A Review of Sexual Selection and Human Evolution:
How Mate Choice shaped Human Nature
Miller, G. F. (1998). How mate choice shaped human nature: A review of sexual
selection and human evolution. In C. Crawford & D. Krebs (Eds.), Handbook of
evolutionary psychology: Ideas, issues, and applications (pp. 87-130). Lawrence
Geoffrey F. Miller
ESRC Research Centre for
Economic Learning and Social Evolution (ELSE)
University College London
Gower St., London WC1E 6BT, England
Running Head: How mate choice shaped human nature
The application of sexual selection theory to human behavior has been the greatest
success story in evolutionary psychology, and one of the most fruitful and fascinating
developments in the human sciences over the last two decades. Ironically, this
development would have seemed absurd only twenty years ago. At that time, many
biologists considered sexual selection through mate choice to be Darwin's least
successful idea: if not outright wrong, it was at most a minor, uninteresting, even
pathological evolutionary process. At that time, any `Darwinization' of the human
sciences would have had to rely on natural selection theory, which bears much less
directly on human social, sexual, and cultural behavior.
Instead, something remarkable happened: sexual selection theory was revived over the
last two decades through the combined efforts of researchers in theoretical population
genetics, experimental behavioral biology, primatology, evolutionary anthropology, and
evolutionary psychology. Today, although natural selection theory serves as the
conceptual and rhetorical foundation for evolutionary psychology (see Tooby &
Cosmides, 1990, 1992), sexual selection theory seems to guide more actual day-to-day
research (see Buss, 1994; Ridley, 1993; Wright, 1994).
This chapter reviews the current state of sexual selection theory, and outlines some
applications to understanding human behavior. Sexual selection theory has been
revived so recently that, while extraordinary opportunities exist for further research,
many old misconceptions persist. These include the mistaken ideas that sexual
How mate choice shaped human nature
selection: (1) always produces sex differences, (2) does not operate in monogamous
species, (3) is weaker than natural selection, and (4) had nothing to do with the evolution
of human intelligence, language, or creativity. One goal of this chapter will be to dispel
some of these myths, and to bring evolutionary psychology up to date with respect to the
biological literature on sexual selection. Sections 2 through 4 review the history and
basic theory of sexual selection. Sections 5 and 6 contextualize human mate choice by
covering sexual selection in primates and hominids. Sections 7 through 9 survey some
possible roles of mate choice in shaping the human body, the human mind, and human
culture. Finally, section 10 concludes with some academic and existential implications of
applying sexual selection theory to understand human nature.
2 History of sexual selection theory
Darwin (1859, 1871) realized that his theory of natural selection through differential
survival could not explain extravagant male traits such as the peacock's tail, because
such traits actually decrease survival ability. Rather, he reasoned that in a sexually-
reproducing species, any heritable traits that help in competing for sexual mates will tend
to spread through the species, even if they compromise survival somewhat. This
process of sexual selection may favor, for example, better sensory and motor abilities for
finding mates, gifts and ornaments to attract them, weapons and bluffs for repelling
same-sex competitors, endurance for lasting through the breeding season, and genitals
and gametes that maximize fertilization rates. Within the sexual selection process,
Darwin distinguished between male competition for female mates (which typically gives
rise to weapons), and female choice of male mates (which typically gives rise to gifts and
ornaments). But he recognized that female choice and male competition are often two
sides of the same coin, because mate choice by one sex usually implies competition by
the other sex, either through direct `interference competition' (e.g. physical fights over
the opposite sex) or through indirect `exploitation competition' (e.g. scrambles to find and
seduce the opposite sex before someone else does). Darwin had no real explanation of
why males usually compete harder for mates than females do — why males court, and
females choose —though he offered a staggering amount of evidence that this pattern
holds from insects through humans (Darwin, 1871).
Sexual selection was a radical idea for several reasons. First, it was a truly novel
concept. Whereas the theory of natural selection had been anticipated by many 18th
and 19th century thinkers such as Jean-Baptiste de Lamarck, Etienne Geoffroy Saint-
Hilaire, Frederic Cuvier, Thomas Malthus, and Robert Chambers (see Richards, 1987),
and was co-discovered by Alfred Russell Wallace (1870, 1889),the notion that mate
choice could shape organic form was without scientific precedent. Second, sexual
selection embodied Darwin's conviction that evolution was a matter of differential
reproduction rather than differential survival. Animals expend their very lives in the
pursuit of mates, against all the expectations of natural theology. Finally, Darwin
recognized that the agents of sexual selection are literally the brains and bodies of
sexual rivals and potential mates, rather than the insensate features of a physical habitat
or a biological econiche. Psychology haunts biology with the spectre of half-sentient
mate choice shaping the otherwise blind course of evolution (see Miller, 1994; Miller &
Todd, 1995). For Darwin, the choice of mates by female animals was no different in kind
from artificial selection by human breeders:
"All animals present individual differences, and as man can modify his
domesticated birds by selecting the individuals which appear to him the most
beautiful, so the habitual or even occasional preference by the female of the
more attractive males would almost certainly lead to their modification; and such
modification might in the course of time be augmented to almost any extent,
compatible with the existence of the species." (Darwin, 1871, p. 750-751).
Because female animals exercised most mate choice, and sexual selection through
mate choice came very close to creative, conscious artificial selection by humans,
Darwin's ideas put females in a very powerful evolutionary role —a role that made most
(male) Victorian biologists deeply uncomfortable. Thus, male competition was widely
accepted by Darwin's peers as an important, necessary, and general evolutionary
process, but the possibility of female choice driving evolution was almost universally
mocked and dismissed (Cronin, 1991).
For example, even Alfred Russell Wallace, the co-discoverer of natural selection
(Wallace, 1870), was deeply skeptical about sexual selection through female choice.
He doubted that the perceptual systems of female animals could shape male courtship
ornaments, and viewed such ornaments as arising from a simple male "surplus of
strength, vitality, and growth-power which is able to expend itself in this way without
injury" (Wallace, 1889, p. 293). Wallace's skepticism is strange because his insightful
analyses of camouflage, mimicry, and warning coloration all presupposed a form of
`perceptual selection' by female (and male) predators hunting prey (see Wallace, 1870,
1889). If female predators could shape the evolution of bright warning colors in their
prey, why couldn't they shape the evolution of bright courtship colors in their males?
Even now, we hear echoes of Wallace's fallacious surplus-of-energy argument in most
psychological and anthropological theories about the `self-expressive' functions of
human art, music, language, and culture. After Darwin (1871), sexual selection received
such a frosty reception from Wallace and others that it was virtually forgotten (Mayr,
1972; Cronin, 1991). The Modern Synthesis of Mendelian genetics and Darwinism in
the 1930s viewed male competition as a sub-class of natural selection, while continuing
to reject female choice. Sexual ornaments were assumed to intimidate other males, or
were `species recognition markers' to help animals avoid cross-species mating (e.g.
Huxley, 1938; Cott, 1940). Sexual selection remained hidden in biology's blind spot for
many decades. The reasons are clear in retrospect. Sexual selection is hard to analyze
mathematically. Behaviorist psychology ignored evolution, denied instincts, and
disregarded the ecological validity of psychology experiments, so was not prone to
doing realistic experiments on mate choice. A fallacious form of group-selectionism
viewed costly courtship ornaments as `bad for the species' and therefore implausible.
Freud's vain attempt to leapfrog past Darwin without really understanding sexual
selection (see Sulloway, 1979) led to psychoanalysis supplanting evolutionary biology as
the early 20th century's leading account of human sexuality. Not least, persistent sexism
in biology denied the power of female choice until the 1970s (see Miller, 1993).
However, during sexual selection's long exile from biology, it was sometimes adopted by
early evolutionary psychologists such as Edward Westermark (1894) and Havelock Ellis
(1905, 1934), who used it to explain many aspects of the human body and mind.
R. A. Fisher (1915, 1930) was one of the few biologists to take sexual selection
seriously. He viewed mate preferences as legitimate biological traits subject to heritable
variation, and this insight led him to postulate a process he called runaway sexual
selection.. In runaway, an evolutionary positive-feedback loop gets established between
female preferences for certain male traits, and the male traits themselves. Given a
nudge in the right direction (e.g. an initial bias in female preferences), Fisher's model
How mate choice shaped human nature
could account for the wildly exaggerated male traits seen in many species, such as the
peacock's plumage (details follow in section 3.3). But Fisher did not explain the
evolutionary origins of female preferences themselves, nor did he develop formal genetic
models of runaway sexual selection. Huxley's (1938) hostile, deeply confused critique of
Fisher's theory and of sexual selection in general consigned the field to continued
neglect until the 1970s.
Sexual selection's revival has been swift, dramatic, and unique: Darwin's idea is, to my
knowledge, the only major scientific theory ever to have been accepted after a century of
condemnation. A centenary volume on sexual selection (B. Campbell, 1972) drew some
attention to Darwin's neglected ideas. Trivers (1972) finally explained why males court
and females choose, when he pointed out that the higher levels of necessary `parental
investment' by females of most species make females a limiting resource over which
males must compete: sex differences in parental investment drive sex differences in the
intensity of sexual selection. Zahavi (1975) set off intense debate with his `handicap
principle', suggesting that the extravagance and costliness of many sexual ornaments
function to guarantee their reliability as displays of genetic quality (e.g. only healthy
peacocks can afford to grow such huge, handicapping tails.) The debate over
sociobiology (Wilson, 1975) attracted interest in the evolution of social and sexual
behavior. The new population genetics models of O'Donald (1980), Lande (1981), and
Kirkpatrick (1982) showed the mathematical feasibility of Fisher's runaway sexual
selection process. New behavioral experiments on animals showed that females of
many species do exhibit strong preferences for certain male traits (e.g. Andersson, 1982;
Catchpole, 1980; Moller, 1988; Ryan, 1985). Important edited volumes appeared on
sexual selection theory (Bateson, 1983; Bradbury & Andersson, 1987), and on sexual
selection in insects (Blum & Blum, 1979) and humans (Betzig, Borgerhoff Mulder, &
Turke, 1987). Eberhard (1985) argued that the only feasible explanation for the wildly
complex and diverse male genitalia of many species is female choice for certain kinds of
genital stimulation. Finally, primatologists began to appreciate the role of sexual
selection in primate social systems (e.g. De Waal, 1982; Dunbar, 1988; Smuts, 1985;
Byrne & Whiten, 1988), and Symons (1979) applied sexual selection theory to humans
more thoroughly than ever before. Once biologists started taking the possibility of
female choice seriously, evidence for its existence and significance came quickly and
ubiquitously (see Andersson, 1994; Cronin, 1991).
