Introduction to The Nature of the Sexes
by Johan M.G. van der Dennen
".. we have evolved a nervous system that
acts in the interests of our gonads, and one attuned to the demands of
reproductive competition. If fools are more prolific than wise men,
then to that degree folly will be favored by selection. And if
ignorance aids in obtaining a mate, then men and women will tend to
be ignorant" (Ghiselin, 1974).
That the human brain developed in the service of the gonads, and not
to solve academic riddles and cross-word puzzles, may at first sight
seem an unusually provocative and even offensive statement by an
alienated armchair theorist. We usually grant that our sexually
dimorphic bodies - our anatomical, morphological, physiological and
neuroendocrine features and characteristics - may have evolved
also in the service of procreation (after all, we are
not sexually dimorphic just for the fun of it), but our precious
brains... our mental faculties... our very mind and soul? Yet, a
moment's reflection reveals that this must indeed be so. The only
currency in the cold calculus of evolution is reproductive success: the
differential contribution of individuals to the gene pool of the
population. That is what is called natural selection ever since Darwin.
Those who do not contribute drop out of the race and their genes
vanish with them (Humans can, of course, more or less consciously
and deliberately decide to drop out). Those who do contribute pass on
their genes to future generations and may enjoy the prospect of genic
immortality. Considering what is at stake, one may deduce that those
genes that have not `programmed' their temporary vehicles with
strong urges to reproduce have been selected against since time
Reproductive success, thus, is the name of the game, and our
individual reproductive success depends on how clever, smart or
intensive we play the game. I use the term `clever' and `smart' here
deliberately, because I mean `clever and smart in relation to the rules
of the game'; there is no intelligence or intent or even consciousness
involved (though in the human case some social intelligence may
come in handy, as may good looks, and status and money, and
whatever is considered attractive by the opposite sex).
And, as will be seen later on, because there are different Grand
Strategies, Gambits and Tactics for males and females in the
Differential Reproduction Game, it must follow that the brain not
only evolved `attuned to the demands of reproductive competition',
but also that the brain belonging to the sexually dimorphic bodies
must itself be sexually dimorphic too. It would be highly negligent of
nature if this were not the case. (I use the word `brain' here as a
shorthand for the neurological as well as the psychological/mental
equipment we are fed up with).
That the human species comes in two flavors or varieties - male and
female - is a fact of life some people regret, and others welcome and
enjoy: "Vive la petite différence". And that
differences between the sexes are not confined to the obvious
anatomical and morphological ones, but also encompass brain
functioning and behaviour, is probably known since the dawn of
mankind, but only recently being acknowledged and incorporated into
scientific theorizing. And only gradually are we unravelling the
evolutionary rationale behind these differences.
At this point some clarification of concepts is in order. I shall use the
concepts of `female' and `male' in a more biological sense (and for
the sake of argument suppose that an organism is either female or
male - excluding, for the time being, the cases of intersexuality,
(pseudo)hermaphroditism, transsexuality, etc.). To all practical
purposes, this is a fair assumption, especially since persons who are
both or neither are nonetheless raised as either female or male in
most human societies, and acquire and display the characteristics
which we generally refer to as `feminine' or `masculine'. What do we
mean when we talk about feminine and masculine? Generally, these
are qualities (or the lack of them) we ascribe and attribute to the
bearers of the two varieties of gonads: when it has
ovaries and a vagina, it is female and supposed to
exhibit feminine behaviour; when it has testes and a
penis, it is male and supposed to display masculine
characteristics. These `gender' attributions, which function as much as
prescriptions (ideas and rules about how men and
women ought to behave, think and feel), are culturally acquired
stereotypes adhering to the social role models of women and men in
a particular society at a particular time - `gender' itself being a
cultural and historical construction. Much of it is rather arbitrary and
fluctuates according to the fashion of the day. But not infinitely so.
