence in its support at present.  Any evolutionary theory that relies on existing data can be dismissed as a 'Just-so story' that just offers post-hoc interpretations of known facts; whereas any that sticks its neck out and makes novel predictions not yet supported by evidence can be dismissed as 'speculative'.  Obviously, the best theories must rely on some evidence and make some predictions, so may appear both post-hoc and speculative.  That quandary is generic to all scientific theories, from physics through psychology.  The real issue is testability (a broader, more realistic, Lakatosian version of Popper's falsifiability criterion), and fortunately, the last two decades of biological research on animal mate choice have produced ever-better methodologies for testing hypotheses about sexual selection, which can also be applied to humans.  Likewise for genetic and neuroscientific evidence supporting the existence of these courtship adaptations: most senior scientists with power to allocate time on the relevant lab equipment won't bother looking for such evidence until someone points out the possible scientific benefits of doing so.

 

63.  Much more empirical research is needed on mate choice for mental traits among humans across diverse cultures, ranging from tribal hunter-gatherers to affluent Westerners.  We need more behavior-genetic work on the inheritance of mate preferences and courtship abilities, and the heritability of variance in those adaptations.  We need more developmental psychology work on the emergence of these adaptations over the life-course, and their facultative sensitivity to changes in personal mate value, mating markets, and parental responsibilities.  The hypothesis that there is a general fitness factor (superordinate to both the g factor and general physical health) needs to be tested with large, representative datasets that include psychometric, anthropometric, health, social status, and other diverse measures of fitness-relevant traits.  More data is needed on the metabolic costs (e.g. glucose burn rates) of different mental activities, to see whether courtship draws disproportionately on the costliest.  Cognitive neuroscience could try to localize courtship adaptations in the brain.  Most importantly, we need much better observations concerning real-life human courtship, including the measurable aspects of courtship that influence mate choice, the reproductive (or at least sexual) consequences of individual variation in those aspects, and the social-cognitive and emotional mechanisms of falling in love.

 

XIV.  References

 

Boas, Franz (1955).  Primitive art.  New York: Dover

 

Boyd, R., & Richerson, P. J. (1990).  Group selection among alternative evolutionarily stable strategies.  J. Theoretical Biology, 145, 331-342.

 

Buss, D. M. (1989).  Sex differences in human mate selection:  Evolutionary hypotheses tested in 37 cultures.  Behavioral and Brain Sciences, 12, 1-49.

 

Cronin, H.  (1991).  The ant and the peacock: Altruism and sexual selection from Darwin to today.  Cambridge U.  Press.

 

Darwin, C.  (1871).  The descent of man,  and selection in relation  to sex (2 vols.).   London: John Murray.   (Reprinted in 1981 by Princeton U. Press.)

 

Eyre-Walker, A., & Keightley, P. D. (1999).  High genomic deleterious mutation rates in hominids.  Nature, 397, 344-346.

 

Fisher, R.  A.  (1930).  The genetical theory of natural selection.  Oxford: Clarendon Press.

 

Hawkes, K. (1991). Showing off: Tests of another hypothesis about men’s foraging goals.  Ethology and Sociobiology, 12, 29-54.

 

Miller, G.  F.  (1993).  Evolution of the human brain through runaway sexual selection: The mind as a protean courtship device.  Ph.D. thesis, Psychology Department, Stanford University.

Miller, G. F. (1997). Protean primates:  The evolution of adaptive unpredictability in competition and courtship. In A. Whiten & R. W. Byrne (Eds.),  Machiavellian Intelligence II, pp. 312-340.  Cambridge University Press.

 

Miller, G. F. (1999).  Sexual selection for cultural displays.  In R. Dunbar, C. Knight, & C. Power (Eds.), The evolution of culture, pp. 71-91.  Edinburgh U. Press.

 

Miller, G. F. (2000).  Mental traits as fitness indicators: Expanding evolutionary psychology’s adaptationism.  Annals of the. New York Academy of Sciences, 907.

 

Miller, G. F. (in press).  Sexual selection for indicators of intelligence.  For J. Goode (Ed.), The nature of intelligence.  Novartis Foundation Symposium 233.  John Wiley, pp. 270-275.

 

Pinker, S.  (1994).  The language  instinct.  London: Allen Lane. 

 

Pinker, S. (1997).  How the mind works.  New York: Norton. 

 

Turing, A. M. (1950).  Computing machinery and intelligence.  Mind, 59, 433-460.

 

Veblen, T. (1899).  The theory of the leisure class.  New York: Macmillan.  Reprinted by Dover.

 

Zahavi, A.  (1975).  Mate selection: A selection for a handicap.  J. Theoretical Biology, 53, 205-214.