Currently, sexual selection is one of the fastest-growing and most exciting areas of
evolutionary biology and animal behavior. Recent biological work permeates the
journals American Naturalist, Animal Behavior, Behavioral Ecology and Sociobiology,
Evolution, Heredity, Journal of Theoretical Biology, Nature, and Science. Research on
sexual selection in humans appears most often in the journals Behavioral and Brain
Sciences, Ethology and Sociobiology, Human Nature, and Psychological Review. The
best recent theoretical and empirical review of sexual selection is Andersson (1994); the
best historical review is Cronin (1991). Darwin's (1871) foundational work The Descent
of Man, and Selection in Relation to Sex still rewards careful and repeated reading.
Accessible introductions to sexual selection in humans include Batten (1992), Buss
(1994), H. Fisher (1992), Daly and Wilson (1988), Ridley (1993), and Wright (1994).
It is important to understand the peculiar history of sexual selection theory because
virtually all of 20th century psychology, anthropology, paleontology, primatology, and
cognitive science, as well as the social sciences and humanities, developed without
recognizing that sexual selection could have played any important role in the evolution of
the human body, the human mind, human behavior, or human culture. Since biologists
have embraced sexual selection, we must face the possibility that most current theories
of human behavior and culture are inadequate, because they may have vastly under-
estimated the role of sexual competition, courtship, and mate choice in human affairs.
3 Mate choice criteria and sexual selection mechanisms
The simplest way to review the current state of sexual selection theory is to explore the
different kinds of criteria that animals can use to choose mates. This is because we can
often view sexual competition within each sex as an outcome of mate choice by the
other sex — if "choice" is understood broadly to include processes both conscious and
unconscious, and both psychological and physiological. Recently, there have been two
major schools of thought about mate choice criteria. Champions of Zahavi's (1975)
handicap principle have emphasized selection for genetic indicators also called "good
genes", "good sense", or "healthy-offspring" selection. Champions of R. A. Fisher's
(1930) runaway process have emphasized selection for aesthetic displays also called
"good taste" or "sexy son" selection. In evolutionary biology, these different mate choice
criteria are often considered competing models of how sexual selection works, but there
is now sufficient evidence for each (see Andersson, 1994) that they can be considered
well-established, often complementary selective forces. Of course, mate choice can
favor many other important qualities, including parental ability and resources (see
Clutton-Brock, 1991; Hoelzer, 1989; Price, Schluter, & Heckman, 1993), fertility (e.g.
sperm quality in males or fecundity in females — see Baker & Bellis, 1995; Singh, 1993),
optimal genetic distance (to avoid inbreeding with close relatives or outbreeding with the
wrong species — see N. Thornhill, 1991, 1993), and similarity in appearance, behavior,
and personality (see Buss, 1985; Rushton, 1989; Thiessen & Gregg 1980). But before
discussing these various classes of mate choice mechanisms, we must understand their
3.1 The origins of mate preferences
Mate choice is the behavioral outcome of mate preferences. These preferences are
usually "mental adaptations" implemented as complex neural circuits, and constructed
through the interaction of many genes and environmental conditions, which bias mating
in favor of individuals with certain perceivable traits. In most species, such systems may
function without conscious awareness, deliberation, or complex aesthetic feelings; but
we might expect mate choice to be among the least unconscious of an animal's
decisions, because it requires the integration of such diverse information, and has such
important fitness consequences. Mate choice operates by rejecting some potential
mates and accepting or soliciting to others. In almost all species, females can effectively
resist copulation attempts by unwanted males, and in many species, females actively
solicit copulations from desired males. Likewise, males actively pursue desired females,
and ignore solicitation attempts by unwanted females. Although sexual harassment of
females is common in nature, `successful' rape seems fairly rare, being reported in only
a small collection of species such as ducks, squid, dolphins, orangutans, and humans
(Brownmiller, 1975; Rodman & Mitani, 1987; R. Thornhill & N. Thornhill, 1992).
Generally, mutual choice and mutual cooperation are necessary for breeding.
How mate choice shaped human nature
Why do these mechanisms for mate choice evolve? Being choosy requires time,
energy, and intelligence, and these costs of mate choice can impair survival and can
decrease the likelihood of sexual selection operating at all (Pomiankowski, 1987;
Reynolds & Gross, 1990). The basic rationale is that random mating is stupid mating. It
pays to be choosy because in a sexually reproducing species, the genetic quality of your
mate will determine half the genetic quality of your offspring. Ugly, unhealthy mates yield
ugly, unhealthy offspring. By forming a joint genetic venture with an attractive, high-
quality mate, one's genes are much more likely to be passed on. Even modern women
who deny the "role of genes in human behavior" tend to choose their sperm donors quite
carefully (see Scheib, 994). Mate choice is simply the best eugenics and genetic
screening that female animals are capable of carrying out under field conditions, with no
equipment but their senses and their brains.
Mate choice mechanisms can evolve through direct selection for mate-choice efficiency
(i.e. better preferences lead to more or better offspring), and through three other less
predictable, less adaptive processes: (1) mutation, (2) genetic drift, and(3) genetic
linkage with another trait that is undergoing genetic drift, natural selection, or sexual
selection. These last three processes will typically produce harmful changes in mate
choice mechanisms, so will usually be selected out. But some changes will persist,
through chance, utility, or Fisher's runaway effect. The unpredictability of these three
processes is important in explaining the diversity of sexually-selected ornaments across
similar, closely-related species (Eberhard, 1985; Miller & Todd, 1995).
The following sections review some of the major kinds of mate choice and sexual
selection. But in addition to the mate choice criteria discussed below, most animals also
have mechanisms to ensure that they mate with partners of the appropriate species,
sex, age, and genetic distance, at an appropriate place and time (see Bateson, 1983;
3.2 Selection for indicators
Probably the most fundamental form of sexual selection is mate choice for various
"indicators" of viability (likelihood of survival) and fertility (likelihood of reproduction).
These can take many forms. Almost any perceivable bodily or behavioral trait can
function as an indicator — revealing age, health, nutritional status, size, strength,
aggressive dominance, social status, disease resistance, or overall vigor. Such
indicators may reveal both heritable genetic traits that would be passed on to offspring
(selection for `good genes'), and chances that the mate will survive to give provisioning,
protection, and support to offspring (selection for `good parents').
No one is surprised when animals avoid mating with the dead, the injured, or the sick.
All such mating decisions must rely on observable cues of viability. The idea of
indicators is that the cues used in such assessments will tend to be exaggerated over
eons of mate choice. Weak, ambiguous, unreliable, incidental cues of being non-dead
and non-sick will become strong, clear, reliable, specially-adapted indicators of being
vigorous and healthy. For example, dead peacocks have rather drab tails. And
peacocks with inferior tails get eaten more often by predators (Petrie, 1992). So tail
quality probably reflects some underlying physiological quality that correlates with
predator-escape ability, and that could be inherited by offspring. This gives an incentive
for peahens to choose males for tale quality, and for peacocks to display large, healthy
tails as vigorously as possible (Petrie, Halliday, & Sanders, 1991). Other classic
examples of indicators include color and condition of bird plumage in other species
(Hamilton & Zuk, 1982; Moller, 1988), loudness and complexity of bird song (Catchpole,
1980, 1987), antler size and symmetry (e.g. Goss, 1983), and raw body size (Ryan,
985). But almost any body part or behavior that is expensive to produce and that varies
in magnitude can serve as an indicator.
The importance of indicators in sexual selection has been emphasized by R. A. Fisher
(1915), Williams (1966), and Zahavi (1975). Indicators, like animal signals in general,
are subject to the handicap principle: the game-theoretic constraint that they must be
costly in order to be reliable because if not, they can be faked too easily (Zahavi, 1975,
1991). Indicators evolve most easily when they are `condition-dependent', such that
healthier animals grow bigger or better indicators (e. g. larger, more colorful tails) or
`revealing', such that healthier animals take better care or make better use of the
indicators they have (e.g. the tails are better-groomed and better-displayed).
Despite initial skepticism about the handicap principle, computer simulations and
mathematical models have helped to convince most biologists that condition-dependent
and revealing indicators are common outcomes of sexual selection. For example,
simulations by Andersson (1986) showed that condition-dependent indicators could
evolve even in perfectly monogamous species, given viability differences of only a few
percent. An important mathematical analysis by Iwasa, Pomiankowski, and Nee (1991)
confirms that indicators can evolve under sexual selection even if mate preferences are
costly, as long as mutations are usually harmful. Other, more recent models suggest
that `good parenting' indicators can evolve to display even non-heritable resources such
as good territories (Grafen, 1990; Heywood, 1989; Hoelzer, 1989; Price et al., 1993).
Thus, not all indicators are necessarily advertising genetic quality; they could simply be
advertising resources and health relevant to raising offspring. Indicators often evolve
better when runaway sexual selection is also operating on the relevant traits and
preferences (Andersson, 1986; Heywood, 1989; Pomiankowski, 1988; Tomlinson, 1988).
However, indicators alone, even without the runaway process, can suffice for the
evolution of extravagant male ornaments and extreme female preferences (Grafen,
1990). See Andersson (1994, chapter 3) for a comprehensive review of indicator models
The idea of genetic indicators has been criticized because of the `lek paradox' (e.g.