There is some kernel of (biological) invariance which can hardly be
transcended. For example, parents do not have to coerce or coax boys
to play with toy weapons, and parents do not have to force or teach
girls to play with dolls. In other words, there is no infinite plasticity;
there are limits to the behavioural malleability of the sexes. The
reason behind that is really quite simple: it would be highly negligent
of evolution (or Nature, or God) to create a sexually dimorphic
species without at the same time `programming' or `wiring', or
otherwise preparing the individuals for the divergent strategies,
functions and roles they have to play during their lifetimes, especially
in the reproductive domain, as will be seen shortly. All this is not
meant to imply that the envisaged sex differences are in any way
`absolute'. For example, many mammalian species are quite capable
of heterotypical sexual behaviour (i.e. behaviour properly `belonging'
to the other sex), although such behaviour is generally more common
in females. In this volume, Peter Meyer, from the university of
Augsburg, Germany, will explore the cultural construction of sex
identities in more detail.
In the view of evolutionary biology, organisms are just temporary
vehicles with only one purpose: to transmit their genes to future
generations. The organism is ephemeral and mortal. The genes are, in
principle, immortal and have the `selfish' interest in spreading as
many copies of themselves as possible. Here is where the `strategy of
the genes' comes in.
But let us return for a moment to the issues of sex and gender, and
the inescapable biological determination many people feel to be
associated with these categories. For the time being it seems sensible
to state that gender differences always presuppose some sex
An important distinction in evolutionary biology, indeed in any
attempt to explain animal and human behaviour, is that between
ultimate (or evolutionary, of phylogenetic) and
proximate (or immediate, or ontogenetic) causes.
Succinctly stated: proximate explanations address the
how of a behaviour, ultimate explanations
why it occurs at all.
A proximate explanation considers the short- and longer-term
causation of that behaviour in psychological or neurophysiological
terms (e.g., psychological states, motivations, previous experience,
stimulus configurations, etc.). An ultimate explanation would ask:
why did this particular behaviour evolve? Did it confer fitness
advantages in the past to the bearer of this particular set of
Many critics of sociobiology and, indeed, many sociobiologists
themselves often fail to distinguish these levels-of-explanation. The
proximate cause of a certain sexual behaviour may be, for example,
the seeking of the thrill of orgasm with a specific partner, `libido', or
whatever we call the urge. An ultimate approach would address the
question how and why in (human) phylogeny sex ever developed:
why was it selected?, had it survival value?, did it contribute to
inclusive fitness?, did it lead to greater reproductive success in those
species that practised it than in those species that did not have it in
their behavioural repertoire?
These and similar questions invoke `natural selection'.
"Natural selection can honestly be described as a process for
the maximization of short-sighted selfishness" as Williams
(1988) states, but as the principle of kin-selection suggests, we are
equally selected to be (short-sighted or not) nepotistically altruistic.
Natural selection operates through the differential reproductive
success of individual members of a population (or rather their
genomes: the strategies of their `selfish genes', of which the
individual is just the temporary vehicle). We may expect all
organisms, including our own species, to be programmed to compete
for differential reproductive success with their conspecifics, and for
the resources and status positions which lead to the enhancement of
reproductive success. But because our next-of-kin also bear replicas
or copies of our own genes, natural selection will also favour those
behavioural strategies which increase the reproductive success of our
next-of-kin. This is called `kin-selection', and is measured in terms of
`inclusive fitness', and its manifestation is nepotism or nepotistic
altruism. A particular behaviour is `adaptive' only in so far as it
contributes to the organism's inclusive fitness. It is rather easy to see
that the concept of `preservation of the species' has no evolutionary
relevance whatsoever. Reproductive success of the individual
organism is the only currency in the cold calculus of evolution.
From these basic considerations a number of other propositions can
be derived. Inclusive fitness considerations also predict strikingly
different reproductive strategies of the sexes (males competing with
other males for the sexual favours and reproductive potential of
females; females being coy, careful and choosy) and the conflict
potential inherent in this (e.g. rape as an attempt to short-circuit the
principle of female choice). Similarly sociobiological reasoning
predicts a conflict potential in every area where there is a relative
difference in coefficients of relatedness, and where the reproductive
interests of individuals are not absolutely identical: parent-offspring
conflict, sibling rivalry, conflict between the sexes (the `battle' of the
sexes), sexual competition and the `double standard', ingroup-
outgroup differentiation and ethnocentrism, and intergroup conflict
along ethnic, `racial', tribal, ideological and other cleavages.