Williams, 1975; Maynard Smith, 1976; Kirkpatrick, 1987; Pomiankowski, 1987, 1995;
Reynolds & Gross, 1990). Leks are aggregations of animals such as sage grouse,
where females pick their mates very carefully from among dozens of males displaying in
large groups, and females receive nothing but sperm from the males they choose. The
most attractive male sage grouse may achieve over 30 matings in a single morning,
while average males usually win none (Boyce, 1990). Under such intense selection for
attractive traits, we might expect the preferred traits to go to fixation (100% frequency) in
the gene pool very quickly (R. A. Fisher, 1930). Once fixated, there would be no further
incentive for females to be choosy, because all of the males should have the same
genes and hence be equally attractive. Indicators would become irrelevant once the
population became genetically homogenous, without any heritable variation in fitness or
However, three processes can maintain heritable fitness variation: temporal variation in
selection, spatial variation in selection, and mutation pressure (see Andersson, 1994).
How mate choice shaped human nature
Temporally varying selection can result from co-evolution between ecological
competitors, between predators and prey, or, perhaps most importantly, between hosts
and parasites (Hamilton & Zuk, 1982; Hamilton, Axelrod, & Tanese, 1990; Low, 1990).
Spatially varying selection in different geographic areas, combined with migration, can
maintain heritable variation in a population. Mutation pressure can also maintain
heritable fitness variation because most mutations are harmful, and give rise to an
excess of low-fitness individuals (Lande, 1981; Charlesworth, 1987; Kondrashov, 1988;
Rice, 1988). Indeed, genetic models show that indicators evolve more easily under
biased mutation (Iwasa et al., 1991). Some recent studies even suggest that sexually-
selected traits have much higher heritabilities and genetic variances than naturally-
selected traits, despite strong directional selection (Moller & Pomiankowski, 1993;
Pomiankowski, 1995; Wilcockson, Crean, & Day, 1995). The importance of heritable
fitness variation is also confirmed by experiments in which females that are allowed to
choose their mates have offspring with higher phenotypic (and, by inference, genetic)
quality than females not allowed to choose (e.g. Partridge, 1980; Reynolds & Gross,
1992). Through female choice, males have been forced to evolve clear windows onto
the quality of their genes, so that females can weed out the bad ones. In this sense,
females shape males to function as a kind of genetic sieve for the species (Atmar, 1991;
Michod & Levin, 1988): out with the bad genes, in with the good.
3.3 Selection for aesthetic displays
Some traits have been shaped as aesthetic displays, sometimes in addition to
functioning as indicators. Aesthetic displays play upon the perceptual biases of
receivers to attract attention, provoke excitement, and increase willingness to mate.
That is, seducers manipulate perceptions. The perceptual biases open to manipulation
can arise in two, often complementary, ways: (1) they may already exist as `latent
preferences' — side-effects of previous evolutionary processes, reflecting basic
psychophysical effects, general principles of perception, or perceptual adaptations to
particular environments — and (2) they may co-evolve with the courtship traits they
prefer, through Fisher's runaway process.
Several species have been shown to have `latent preferences' for particular ornaments,
even though the ornaments have not yet evolved in the species. Burley (1988) showed
that female zebra finches prefer males whose legs have been experimentally decorated
with red or black plastic bands, but males with blue and green bands were rejected.
Basolo (1990) showed that female platyfish prefer males with colorful plastic `swords'
glued on the ends of their tails, suggesting that this preference also pre-dated the
evolution of such ornaments in their close relatives the swordtails. Ryan (1985, 1990)
found that females frogs of some species prefer the courtship calls (deep "chuck"
sounds) of male frogs if they are played back at artificially lowered frequencies, as if
produced by extra-large frogs. Ridley (1981) suggested that tails with multiple eye-
spots, such as those of the peacock and the Argus pheasant, play upon a widespread
responsiveness to eye-like stimuli in animal perception.
In response to such findings, several theorists have emphasized the role of perceptual
biases in sexual selection, using terms such as `sensory drive' (Endler, 1992, 1993),
`sensory trap' (West-Eberhard, 1984), `sensory exploitation'(Eberhard,1985; Ryan,
1990; Ryan & Keddy-Hector, 1992), `signal selection' (Zahavi, 1991), and `the influence
of receiver psychology on the evolution of animal signals' (Guilford & M. S. Dawkins,
1991; see also Enquist & Arak, 1993). As any perceptual psychologist might predict,
animals typically prefer displays that are louder, larger, more colorful, more frequent,
more varied, and more novel than average (Ryan & Keddy-Hector, 1990; Miller, 1993).
But such perceptual biases may also vary substantially across species, in accord with
ecological specializations of the perceptual systems. For example, birds that eat blue
berries may evolve blue-sensitive eyes, which would tend to favor blue ornaments;
whereas birds that eat red berries may evolve red-sensitive eyes that favor red
ornaments. These perceptual specializations may help explain the rapid divergence of
sexually-selected traits across closely-related species (Endler,1992, 1993). The
effectiveness of aesthetic displays in courtship supports the R. Dawkins and Krebs
(1978) theory that animal signals often evolve to manipulate receivers in the signaller's
interest, not to communicate truthful information (as indicators do) for the benefit of both.
But latent preferences are not necessary, according to R. A. Fisher's (1930) runaway
theory. Even chance fluctuations in mate preferences, combined with a strange kind of
evolutionary positive-feedback loop, could produce quite extreme mate preferences and
quite exaggerated courtship traits (see Miller & Todd, 1993; Todd & Miller, 1993).
Suppose that mate preferences vary somewhat randomly within a bird population, so
that in one particular generation, some females happen to prefer long tails on males,
while others don't care. Suppose male tail length also varies randomly. Could the
preference (for long tails) and the trait (of having a long tail) evolve together in a positive-
feedback loop? This possibility was first considered and dismissed by T. H. Morgan
(1903) to ridicule Darwin's sexual selection theory:
"Shall we assume that ... those females whose taste has soared a little higher
than that of the average (a variation of this sort having appeared) select males to
correspond, and thus the two continue heaping up the ornaments on one side
and the appreciation of the ornaments on the other side? No doubt an
interesting fiction could be built up along these lines, but would anyone believe it,
and if he did, could he prove it?"
R. A. Fisher (1930) believed it, but couldn't prove it:
"The two characteristics affected by such a process, namely plumage
development in the male and sexual preference in the female, must thus
advance together, and so long as the process is unchecked by severe
counterselection, will advance with ever-increasing speed."
Recent population genetics models (e.g. Lande, 1981; Kirkpatrick, 1982; Pomiankowski,
Iwasa, & Nee, 1991) have finally proved it:
"Females that prefer to mate with long-tailed males will mate with such males
more often than females that prefer short-tailed males. Following mating and
genetic recombination, the genes for long-tail preference and the genes for the
long tail itself will become correlated: an individual carrying a gene for long tails
will tend to carry a gene for the corresponding preference."(Kirkpatrick,1987, pp.
The argument looks a bit circular, but then all positive-feedback processes look a bit
circular. The only thing keeping runaway going is the `momentum' conferred by genetic
linkage and the risk to individuals of failing to display exaggerated traits or choosy
How mate choice shaped human nature
preferences given that momentum. The peacock's tail grows longer and longer because
of a despotic treadmill of fashion: "Each peahen is on a treadmill and dare not jump off
lest she condemn her sons to celibacy" (Ridley, 1993, p. 135). The treadmill doesn't go
on forever though: eventually, runaway would be counter-acted by the survival costs of
elaborate ornaments (R. A. Fisher,1930). At evolutionary equilibrium, the survival costs
of an ornament should balance the reproductive advantages (Kirkpatrick, 1982).
Runaway can happen in any sensory modality. Animals' eyes respond to color and form
on tails and faces; ears respond to loud complex songs by birds and whales; noses
respond to intense pheromones such as musk deer scent; skin responds to grooming,
foreplay, and genital stimulation. Electric fish may even respond to galvanic courtship
(Kramer, 1990). But there is much more to animal cognition than low-level sensation, so
courtship behaviors may have evolved to play on higher-level mental processes of
categorization, symbolism, memory, expectation, communication, and curiosity (Miller,
Runaway is a fairly robust and pervasive force that emerges even in genetic models of
indicators (Pomiankowski et al., 1991; Kirkpatrick, 1992), but it also a highly stochastic
process, quite sensitive to initial conditions and therefore capable of explaining the
capricious divergence of sexual ornamentation observed across species (Eberhard,
1985; Miller & Todd, 1993, 1995). The three basic assumptions of Fisher's model have
been moderately well-supported by recent empirical work (see Andersson, 1994): (1)
individuals with large sexual ornaments have higher mating success but lower survival
than those with smaller ornaments (all else being equal), (2) the relevant traits and
preferences show heritable genetic variation, and (3) there is genetic linkage between
the relevant traits and preferences (e.g. Bakker, 1993; Houde & Endler, 1990). The
runaway process is also supported by findings that some animals copy each others'
mate choices, as if following an arbitrary fashion rather than a reliable indicator
(Balmford, 1991; Dugatkin, 1992; Pruett-Jones, 1992).
3.4 Selection for sperm competition
Sexual selection does not stop when copulation begins. Indeed, gonads and genitals
are the clearest expressions of sexual selection, because they are most directly
responsible for fertilization, and they typically serve no survival functions. The traditional
view that `primary sexual characters' such as penises are "necessary for breeding and
hence are favored by natural selection" (Andersson, 1994, p. 14) is misleading. If sexual
competition and mate choice can affect genitals, then genitals can be shaped by sexual
In many species, females mate with more than one male, so sperm competition
becomes important: males evolve larger testicles, larger ejaculates, faster-swimming
sperm, various devices to remove previous competitors' sperm from the female
reproductive tract, and various plugs to keep future competitors' sperm excluded entirely
(Smith, 1984; G. Parker, 1984). The results can be dramatic: the male North Atlantic
right whale reputedly has 2000-pound testicles to pump out gallons of semen and
billions of sperm per ejaculate.
In primates, testicle size increases with intensity of sperm competition across species
(Harcourt & Harvey, 1984); female chimpanzees are highly promiscuous, so male
chimpanzees have evolved large 4-ounce testicles. Male humans have medium-sized
testicles by primate standards, and produce a respectable 400 million sperm per
ejaculate, suggesting that ancestral females had multiple lovers within a month fairly
often (Baker & Bellis, 1995).
Female choice does always not stop when copulation starts, either. Eberhard (1985)
has argued that male genitals often function as `internal courtship devices' to stimulate
females into accepting sperm from the copulating male. The length, variety, and vigor of
human copulation suggests that this type of internal courtship has been highly
elaborated in our species. Human female orgasm may function partially to suck sperm
into the uterus, thereby promoting fertilization by sexually exciting males (Baker & Bellis,
3.5 Selection for provisioning, territories, and protection
Females can gain nongenetic benefits from mate choice by favoring males that offer
material gifts (Searcy, 1982). The main examples of such provisioning come from male
insects giving nuptial gifts such as spermatophores or caught prey (see R. Thornhill &
Alcock, 1983), male birds provisioning offspring and building nests in socially
monogamous bird species (see Clutton-Brock, 1991), and sex-for-meat exchanges (e.g.
prostitution and marriage) in humans(H. Fisher, 1982, 1992). Male provisioning is
useful to females because it eases the nutritional and energetic burden of producing
eggs, gestating young, and feeding them. But male provisioning of females during
courtship is not common across species, and male provisioning of offspring after birth is
quite rare except in monogamous birds (Clutton-Brock, 1991). Often, male provisioning
may represent mating effort more than paternal effort, if females prefer males that have
provisioned previous offspring (Seyfarth, 1978; Smuts, 1985). Biologists may often
mistake the grudgingly generous step-father for the committed dad.
Selection for direct provisioning must not be confused with the more common pattern of
selection for good territories that happen to be defended by particular males.
Socioecologists have long recognized that female animals tend to distribute themselves
around their habitat to exploit the available food resources and protect themselves
against the local predators, and the males distribute themselves to exploit the available
females as reproductive resources (Davies, 1991; Dunbar, 1988). In such cases, males
often fight to exclude competitors from prime territories, and females prefer to mate with
males that hold prime territories. Such systems are called `resource defense polygyny',
because males that are successful at excluding other males from areas desired by
females will reap a disproportionately high number of offspring with multiple females.
Male territoriality can be viewed in two ways: as female choice in favor of sexy, healthy,
high-status, land-holding aristocrats, or as female acquiescence to a Machiavellian
protection racket, where violent, harassing males extort sex for access to food, and then
leave females with all the burdens of parenting.
Males can sometimes serve as convenient if unreliable protectors from predators or from
other males. Thus, mate choice in favor of protectors is especially favored in species
where females and/or infants are subject to strong predation risk or strong risk of
infanticide by rival males (see Hausfater & Hrdy, 1984). Again, the protection racket
metaphor may be apt: males extort sex in exchange for a commitment not to kill a
female's offspring, and a willingness to keep other males from killing them. Moreover,
How mate choice shaped human nature
much of what appears to be `protection' behavior by males may function as mate-
guarding to minimize sexual competition from rival males, and may not reflect female
choice. Biologists have recently begun taking a darker view of male provisioning,
territoriality, and protection behavior — a view surprisingly concordant with recent
feminist analyses of human patriarchy, prostitution, marriage, sexual exploitation, and
the economic oppression of women (see Brownmiller, 1975; Buss & Malamuth, in press;
Haraway, 1989; Lancaster, 1991; Smuts, 1991).
3.6 Summary of sexual selection modes
The scope of sexual selection through mate choice is rather broad: it can operate in
almost any animal species capable of making discriminations among potential mates
and in responding more positively towards some than towards others. Mechanisms that
cause selective mating can arise from several sources, both as adaptations in their own
right and as side-effects of other adaptations (e. g. as sensory biases). Once in place,
these mechanisms can influence the evolution of sexual ornamentation and courtship
behaviors. If the selected trait correlates with general viability as a conditional or
revealing handicap, and if genetic variance in viability is maintained somehow, e.g. by
biased mutation or coevolution, then Zahavi's handicap principle will work to elaborate
both trait and preference (Iwasa et al., 1991). Even if the selected trait is purely
ornamental and does not correlate with general viability, as long as genetic variance in
the trait is maintained, then Fisher's runaway process can elaborate both trait and
preference (Pomiankowski et al., 1991). Often, Zahavi's and Fisher's processes will be
mutually reinforcing, such that a trait is elaborated both as a viability indicator that
increases offspring survival rates and as an aesthetic ornament that increases offspring
attractiveness. For example, "a peacock's tail is, simultaneously, a testament to
naturally selected female preferences for eye-like objects, a runaway product of despotic
fashion among peahens, and a handicap that reveals its possessor's condition."
(Ridley,1993, pp.161-162). These processes can operate even in the face of substantial
natural selection to evolve costly male traits and costly female preferences. Sexual
selection will work in pseudo-monogamous and polygynous species through differential
mating success, and will work in truly monogamous species if animals mate assortatively
with respect to viability-indicators, or if animals that mate earlier have more offspring
(Darwin, 1871; Kirkpatrick, Price, & Arnold, 1990).
4 Sex differences and sexual selection
Sexual selection through mate choice would be expected to operate in any sexually-
reproducing lineage, regardless of whether there were distinct sexes such as males and
females. If hermaphrodites exercise mate choice, they can evolve sexual ornaments.
Thus, sexual selection does not necessarily require or produce sex differences.
However, in almost all sexually-reproducing lineages on earth, distinct sexes have
evolved, consisting of `males' that produce small gametes called sperm and `females'
that produce large gametes called eggs. Bateman (1948) and Trivers (1972) pointed out
that since females invest more matter and energy into producing each egg than males
invest in producing each sperm, eggs form more of a limiting resource for males than
sperm do for females. Thus, males should compete more intensively to fertilize eggs
than females do to acquire sperm, while females should be choosier than males. Males
compete for quantity of females, and females compete for quality of males. In short,
males court, and females choose (see Daly & Wilson, 1983; Reynolds & Harvey, 1994;
In female mammals the costs of internal fertilization, gestation, and long-term lactation
are especially high, leading to even more striking differences between male
competitiveness and female choosiness. For example, the minimum parental
investment by female humans under ancestral conditions would have been a harrowing
9-month pregnancy followed by at least three years of breast-feeding and baby-carrying
(Shostak, 1981); whereas the minimum paternal investment would have been a few
moments of copulation and a teaspoonful of semen (Symons, 1979). The result is an
enormous difference in maximum lifetime reproductive success. King Moulay Ismail the
Bloodthirsty, a medieval despot of Morocco, sired over 800 children by the women in his
harem, and the first emperor of China, around 3000 years ago, was reputed to have
sired even more through his much larger harem (Betzig, 1986). By contrast, the world
record for a woman is 69 children, many of which were triplets (Daly & Wilson, 1983).
Even under relatively egalitarian tribal conditions, some men can father several dozen
children by several different women, whereas no woman bears more than 10 or so
children (Chagnon, 1983).
Thus, a man's reproductive success generally increases with his number of sexual
partners (in the absence of contraception), whereas a woman reaches her reproductive
limit rather quickly as her number of sexual partners increases. This is because males
can opt out of parental investment in a way that women cannot — nature can't enforce
child support laws any better than modern governments. Of course, women under
ancestral conditions probably used abortion and infanticide to avoid maternal investment
during difficult times (see Hausfater & Hrdy, 1984), but they could not induce another
woman to bear a child for them. Maternal investment was obligatory in hominids;
paternal investment was not.
There are usually trade-offs between courtship effort and parental effort. Males usually
invest more in the former, and females more in the latter. In females, the marginal costs
of sexually-selected traits will be higher (because the demands of maternal investment
push females closer to their physiological limits), and their benefits will be lower
(because males are less choosy), so females often invest less time and energy in
growing and displaying such traits than males do. The result is sexual dimorphism: a
sex difference in the expression of the courtship or reproductive trait. The most ancient
and reproductively central sexual dimorphisms are usually qualitative: males have
testicles, whereas females have ova. More recently evolved courtship traits usually
retain only quantitative dimorphism: many male birds have longer, brighter feathers than
females, but females often retain some discreet ornamentation. Female mammals have
breasts, but males retain nipples. All the qualitative sexual dimorphisms started out as
Sexual dimorphism is a common but not necessary outcome of sexual selection. Two
major factors limit sexual dimorphism: the mutuality of mate choice, and `genetic
linkage' between the sexes. The effects of mutual choice are easy to understand: if
both males and females are somewhat choosy and somewhat competitive, as in many
monogamous species, then sexual selection will apply to males and females roughly
equally, and sexually selected ornaments and indicators will evolve to similar
magnitudes in each. Whenever males must invest time, effort, and energy in courtship,
How mate choice shaped human nature
they have incentives to be at least slightly choosy about which females they choose to
court — but male choice has been studied only rarely, and may have often been
overlooked. For example, Trail (1990) observed that in about a quarter of lek-breeding
birds (which provide the best opportunities for female choice), ornaments are equally
elaborate in males and females, suggesting that male choice was operating as well.
Also, whenever high-quality males are in short supply, females have an incentive to
compete with each other to attract and retain such males. Competition to retain the
paternal investment and protection of male partners will also lead to substantial variance
in the number of offspring raised to maturity by females; measuring variance in number
of offspring born would completely miss a major stage of female reproductive
competition, which occurs after birth (Dunbar, 1988; Miller, 1993). Males also vary less
in their lifetime reproductive success than in their day-by-day success, because male
success follows a typical life-history trajectory (adolescent frustration, young adult
violence, older adult coalition-building, and gradual senescence). Therefore, short-term
measures will over-estimate variance in male reproductive success and under-estimate
female variance (Dunbar, 1988). Thus, sexual selection often applies to both sexes, and
can drive the evolution of indicators and aesthetic displays in both sexes.
Especially under monogamy, mutual mate choice can yield strong sexual selection
without much sexual dimorphism. Sexual selection can work in monogamous species if
the sex ratio is skewed, if extra-pair copulations undermine the putative monogamy, or,
most importantly, if mates differ in genetic quality (Darwin, 1871; R. A. Fisher, 1930).
For example, if animals mate assortatively with respect to quality (e.g. the healthy marry
each other, leaving the unhealthy no option but to marry other unhealthies), then
indicators of genetic quality can still evolve under sexual selection (see Darwin, 1871).