Indeed, field observations of a great number of species have
confirmed these sociobiological predictions: feticide, infanticide,
siblicide, homicide, and rape, as the most extreme and gory forms of
`conflict-resolution', are much more widespread than was ever
envisaged by the first generation of ethologists, such as Lorenz, who
thought that animals had innate inhibitions against killing
conspecifics, and that Man was a biological freak because he
apparently lacked those inhibitions.
Basically, the sociobiological reasoning behind the `battle of the
sexes' is simple and straightforward: men and women invest
differently in their reproductive success. For mammalian species, such
as we ourselves are, female reproductive success is limited only by
the amount of resources (time, energy, nutrients, etc.) she has to
invest in offspring. But for the male, the female herself is the limiting
resource: one male can inseminate many females and male
reproductive success is only limited by the number of matings a male
can achieve. Even in species where males typically invest in their
offspring - and human males certainly do - the temptation of
enhancing reproductive success by means of securing extra-pair
copulations and inseminating other females without further
investment tends to select for a mixed male strategy of pair-bonding
and philandering, and a female counter-strategy of carefully assessing
the male's potential and willingness to invest in her offspring. Other
consequences of this basic asymmetry in female and male parental
investment will be considered in greater detail in the chapters to
The Development (Ontogenetic Determinants) of Sex and
Once the gametes from the mother- and the father-to-be have fused
the resulting fetus normally displays chromosomal dimorphism
(chromosomal sex: XX or XY) which influences the fetal gonadal
sex: the undifferentiated gonad develops into testis or ovary.
In the case of a male (XY), the testicular secretions or hormones
produced by the fetus are necessary for the continuing differentiation
of the gonads and genitals in a male direction. The basic anlage (the
`default form') of the fetus/embryo is female, and unless counteracted
by a Y chromosome and testicular hormones the embryo will further
develop in a female direction. Maleness is, one could say, a deviation
from this basic pattern. In the absence of testicular hormones the
fetus will continue to differentiate and develop the reproductive
anatomy of a female. Moreover, there is by now abundant evidence
that these testicular secretions not only influence genital dimorphism
but also have a priming effect on brain dimorphism, especially on
those limbic and hypothalamic structures that will subsequently, much
later in life, influence many aspects of sexual and agonistic
behaviour. For the understanding of sex differences as discussed in
this volume, it is important to realize that these sexually differentiated
brains interact with the prenatal, perinatal and postnatal environment
differently from the very beginning; they create/construct, as it were,
a sexually dimorphic world. Normally, when no errors occur in this
complicated, unfolding program - and many things can go wrong at
every stage of the process: chromosomal mosaicism,
(pseudo)hermaphroditism, gonadal dysgenesis, genital hypoplasia,
female adrenogenital syndrome, male androgen insensitivity
syndrome, to mention only the most frequent construction failures -
girls and boys soon after birth develop stable body images and gender
identities as either female or male, with a clear demarcation of the
boundaries of the gender roles, and start their journey on the long and
winding road toward puberty and adulthood. The surges of androgens
during puberty have dramatic effects on the morphology and
psychology of the maturing boy, while menarche and breast
development have an equally dramatic impact on the girl-turning-
woman. Androgen, especially testosterone, does not only affect
aggressive behaviour in males, but also acts as the libido hormone in
both men and women; thus there is evidence of a neurophysiological
as well as an endocrinological interaction between the sexual and the
agonistic behavioural systems in the males of the species. This
pattern is very probably an ancient and general one in mammalian
species, in which males commonly have to engage in vigorous
agonistic interactions with other males in order to establish mating
opportunities with one or more females.
"Stated in nontechnical terms, the lesson from embryonic
anatomy is that it is easier for nature to make a female than a male.
The familiar embryonic and fetal rule is that something must be
added to produce a male. Quite possibly, the same paradigm may
apply also to gender-identity differentiation, though there is as yet no
conclusive proof of this hypothesis". It would, however, help to
explain why paraphilias (formerly called `perversions') are almost
exclusively male phenomena: "The majority of the paraphilias
are found as distortions exclusively on man's gender identity, not
woman's. Thus they provide still further evidence that nature has
more difficulty in differentiating the gender identity of the male than
the female. Nature makes more errors in the male" (Money
& Ehrhardt, 1972).