Thus, traits that improve the ability to compete for mates will be favored even under strict
monogamy (Jones & Hunter, 1993; Kirkpatrick et al., 1990). Moreover, female
competition over mates will be stronger under monogamy, so females may evolve
ornaments as extravagant as those of males. Mate choice must also be somewhat
mutual in species that use interactive courtship displays (which ethologists used to call
`pair-bonding rituals'), such as coordinated dances, song duets, mutual sexual foreplay,
and conversations (Miller, 1993). Only the pure Fisherian runaway process is
undermined by monogamy, because it depends on some individuals obtaining a
disproportionate number of mates.
Genetic linkage between the sexes also constrains the evolution of sexual dimorphism.
Because males and females within a species grow from very similar genes and
developmental mechanisms, most traits are homologous (developmentally and
anatomically similar) across sexes, and the male trait cannot initially evolve separately
from the female trait. This constraint holds for any traits that still have quantitative rather
than qualitative sexual dimorphism. Sons will tend to inherit their mothers' mate
preferences, and daughters will tend to inherit their fathers' sexually-selected traits.
Darwin (1871) called this the "Law of Equal Inheritance": all else being equal, even if
only one sex is exercising selective mate choice, both the selected traits and the
selective preferences will tend to be expressed in both sexes. For example, if female
choice favored large penises over many generations in some species, the clitoris
(female homolog of the penis) would tend to enlarge along with the penis, assuming no
other selection operated on the clitoris.
Lande (1980, 1987) showed that this sort of genetic linkage between the sexes makes
the evolution of sexual dimorphism a very slow process. Typically, sexual dimorphism
evolves a few orders of magnitude slower than sexually-selected traits themselves do.
For example, Rogers and Mukherjee (1992) applied Lande's model to data on the cross-
sex heritability of human height and other body dimensions, and found that if female
choice alone were favoring tall males, and males were not selecting females for height,
sexual dimorphism in height would evolve around 65 times slower than height itself. That
is, female height would increase over 98% as fast as male height increases, purely as a
correlated response to selection on males. This argument also applies to sexually-
selected behavioral and mental traits: any female choice for some courtship capacity in
the male would be expected to produce a correlated response in the female. In an
extraordinary passage, Darwin revealed his belief in the importance of mate choice in
human mental evolution, and in the importance of genetic linkage between the sexes:
"It is fortunate that the law of equal transmission of characters to both sexes
prevails with mammals; otherwise it is probable that man would have become as
superior in mental endowment to woman, as the peacock is in ornamental
plumage to the peahen." (Darwin, 1871)
Now that we have reviewed sexual selection theory, we can explore how that theory
applies to primates, hominids, and modern humans.
5 Sexual selection in primates
To a first approximation, ecological circumstances determine mating patterns in
primates. Generally, the distribution of food determines the distribution of females, and
the distribution of females determines the distribution of males. When females must
forage on their own, males disperse to pair with the lone females, giving rise to
monogamy; this pattern is fairly rare in primates, being restricted to gibbons, some
lemurs, and some African and South American monkeys. When females can afford to
forage in small groups to protect each other against female competitors, predators
(Dunbar, 1988) and infanticide by strange males (Hrdy, 1979), a single male can exclude
other males from each female group, giving rise to the common `harem system' of
unimale polygyny, as in hamadryas baboons, colobus monkeys, some langurs, and
gorillas. Unimale polygyny usually imposes strong sexual selection for aggressiveness,
including male size, strength, and weaponry (e.g. large canine teeth), resulting in high
degrees of sexual dimorphism in body size and behavior. When females can forage in
larger groups (of more than 10 or so) males must usually form coalitions to exclude other
males from the female group, resulting in a complex system of multimale polygyny, as in
some baboons, macaques, ring-tailed lemurs, howler monkeys, and chimpanzees. In
multimale polygyny, males compete at several levels: female promiscuity leads to sperm
competition; female preferences for dominant males lead to status competition,
individual aggressiveness, and coalition-formation; and female preferences for nice
males lead males to groom females, protect their offspring, and guard them from other
males (see De Waal, 1989). Hominids and humans probably evolved in fairly large
groups under multimale polygyny, so we will focus here on sexual selection in large-
Male primates fight more often and more intensely when estrus females are in their
group. These fights usually result in a linear dominance hierarchy among the males,
with high-ranking males usually obtaining more matings because they can chase lower-
ranking males away from estrus females (Silk, 1987). However, lower-ranking males
How mate choice shaped human nature
can use a number of alternative mating strategies, because females often prefer novel
males, long-term friends, and ex-dominant older males, to the currently dominant male
(Smuts, 1985, 1987). Sometimes, these alternative strategies are as successful as
achieving high dominance rank, though they may often be making the best of a bad
situation. Males can also form coalitions to take over groups, repel outside males,
achieve higher dominance rank within groups, and acquire estrus females (Smuts,
1987). Male primates often use different strategies at different ages, as their physical
and social powers wax and wane (Dunbar, 1988).
Given multi-male, multi-female primate groups, how does mate choice work? Female
primates can exercise choice by joining groups that contain favored males, initiating sex
with them during estrus, supporting them during conflicts, and developing long-term
social relationships with them. Females can reject disfavored males by refusing to
cooperate during copulation attempts, driving males away from the group, or leaving the
group. But female mate choice criteria remain obscure for most primate species. In
contrast to modern humans, female primates rarely favor males that can provide
resources or paternal care of offspring. The sporadic male care that is observed, such
as watching, carrying, and protecting infants, may represent mating effort rather than
paternal investment (Seyfarth, 1978; Smuts, 1985), because it is often performed by a
male unlikely to be the father of the infant, who is interesting in mating with the infant's
mother. Rather, the only consistent female preferences observed have been for (1)
high-ranking males capable of protecting females and offspring from other males, (2)
specific males with whom a special short-term consortship or long-term friendship has
been formed through mutual grooming and affiliation, and male food-giving and infant
protection, and(3) new males from outside the group, to avoid inbreeding and protect
against the infanticide they might commit if they knew that none of the local offspring
were theirs. Clearly, these criteria conflict somewhat: high-ranking males have
insufficient time to maintain special friendships with all local females; and new males by
definition cannot yet be long-term friends, nor can they attain high-rank immediately. In
addition, females may choose to mate promiscuously, to maximize sperm competition
and to confuse paternity, thereby inducing several males to protect the offspring, and to
guard against infanticide (Hrdy, 1979; Small, 1993). Despite Darwin's (1871) discussion
of sexual selection for the various beards, tufts, and colorful hair styles that adorn male
primates, female choice for aesthetic displays and indicators has rarely been
investigated in primates, perhaps because the relevant sexual selection theory has been
developed only recently. However, female primates often exhibit preferences that
cannot be accounted for on the basis of male rank, age, novelty, grooming effort, or
protection effort. Sometimes, primates just seem to like each other based on
appearance, behavior, and personality.
Although primates follow the general animal pattern of male sexual competition and
female choosiness, female competition and male mate choice is also important (Smuts,
1987). In monogamous callitrichids such as marmosets and tamarins, female compete
to form pairs with quality males and drive off competing females. In unimale polygynous
systems, the dominant male's sperm becomes a limiting resource, and high-ranking
females prevent low-ranking females from mating through aggression and harassment
(Small, 1988). In multimale groups, females compete to form consortships and
friendships with favored males. Such patterns of female competition suggest some
degree of male mate choice. When the costs of sexual competition and courtship are
high, as they are for most primates, males have incentives to be choosy about how they
allocate their competitive effort, courtship effort, and sperm among the available females.
Males compete much more intensely for females that show signs of fertility such as
sexual maturity, estrus behavior, absence of lactation, and presence of offspring (Smuts,
1987). Male primates almost always avoid adolescent, low-ranking, nulliparous (no-
offspring) females and prefer older, high-ranking, multiparous (several offspring) females
who have already demonstrated their fertility, viability, social savvy, and mothering skills.
Marriage (i.e. legally-imposed, life-long monogamy) has overturned this male mate
choice pattern in modern human societies by pushing males to compete for unmarried,
nulliparous young women of unproven fertility and uncertain status — a recent pattern
that Symons (1979), Buss (1989, 1994), and others have projected into the ancestral
past. Like females, male primates also show strong individual preferences for particular
mates with whom they have developed special relationships (Smuts, 1985). The myth
that romantic love is a recent invention of Western patriarchy denies not only the warm
sexual relationships of humans in other cultures and historical epochs, but also those of
other primate species.
In summary, sexual selection in multi-male, multi-female primate groups is intense
because the social context of mating is so complex and dynamic. Both sexes compete,
both sexes are choosy, both sexes have dominance relations, and both sexes form
alliances. Sexual relationships develop over weeks and years rather than minutes.
Under these relentlessly social conditions, reproductive success came to depend on
mental capacities for "chimpanzee politics" (De Waal, 1982, 1989), "Machiavellian
intelligence" (Byrne & Whiten, 1988), "special friendships" (Smuts, 1985), and creative
courtship (Miller, 1993) rather than simple physical ornaments and short-term courtship
behaviors as in most other animals.
6 Sexual selection in hominids
It is hard to reconstruct sexual selection patterns in extinct animals because mate
preferences and courtship behaviors don't fossilize. However, it seems reasonable to
suppose that the primate tradition of intense sexual selection within highly social groups
persisted in our hominid ancestors, with ever-larger group sizes, and ever-more-complex
relationships and sexual strategies. We are the products of this primate heritage
refracted through a unique hominid sequence of habitats and econiches (Foley, 1987),
combined with the unpredictable effects of runaway social competition for Machiavellian
intelligence (Byrne & Whiten, 1988; Dunbar, 1992), and runaway sexual selection for
various courtship behaviors (Miller, 1993).
Fossils and genetic markers suggest that hominids diverged from other anthropoid apes
around 6 million years ago (mya), leading to increasing bipedalism, group size, and
omnivory in the sequence Australopithecus ramidus , A. afarensis, and A. africanus.
By two mya, hominids had divided into two main branches (see Foley, 1987):
Paranthropus (also known as Australopithecus), including P. robustus and P. boisei,
and Homo, including successively Homo habilis (2.0-1.8 mya), Homo erectus (1.8-0.5
mya), and Homo heidelbergensis (400,000-120,000 years ago). This latter type split into
two species (see Stringer & Gamble, 1993): the Neanderthals (Homo neanderthalensis,
200,000-40,000 years ago), and modern Homo sapiens (120,000 years ago to the
present). Because the Pleistocene period covers the era from two mya until recently,
and Homo sapiens probably evolved and migrated out of Africa quite recently (see
Gamble, 1993), hominids and humans are largely a product of Pleistocene Africa.
How mate choice shaped human nature
Mating among our ancestors probably occurred in the context of small, mobile hunter-
gatherer tribes. As with most primates, social life was probably centered on matrilines
(female kin groups and their offspring), with the males largely fending for themselves,
hovering around the periphery, and trying to insinuate themselves into the powerful
female bands (see Dunbar, 1988). Under these conditions, the central mating problem
for males was inseminating mature, attractive, viable, fertile females (Buss & Schmidt,
1993). The central mating problem for females was obtaining good sperm and good
genes from high-quality males, and perhaps some provisioning and protection from a
few males whose presence was not more trouble than it was worth. Equally unlikely are
the tough-minded view of the Pleistocene as a brutal, male-dominated era of continuous
warfare, frequent rape, and anarchy (e.g. Ardrey, 1976), and the tender-minded picture
of life-long pair-bonded monogamy and heavy male investment (Lovejoy, 1981). Male
scientists have been reluctant to recognize that, for the most part, adult male hominids
must have been rather peripheral characters in human evolution, except as bearers of
traits sexually-selected by females for their amusement value or utility.
Hominids probably did not live in discrete tribes with mutually-exclusive and stable
memberships, well-defined territories, or coherent group movements. Social
organization was more complex and multi-layered, as it is in other primates (Dunbar,
1988). Thus, mates may have been chosen not from within the small bands that
characterize day-to-day foraging, but from the much larger congregations that occurred
at special times (e.g. food-rich seasons, or “harvest carnivals”) and places (e.g. water
sources, or “beaches”). Social and sexual relations were probably at least as fluid,
complex, and ad hoc as they are today, with plenty of polygamy, serial monogamy and
infidelity(see H. Fisher, 1992; Ford & Beach, 1951; Lockard & Adams, 1991; Shostak,
1981). Without marriage, mortgages, or money, why stick with just one lover during a
lifetime? Given this social complexity and fluidity, each sex probably evolved a multitude
of flexible strategies for pursuing their mating goals (Buss & Schmidt, 1993; Simpson &
Gangestad, 1992). An individual's current strategy might depend on their personal
attributes (e.g. age, health, attractiveness, parenting skill, social skills, and seduction
skills), the state of their kin network and social network(e.g. number of dependable
child-care helpers), and various ecological conditions(e.g. reliability and patchiness of
resources, foraging costs and dangers) and demographic conditions (e.g. operational
Primates and especially hominids are extremely `K-selected' taxa: we have much
slower development, larger bodies, fewer offspring, higher survival rates, and longer
lifespans than more `r-selected' taxa such as insects, fish, or rodents (Harvey, Martin, &
Clutton-Brock, 1986). The more K-selected the species, the more important sexual
selection usually becomes compared to natural selection (Miller & Todd, 1995). We
might expect that as hominids evolved to be more and more K-selected, the relative
importance of sexual selection increased. K-selection usually reduces the relative
energetic demands of reproduction on the female and almost eliminates the need for
male help, because slow gestation spreads maternal investment over a longer period,
and small litters of large, well-developed offspring are easier to care for. However,
human brains grow so large that infants must be born relatively immature to fit through
the female pelvic canal: "human gestation is really 21 months long, with 9 months in the
uterus followed by 12 months in the mother's care" (Martin, 1992, p. 87). The
helplessness and expense of human infants increases both the nongenetic and genetic
benefits from mating: choosing males for their provisioning and protection ability eases
the energetic burden of motherhood, but choosing males for their indicators of genetic
quality and their aesthetic displays reduces the risk of producing sickly, unattractive
offspring that may never reproduce. Thus, whereas infant dependency favors male
provisioners, infant expense favors males with good genes and good displays. Foley
(1992) provides life-history and nutritional evidence that the latter was more important:
human infants do not grow using more energy per month than other ape infants, as
paternal provisioning would have made possible — they simply grow for a longer time.
Such data undermine the common assumption that male hunting was somehow
important in feeding infants and mothers, and in supporting the energetic costs of
encephalization (cf. Buss, 1992, 1994; Lovejoy, 1981; Knight, Power, & Watts, 1995).
Many people assume that the opportunities for mate choice would have been severely
limited under ancestral conditions, due to the supposed prevalence of arranged
marriages, the exchange of women as chattel between families and tribes, the influence
of cultural rules concerning incest, outbreeding, marriage, monogamy, and adultery, and
the generally low status of women under patriarchy. But there is good archaeological
and ethnographic evidence that many of these factors arose within the last 10, 000
years, where they arose at all (see H. Fisher, 1992). The economic and geographic
demands of agriculture distorted human mate choice patterns, because agriculture
requires long-term investment in preparing and maintaining a plot of land, and thereby
reduces the physical and social mobility that underlay the free choice of sexual mates in
hunter-gatherer tribes. Modern mating behavior may not accurately reflect ancestral
patterns of sexual selection. But we will turn next to modern human morphology —
which, being less influenced by culture, is more reliable evidence of ancestral mate
7 Sexual selection and human morphology
Humans show sexual dimorphism in several traits. Compared to females, males on
average have more height and mass, more upper-body strength, higher metabolic rates,
more facial and bodily hair, deeper voices, larger brains, and riskier life histories, with
higher juvenile mortality, later sexual maturity, and earlier death (Ankney, 1992; Daly &
Wilson, 1983, 1988; Ghesquiere, Martin, & Newcombe, 1985; Rushton, 1995; Short &
Balaban, 1994). Our moderate size dimorphism is consistent with our species having
evolved under a moderately polygynous mating system, with more intense sexual
competition between males than between females (Fleagle, Kay, & Simons, 1980;
Martin, Willner, & Dettling, 1994). But human bodies reveal much more than just the
degree of ancestral polygyny; they indicate a wide array of mate choice criteria used by
our male and female ancestors.
Compared to other anthropoid apes, humans have less hair on our bodies, more on our
heads, whiter eyes, longer noses, larger ear lobes, more everted lips, smaller and safer
teeth, more expressive faces, more dextrous hands, and better-developed pheromone
systems (Margulis & Sagan, 1991; Miller, 1993; Morris, 1985; Napier, 1993; Stoddart,
1990). Also, male humans have rather long, thick, and flexible penises compared to
other primates, larger beards, and sometimes baldness later in life; female humans
have greatly enlarged breasts and buttocks, a greater orgasmic capacity, and continual
`sexual receptivity' throughout the monthly cycle. Many of these traits show hallmarks of
having evolved under the capricious power of sexual selection: they are uniquely
elaborated in our species, show considerable sexual dimorphism, are grown only after
puberty (sexual maturity), become engorged and displayed during sexual arousal, are
How mate choice shaped human nature
manifestly valued as sexual signals, and are selectively elaborated through ornament
and make-up (Miller, 1993; Morris, 1985). Such traits probably evolved both as
indicators (of fertility, viability, age, health, and lack of infestation by pathogens and
parasites) and as aesthetic displays (that play upon pre-existing or co-evolved
perceptual biases). Sexual selection research has focused particularly on the human
face, the breasts and buttocks, the penis, and the clitoris. These will be examined in
The human face is a major target of selective mate choice during all stages of courtship,
from flirtation through face-to-face copulation. Research on human facial aesthetics has
boomed in the last few years (Alley & Cunningham, 1991; Brown & Perrett, 1993;
Langlois & Roggman, 1990; Perrett, May, & Yoshikawa, 1994), revealing that average
faces are attractive, but that females with more `neotenous' (child-like) faces, including
large eyes, small noses, and full lips, are still more attractive, as are males will
testosterone-enlarged features such as high cheekbones, strong jaws, strong chins, and
large noses (R. Thornhill & Gangestad, 1993). Bilateral symmetry is another important
determinant of facial beauty, because symmetry correlates with "developmental
competence"— resistance to disease, injury, and harmful mutations that cause
"fluctuating asymmetry" during development (Moller & Pomiankowski, 1993; R. Thornhill
& Gangestad, 1993). Also, as Darwin (1872) emphasized, human facial musculature is
uniquely well-developed for displaying a variety of expressions, many of which are used
Darwin (1871) assumed that genitals evolve purely through natural selection for
fertilization ability, but Eberhard (1985, 1991) has demonstrated a substantial role for
female choice in the evolution of male genitalia. The human penis is a prime example:
men have the longest, thickest, and most flexible penises of any living primate. Gorillas,
orangutans, and chimpanzees have very thin `filiform' penises less than three inches
long when fully erect, and made rigid by muscular control combined with a baculum
(penis bone). By contrast, human penises average over 5 inches long and one and a
quarter inches in diameter, and use an unusual system of vasocongestion (blood
inflation) to achieve erection(Sheets-Johnstone, 1990; Short, 1980). The size and
flexibility of the human penis is more likely the result of female choice than sperm
competition because sperm competition generally favors large testicles, as in the small-
penised chimpanzee (Baker & Bellis, 1995; Harcourt & Harvey, 1984; G. Parker, 1984;
The female clitoris is anatomically homologous to the male penis, and although its
structure probably did not evolve directly under male mate choice, clitoral orgasm has
two important roles in sexual selection. First, as a female mate choice mechanism,
clitoral orgasm favors males capable of providing high levels of sexual stimulation. Over
the short term, orgasm promotes vaginal and uterine contractions that suck sperm into
the uterus and minimize post-coital `flowback' therefrom (Baker & Bellis, 1995). Over
the long term, pleasurable orgasms promote future copulations with the favored male
through reinforcement learning and emotional attachment. Some male scientists (e.g.