For most men most of the time, sex is a short, sharp pleasure. For
many women sex is a lifelong-lasting lamentable burden, or, at least,
a nuissance, a curse of nature. For young adolescent males, sex (the
`fire of the loins') is more often than not a source of chronic
frustration, but always fascinating to the point of obsession (One is
reminded of Huxley's definition of an intellectual as someone who
manages not to think of sex for full five minutes). For some young
adolescent females sex is a horrifying prospect, or a commodity for
exchange or exploitation, or a few minutes of agony for a bundle of
joy, but rarely the exhilarating titillation it is to males. In romantic
relationships sex is as often a source of traumatic conflict as it is of
ecstasy and bliss. Relentless passion may ruin a love relationship, and
There is evidently a wide experiential gap between the sexes. Many
girls and women experience sex literally as violence, as a violation of
their corporeal integrity, a penetration not only of the body but also
of the personality and free will. For those women who are subjected
to clitoridectomy, infibulation or pharaonic circumcision - a genital
mutilation in which the clitoris is cut away and the vaginal labia have
been sewn together in order to preserve the woman's virginity intact -
sex is probably everything but a pleasurable experience once they
have been cut open by their owner-husband. For the girls who have
been sexually abused in their youths by adult males in power
positions, or who have been raped, sex in later life is not an intimate,
self-transcending sharing of mutual satisfaction but more probably a
sordid traumatic affair, if, that is, they manage to engage in intimate
erotic relationships at all.
For those women battered by jealous husbands or possessive lovers, it
is probably of little solace to learn that the ultimate concern of the
man involved is his `confidence of paternity': Most wife beating and
battering is the result of suspicion or knowledge of marital infidelity
(Daly, Wilson & Weghorst, 1982), and as such it is part of the
array of male anticuckoldry strategies; as is the universal double
standard in matters sexual.
"The biological irony of the double standard is that males could
not have been selected for promiscuity if historically females had
always denied them the opportunity for expression of the trait"
(R.L. Smith, 1984). Paradoxically, the same may be true for male
homoeroticism, homosexuality and androgyny (Rancour-
Then why this suffering and conflict and frustration on account of
sex? Why are we a sexually dimorphic species? Why are we a sexual
species to begin with?
Sociobiology could be described as natural history - the evolutionary
history of organic life - from the point of view of the genes. Genes
are potentially immortal, while the organisms are merely temporary,
mortal vehicles in the service of the replication of the genes. Many
organisms, such as protozoans, have reproduced, and still reproduce,
asexually, by means of fission or parthenogenesis, and they have been
doing so successfully for millions of years. Then what is so special
about sexual reproduction (needing two different
morphs who each contribute only one half of their genes to the
offspring): the way all animals best known to us, the mammals, and
we ourselves do it? This question becomes especially enigmatic when
we realize the costs involved in sexual reproduction. Sex is wasteful
and inefficient: it uses energy, materials and time; and the highest
cost of all is the damage, wounds and even death incurred in the cut-
throat competition for mates. What could the possible benefits be that
transcend the substantial costs involved? Why did sex evolve? What
do we mean by `sex' in the first place?
What is `Sex'?
The term `sex', as one might expect, is highly ambiguous and is used
in a wide variety of meanings. In everyday parlance it usually refers
to copulation (coitus) and its preliminaries and paraphernalia (which
in the case of humans involve an extensive repertoire of play/pleasure
components and erotic elaborations, e.g. mutual masturbation, fellatio,
cunnilingus, etc. in the service of orgasm), and is equivalent to
`making love' or `sleeping together' or `carnal knowledge' or `doing
it' or `intercourse' or `mating' or `fucking' or any other of the
hundreds of euphemistic, clinical, legal or obscene terms we have
invented to designate the genital union of two bodies. In contrast to
the hedonic inventiveness of humans in matters sexual (carefully
classified in such treatises as the Kama Sutra and
The Perfumed Garden), copulation is most animal
species is a brusque "Wham bam, thank you Ma'am"
affair (involving little affection or empathy, let alone love).
Besides this primary meaning of `sex', we also use the term roughly
as an equivalent of `gender', male/female, masculine/feminine, as in
`sex differences', `sex roles', `battle of the sexes', etc.
In the third place, `sex' is associated with the realm of vicarious
gratification: wet dreams, masturbation fantasies, erotic imagery, in
which we lustfully project what we would like to do to a particular
person, or rather what we would like that particular person to want us
to do to her/him. The `appeal' in `sex appeal' refers particularly to
this meaning of `sex'.