Gould, 1987; Symons, 1979) have questioned whether human female orgasm is an
adaptation at all, because it can be hard to achieve. But is makes sense for a `choosy
clitoris' to produce orgasm only given substantial foreplay and emotional warmth,
because this would reinforce only sex with males who have the willingness and skill to
provide the right kinds of sexual stimulation. Thus, the sexual dimorphism between
penis and clitoris could be viewed as a direct physical manifestation of the two
components of Fisher's runaway process: a highly elaborated male trait (the penis)
designed to stimulate, and a highly discerning female preference (the clitoral orgasm)
designed to respond selectively to skillful stimulation. The second role for orgasms is to
advertise happiness to lovers. Given the fact that orgasms come hard, only when sex is
long, varied, and exciting rather than brief, mechanical, and perfunctory, orgasms can
serve as fairly reliable indicators of female sexual satisfaction, commitment, and fidelity.
Thus, some aspects of female orgasm may have evolved through male mate choice to
promote male certainty of paternity (and hence male protection and investment). If so,
we can understand why females advertise their orgasms through clear tactile, visual,
and auditory signals such as strong vaginal contractions and hip movements, the sexual
blush over face and chest, and passionate vocalizations (see Morris, 1985).
Female human breasts and buttocks have undergone sexual elaboration through mate
choice by males. These organs store substantial amounts of fat, so could function as
indicators of female nutritional status and hence fertility (Low, Alexander, & Noonan,
1987; Szalay & Costello, 1992). Singh (1993) showed that males prefer women who
display a low waist-to-hip ratio (WHR), ideally about 0.70, concordant with enlarged
buttocks indicating sufficient fat reserves, and a narrow waist indicating non-pregnancy.
Permanent enlargement of breasts and buttocks is also fairly effective at concealing
ovulation (Margulis & Sagan, 1991; Szalay & Costello, 1992). Females who do not
reveal their menstrual or lactational cycles may benefit from male uncertainty by being
able to solicit male attention and investment even when they are not really fertile: "From
hairy, flat-chested ape to modern buxom woman ... males were kept guessing about
when females were ovulating" (Margulis & Sagan,1991, p. 96). More generally, the loss
of a specific estrus period, combined with `concealed ovulation' and `continuous sexual
receptivity', may have allowed females to attract more continuous attention (e.g.
protection, provisioning, social support) from males even when they were not ovulating
(Alexander & Noonan, 1979; H. Fisher, 1982; Hrdy, 1981, 1988; Hrdy & Whitten, 1987;
Sexually-selected morphological features are important to the study of evolution and
human behavior for three main reasons. First, there is no sharp division between body
and brain: apparently simple bodily adaptations also have physiological, neurological,
and psychological features. The richly-innervated penis, clitoris, nipple, and mouth are
as much psychological organs as physical objects. Second, the mate choice
mechanisms that assess bodily features are easy to study experimentally and may lead
to insights about mate choice with respect to more complex mental and behavioral traits.
Finally, body features reveal patterns of ancestral mate choice relevant to understanding
human mental evolution. Mate choice by males has shaped female breasts, buttocks,
and orgasms; mate choice by females has shaped male body size, beards, and penises.
Mutual mate choice has probably influenced human hair, skin, eyes, lips, ears, face
shape, hands, and pheromones. If our male and female ancestors were both selecting
for bodily traits, it seems likely that they were also both selecting for mental and
behavioral traits. By overcoming the Cartesian split between body and mind, we can
better appreciate the role of mate choice in shaping both.
8 Sexual selection and human mental evolution
"Most evolutionary anthropologists now believe that big brains contributed to
reproductive success either by enabling men to outwit and outscheme other men
How mate choice shaped human nature
(and women to outwit and outscheme other women), or because big brains were
originally used to court and seduce members of the other sex."(Ridley,1993, p.
Could sexual selection have shaped not only the human body, but the human mind?
Darwin (1871) clearly thought so, but most 20th century theorists have viewed natural
selection as the exclusive director of human mental evolution. Even those who granted
a role to sexual selection focused more on male sexual competition than on mate
choice. Chance (1962) suggested that sexual selection would have favored young
males that show intelligence and caution in challenging dominant males, and in forming
coalitions to take territories and intimidate females. Fox (1972) argued along similar
lines that sexual selection would have favored male hunting prowess, leadership, and
tool making. Alexander (1971) viewed organized warfare for possession of females and
mating-relevant resources as a major force in human evolution. Caspari (1972)
considered oratory as an arena of male competition and suggested a role for sexual
selection in the evolution of language. In a fairly sketchy but provocative paper, S.
Parker (1987) proposed that sexual selection could help account for the evolution of
bipedalism, canine reduction, tool-making, fire-using, shelter-construction, and language.
This emphasis on male competition made sense when Darwin's theory of female choice
was still considered unfounded. But given the resurgence of interest in mate choice in
other species, perhaps the role of mate choice in human mental evolution deserves
But why bother with sexual selection? What's wrong with the traditional story that
natural selection just generally favored intelligence, learning, tool-making, and culture?
The problem is that the evolution of big brains is so rare, so recent, so capricious, and
seemingly so unrelated to the demands of habitat or econiche (Miller, 1993). Brain size
in our lineage has tripled over the last two million years, reflecting the evolution of
unprecedented mental and behavioral capacities. Over three million years ago, our
ancestors were already successful, social, fairly bipedal, tool-making hunter-gatherers
on the African savanna — and they had brains only slightly larger than the
chimpanzee's. Then, two million years ago, for no apparent reason, brain size started
growing exponentially in our lineage but not in other closely-related hominid species who
shared the same habitat, such as Paranthropus boisei and robustus. Encephalization
then stopped about 100, 000 years ago, again for no apparent reason, long before the
Neolithic revolution in technology and art 40, 000 years ago. Extreme encephalization
also happened in some species of cetaceans (dolphins and whales) and proboscids
(elephants) living in quite different environments, but has not occurred in other primates
living in quite similar environments (e.g. baboons, chimpanzees, Paranthropus
The speed, uniqueness, and capriciousness of this encephalization process has
prompted many theorists to accept that human mental evolution must have been driven
by some sort of positive-feedback process that is sensitive to initial conditions. There
have been two traditional contenders. In the runaway social competition model (Byrne &
Whiten, 1988; Humphrey, 1976; Whiten, 1991; also see Miller, in press), hominids got
smarter to predict and manipulate each others' behavior, leading to a social-intelligence
arms race between mind-reading and deception. In the runaway gene-culture co-
evolution model, hominids got smarter to learn and use material culture (e.g. tools and
survival techniques), which was itself evolving (Durham, 1991; Lumsden & Wilson, 1982;
Wills, 1993). Yet these theories overlook the clearest and best-established case of
positive-feedback evolution in nature: runaway sexual selection. The runaway process
is a good fit to the human evolution data because it begins and ends unpredictably,
without much relation to the external environment, but it is extremely powerful and
directional once underway (Miller, 1993; Miller & Todd, 1993).
As we saw in section 6, hominid social life probably allowed considerable scope for mate
choice by both males and females. Our ancestors lived in hunter-gatherer tribes that
probably had rather fluid, complex, and polygynous mating patterns, rather different from
the modern ideals of lifelong monogamy and nuclear family. The mate choice patterns
permitted by tribal life could have favored several classes of courtship behaviors that
function as indicators: `viability-indicators' that demonstrate physical health, energy, and
freedom from disease, deformity, or deleterious mutation, `age-indicators' that reveal
age, reproductive status, and survival prospects, `social-success-indicators' that reveal
social skills for dominance, competition, aggression, deception, peace-making,
communication, and unpredictability, and `cognition-indicators' that reveal mental
capacities for perception, attention, memory, planning, and creativity. In addition,
perceptual biases in mate choice would have favored `aesthetic displays' of complex,
interesting, innovative behaviors that are less closely correlated with fitness in other
domains. Together, these forms of mate choice could have set up runaway sexual
selection for more complex and creative `behavioral courtship displays' such as stories,
myths, jokes, rituals, dance, music, art, and sexual foreplay.
If the brain evolved through runaway sexual selection, what were the relevant traits and
preferences? Two uniquely elaborated aspects of the human brain are its creativity
(Boden, 1991, 1994; D. Campbell, 1960; Freyd, 1994) and its neophilia, or love of
novelty (Zuckerman, 1984). Perhaps creativity itself became a trait subject to sexual
selection by neophilia as a mate preference. More technically, mental capacities for
generating `protean' (adaptively unpredictable) courtship displays may have been
subject to `neophilic' mate preferences in both sexes (on proteanism see Driver &
Neophilia influences mate choice in many species. Darwin (1871) observed that “mere
novelty, or slight changes for the sake of change, have sometimes acted on female birds
as a charm, like changes of fashion with us.” Males of many species are more sexually
excited by novel females (Dewsbury,1981). Females of several bird species prefer
males who display larger song repertoires with greater diversity and novelty (Catchpole,
1980, 1987; Podos, Peters, Rudnicky, Marler, & Nowicki 1992). Such neophilic mate
choice may account for the creativity of male blackbirds, nightingales, sedge warblers,
mockingbirds, parrots, and mynahs. Small (1993) emphasized neophilia in primate
mate choice: "The only consistent interest seen among the general primate population
is an interest in novelty and variety." Neophilia (termed `openness') is one of the `Big
Five' personality traits in humans (see Buss, 1991), and shows moderate heritability
(Plomin & Rende, 1991; Zuckerman, 1984). Of course, in modern society, human
neophilia is the foundation of the art, music, television, film, publishing, drug, travel,
pornography, fashion, and research industries, which account for a substantial
proportion of the global economy. Before such entertainment industries amused us, we
had to amuse each other on the African savanna — and our neophilia may have
demanded ever-more creative displays from our mates. This hypothesis can explain the
mysterious `cultural' capacities that are universally and uniquely developed in humans,
such as language, music, dance, art, humor, intellectual creativity, and innovative sexual
play. These are all highly valued during mate choice and highly useful during courtship.
How mate choice shaped human nature
Such displays all use a uniquely human trick: the creative recombination of learned
semantic elements (e.g. words, notes, movements, visual symbols) to produce novel
arrangements with new emergent meanings (e.g. stories, melodies, dances, paintings).