And lastly, though we know reproduction to be result of sexual
activity, it is not the first thing we usually associate with the term.
The ultimate imperative of reproductive success has
resulted in emphasis on the lustful components of the
proximate behaviours, at least in humans and
probably quite a number of primate species.
To an evolutionary biologist, however, the term `sex' means quite
different, and much less romantic and erotic, things. In the first place
it refers to mixis (literally `mingling'): the
reorganization, recombination or reshuffling of the genome. "It
is readily apparent why this phenomenon might be inadequately
distinguished from others that accompany it. Sex is often confused
with reproduction. The two commonly go together, and the reason is
that the occasion of producing a new organism is an appropriate time
for producing one that is different" Ghiselin (1974) explains.
And he continues: "The commonly used expressions `asexual
reproduction' and `sexual reproduction' treat sex as if it were a subset
of reproduction. We do have reproduction without sex: not only the
usual sorts of vegetative growth, budding and fission, but also
reproduction with gametes but lacking genetic recombination (amictic
parthenogenesis). On the other hand we also have sex unconnected
with reproduction - what might be called `areproductive sexuality'.
This occurs in the `automixis' of many protozoans... Likewise, the
physical union of individuals is often confused with what will here be
called `sex' in the sense of recombination. Sex often means the
genital intercourse of multicellular organisms, or the fusion of
gametes or protozoa... For modern biology it is more important that
we draw a distinction between sex and the more particular form of it
called `amphimixis'... , or the formation of new kinds of genomes
from those of different individuals. Thus we have
mixis and its two subclasses:
automixis and amphimixis"
It is by no means clear what exactly it is that we want when we want
sex, when the libido is playing havoc with our peace-of-mind. Is it
comfort and solace, escape from loneliness, relief of tension,
acceptance, love, pair-bond, intimacy, body contact, lust, orgasm,
erotic pleasure, satisfaction, tenderness, passion, ecstasy, romance,
regression, hugging and caressing, mutuality, submission, conquest,
dominance, or all of these in various combinations? In view of the
many deceptive strategies and tactics men and women employ in their
mating games, one might deduce that for males conquest, possession
and control itself is a powerful incentive, while for females the
intimacy of the relationship, which signals a long-term commitment,
is prevalent in sexual satisfaction.
Why Did Sex Evolve?
The usual textbook answer to this vexing question is: "For the
preservation of the species, of course". And, like so many
textbook answers, it is wrong. The answer is based on the, still very
`en vogue', so-called `group-selection' paradigm, which states in brief
that something (in this case sexual reproduction) evolved because it
was good, or advantageous, or functional, or adaptive for the group
or the species. A recurrent problem with the group-selection paradigm
is that something, be it somatic or behavioural, which evolves for the
good of the individual organism always overrules that which may
evolve for the good of the species (for technical details see NOTE 1).
One example may suffice to illustrate this important principle of
evolutionary biology: Induced abortions, intrauterine resorption of
embryos, cannibalism and kronism of offspring, nest desertion and
infanticide exist as evolved mechanisms and behaviours in many
species. These mechanisms and behaviours can hardly be construed as
good for the (preservation of the) species. Yet, they have evolved,
and virtually all of these mechanisms and behaviours can be shown to
be adaptive in terms of reproductive success, not for the species but
for the individual organism practising them. So, once again, why did
sex(ual reproduction) evolve if not for the good of the species?
Although some asexual species have apparently survived for very
long times, this is not the general rule. Asexually reproducing species
may well have immediate evolutionary advantages, but are less
successful in the long run. The prevailing view is that asexual species
are less able to keep up with environmental changes than are sexual
The three arguments for the evolutionary advantages of sexual
reproduction are (1) adjusting to a changing environment, (2)
incorporating beneficial mutations, and (3) getting rid of deleterious
mutations (Crow, 1988).
Why Only Two Sexes?
Biologically speaking, a female is by definition that sex that
specializes in the production of a few, large, nutritious, and relatively
immobile gametes (ova), while the male is by definition that sex that
specializes in the production of a huge quantity of small, non-
nutritious, motile gametes (sperm). Imagine an initial isogamic
population in which individuals differ slightly in the size of gametes
they produce. Some individuals will produce large gametes with high
prospects of survivorship as zygotes, others will produce many
smaller gametes with poorer prospects of survival. Most individuals
under the normal distribution curve produce intermediate gametes.