This trick allows human courtship displays not just to tickle another's senses, but to
create new ideas and emotions right inside their minds, where they will most influence
The gradual evolution of language was especially important, because it allowed
hominids to display complex ideas and images to one another using an increasingly
complex, structured, open-ended, combinatorial system (Pinker, 1994). Language gave
potential mates a unique window into each other's minds, so allowed much more direct
sexual selection on the mind itself. Also, language permits gossip, which can transform
mate choice from an individual decision to a social decision that integrates information
from family and friends. With language and gossip, courtship displays need not be
observed directly; they need only be witnessed by someone who can talk later to
potential mates. The feedback loop between sexual selection, language complexity, and
mental complexity was probably the mainspring of human mental evolution.
The lack of sexual dimorphism in human mental capacities is not a fatal problem for this
sexual selection theory. We would expect men and women to have similar minds given
the genetic linkage between the sexes, the mutuality of mate choice, the interactiveness
of courtship behaviors (e.g. conversation, dance, and musical dueting), and the overlap
between perceptual capacities for judging complex behaviors (e.g. understanding
language) and motor capacities for generating complex behaviors (e.g. speaking
language). The general notion of mental evolution through mate choice has been
presently more fully elsewhere (Miller, 1993, 1994, 1995, in press; Miller & Pratto, 1992;
Miller & Todd, 1993, 1995; Todd & Miller, 1993).
A methodological problem arises: how could one demonstrate that a mental adaptation
really evolved through mate choice? As section 5 showed, sexually-selected human
bodily traits can be identified by being uniquely elaborated in our species, growing only
after puberty, becoming engorged and displayed during sexual arousal, being selectively
elaborated through ornament and make-up, being manifestly valued as sexual signals,
and showing sexual dimorphism. Similar criteria for special design features can also be
applied to mental and behavioral traits. If a behavior is uniquely human, is selectively
displayed by adult humans during courtship and sexual competition, is displayed in
different forms and frequencies by males and females, and is clearly valued as a sexual
display, then it is worth investigating as a sexually-selected adaptation. By these
adaptationist criteria, many aspects of human cognition and culture would thus fall
under the rubric of courtship behavior: language, art, music, humor, acting, mimicry,
metaphor, sports, games, ritual, myth, ideology, religion, politics, and science. More
generally, sexually-selected adaptations are expected to show complex organization
specially attuned to reliably, efficiently, and flexibly perform certain functions in sexual
competition and/or courtship (on adaptations, see Williams, 1966; Tooby & Cosmides,
1990). Such adaptations can also be identified through the comparative method (Harvey
& Pagel, 1991) by examining the distribution of traits across related species with known
phylogenies, to discern when and where evolutionary innovations occurred. New
methods in cognitive neuroscience (see Gazzaniga, 1995) should also allow localization
of the mental adaptations underlying these courtship capacities, and comparison to
homologous structures in other primates.
One might also check whether such adaptations are currently under sexual selection, by
seeing whether (1) the trait shows heritable variation (e.g. a moderate coefficient of
additive genetic variation — see Moller & Pomiankowski, 1993), and whether (2)
individuals exhibiting one form of the trait have greater mating success (e.g. number of
copulations, partners, or offspring) than individuals exhibiting other forms. One might
also show that individuals can (consciously or unconsciously) discriminate among
variants of the trait, and do exhibit a preference for one variant. To further establish that
a trait functions as an indicator, one must show that variants of the trait correlate with
some indicated quality such as age, health, fertility, or social status. To establish that a
trait is evolving at least partially under the runaway process, one must show genetic
linkage between the trait and the corresponding preference. However, all such
questions of utility in current societies are a bit tangential to the question of adaptive
function under ancestral conditions.
9 Sexual selection and human culture
Theories of human mental evolution are theories of human nature, and theories of
human nature are the foundation of psychology, the social sciences, and the
humanities (Tooby & Cosmides, 1992). So, if sexual selection played a major but little-
appreciated role in shaping human evolution, and if sexually-selected traits are the most
central, distinctive, and long-overlooked components of the human mind, then the
standard model of human nature used in the social sciences and humanities probably
focuses too heavily on the economics of survival and not enough on the mental,
material, and cultural demands of courtship. Sexual competition probably underlies
many political, economic, sociological, anthropological, criminological, cultural,
ideological, religious, moral, and artistic phenomena (e.g. see Barkow, 1989; Betzig,
1986, 1992; Daly & Wilson, 1988; L. Ellis, 1993; Frank, 1985; Ridley, 1993; Wright,
1994), but it has been almost entirely overlooked as an explanatory principle. Instead,
`culture' has become the dominant explanation for all human social and communicative
behavior, despite its vagueness as a scientific concept (see Cosmides & Tooby, 1994;
Sperber, 1994; Tooby & Cosmides, 1992). Rather than viewing culture as the reason for
individual human behavior, we might view culture as an emergent phenomenon arising
from sexual competition among vast numbers of individuals pursuing different mating
strategies in different display arenas.
For example, only sexual selection theory can provide a coherent, non-circular account
of `cultural dimorphism': why have males always dominated political, economic, and
cultural life in every known society? Most feminist theories of `patriarchy' simply beg the
question by viewing male power as a self-sustaining tradition, without offering any
plausible explanation of its origins. Traditional religious, reactionary, and sexist
ideologies also beg the question, by invoking unexplained `natural' or divinely ordained
sex differences. But if most `economic behavior' is mating effort by males to acquire
material resources for attracting and provisioning females, and if most `cultural behavior'
is male mating effort to broadcast courtship displays to multiple female recipients, then
cultural dimorphism is easily explained by sexual selection.
The age and sex demographics of cultural production are almost the same as the
demographics of homicide (Miller, 1995; see Daly & Wilson, 1988): males produce about
an order of magnitude more art, music, literature, and violent death than women, and
they produce it mostly in young adulthood. This suggests that, like violent sexual
How mate choice shaped human nature
competition, the production of art, music, and literature functions primarily as a courtship
display. For males, the mating benefits of public cultural displays are large because
every additional short-term mating achieved through impressing some receptive female
represents a substantial increase in expected fitness. Because male reproductive
success can be virtually unlimited, the amount of energy and time that talented men are
motivated to invest in cultural displays should be virtually unlimited. For example,
although the gifted guitarist Jimi Hendrix died at age 27 from a drug overdose, he had
affairs with hundreds of groupies, and fathered children in the U. S., Germany, Britain,
and Sweden. Composer J. S. Bach fathered 8 children by his first wife and 11 by his
second. The sexual conquests of Picasso, Chaplin, and Balzac are legendary. As every
teenager knows and most psychologists forget, cultural displays by males increase their
But for females, the genetic benefits of public cultural displays are smaller, because their
maximum reproductive success is constrained directly by their maternal investment
ability (i.e. the time required for pregnancy and lactation), not by the number of short-
term matings they can achieve. Rather than broad-casting her courtship displays to all
males indiscriminately and risking sexual harassment from undesirables, it may be more
effective for a woman to narrow-cast her courtship displays to a few select males who
are capable of giving her the long-term care, attention, and resources she wants. This
could be called the `Scheherezade strategy', after the woman who retained a sultan's
intellectual attention, sexual commitment, and paternal investment by inventing fantastic
stories throughout a thousand and one nights. Thus, cultural dimorphism is much more
likely to reflect a difference in motivation and sexual strategy than a difference in basic
In sexually-reproducing species, all genes must propagate through the gateway of sex,
and mate choice is the guardian of that gateway. For this reason, sexual courtship was
probably central in human evolution, and remains central in modern human life.
However, sexual selection has long been overlooked in the human sciences, partially
because evolutionary biologists themselves were skeptical about Darwin's most
innovative theory until quite recently, and partially because various ideological biases
kept sex marginalized as a topic too messy, too mystical, too embarrassing, and too
arousing for scientific analysis. We have to face the possibility that if human evolution
was a film, it would be X-rated.
This chapter has reviewed the history of sexual selection theory, the diversity of mate
choice criteria (selection for indicators, aesthetic displays, sperm competition,
provisioning, territories, and protection), the logic and limits of sex differences, the
patterns of sexual selection in primates and hominids, and the parts of the human body,
the human mind, and human culture that have probably evolved through sexual
selection. But this is all only the tip of the sexberg: a snap-shot of the sometimes eager,
sometimes resistant human sciences trying to absorb an unexpectedly large and potent
body of biological theory and evidence. The rapture will be mutual though, because a
new appreciation of sexual selection allows the tightest possible fit between (1) well-
established biological theory and data, (2) universal and important aspects of human
nature and human psychology, and(3) universal and important aspects of human culture
and social life. This integration, though necessary for future progress, will be difficult for
the social sciences and the humanities because it undermines and replaces some of
their cherished models of human nature(e.g. Freud, Marx, social-constructivism), and
because it demands research concerning the adaptive functions rather than just the
proximate mechanisms of human social, sexual, and cultural behavior. But if we
recognize the role of sexual selection in the evolution of human intelligence, creativity,
and culture, perhaps some of the old dichotomies — passion/reason, mind/body,
nature/culture, sex/science — can finally be reconciled.
Future histories of science will probably look back at our era as a critical point during
which human self-understanding was challenged and re-cast more deeply than ever
before. While the conceptual novelties of Copernicus, Adam Smith, Marx, Einstein, and
Freud have lost their revolutionary edge, the Darwinian revolution continues to dig
deeper and more sharply into the human soul (see Dennett, 1995; Ridley, 1993; Wright,
1994). Just when we thought we were comfortable with the idea of blind natural
selection shaping human nature, the eerie, half-sentient process of sexual selection
came back from the dead, more powerful and ubiquitous than ever. A full recognition of
the role of mate choice and sexual competition in human affairs and human evolution
may shake not only our psychology, but our psyches. It remains to be seen whether we
have the intellectual creativity, the sexual self-confidence, and the existential courage to
pursue these inquiries to their completion.
The author's research was supported partly by NSF-NATO Post-Doctoral Research
Fellowship RCD-9255323. For institutional support, thanks to the University of Sussex,
the University of Nottingham, the London School of Economics, and the Max Planck
Society. For guidance and illuminating discussions, thanks to Rosalind Arden, David
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