With random fusion of gametes, such a system stabilizes over time -
via a transient bimodal distribution - with mainly two genotypes
(males and females) in a 1:1 ratio. As Parker (1984) explains:
"The third genotype tends not to be represented either because
it arises as the rather inviable product of the fusion of two `proto-
sperm' or because it is formed by the fusion of two `proto-ova',
depending on dominance. This latter fusion is rare because most
gametes fuse with small gametes because of their numerical
predominance. After several generations, the population is essentially
anisogamous as a result of disruptive selection". This means
that the final result of this process is the existence of two discrete and
non-overlapping distributions of ovum-producers (females) and
sperm-producers (males). In other words, the Evolutionary Stable
Strategy (ESS) in this case is the stable coexistence of two morphs
for gamete size. "Sperm producers survive by parasitizing the
investment of ovum producers; the ESS solution (stable anisogamy) is
essentially driven by sperm competition" (Parker, 1984).
("Oh God", my wife, who is reading over my shoulder,
now cries out in desperation, "the feminists were right after all:
males are parasites". And I shamefully
But what if for whatever reason a third breeding morph arose in such
a population, or that there would be three sexes: X, Y and Z, with
mating patterns XZ, YZ, and XY? In that case a minimal selective
advantage of one sex or zygote would eventually outcompete the
others (simply by being more prolific), and again the final result
would be two sexes.
Why is there a fifty/fifty sex ratio; why are there approximately equal
numbers of each sex? This puzzle was in principle solved only in the
1930s by R.A. Fisher, and is accordingly called `Fisher's principle'.
When one sex numerically predominates in a population, it becomes
advantageous for parents to invest in the opposite sex because that
maximizes their offspring's reproductive success. With a
preponderance of females, investment in males will ensure high
opportunities for fertilization; and, conversely, with a preponderance
of males, investment in females will produce the highest pay-off. This
mechanism eventually stabilizes the sex ratio around 1:1. Fisher
stated his solution in genetic terms, but the potential of parents to
actually vary and manipulate the sex of their offspring may also be
involved (Trivers-Willard hypothesis).
I am acutely aware that many people feel some kind of moral
indignation, as if insult were added to injury, when their behaviour is
compared to that of dragonflies, sticklebacks, rats or chimpanzees.
"Man is the creature with culture and language and intelligence
and free will", so the major objection goes, as if such Crown-
of-Creation-argument would effectively invalidate or exclude any
view of the human species as one species among a plethora of others,
and having an evolutionary history just like the others. Selection
thinking does not and cannot consider one species superior vis-
à-vis others. For selection thinking or evolutionary theory
Homo sapiens sapiens (as he calls himself somewhat
arrogantly) is no more or less unique than other species.
The animal examples in this volume should be understood primarily
as illustrations of principles, of strategies and solutions the genes
have `invented' or stumbled upon to solve common and recurrent
problems of survival and procreation. If one reads that in a certain
species females habitually cannibalize the males in
copulo, such a consumption pattern is not recommended for
human females. Conversely, if one reads that in a certain species the
males habitually rape the females of the species, that does not imply
that such a strategy is recommended for human males either. It only
means that in that particular species rape may have evolved as one
male counter-strategy to short-circuit the strategies of female choice.
There is no moral stance or ethical precept involved.
The `battle of the sexes' is, of course, a metaphor. But it is a
metaphor based on a universal human experience: that there is not
only harmony and peace and roses and honeymoons and bliss
between the sexes, but also - and sometimes quite unintendedly and
therefore the more mystifying ("Is it something I said?") -
a lot of frontal collisions, misunderstandings, conflict, antagonism
and even violence: intimate enemies on their own private, home-made
battlefield. The clever reader might object that this is the universal
human condition and not specifically confined to the relations
between the sexes. And that is, of course, partially true. The
evolution of anisogamy and it sequelae has, however, resulted in a
whole new and specific category of potential conflicts over and above
those which already plague an obligatorily social species such as we
are. Not every battle is a `battle of the sexes'. In this volume, Robin
Russell and his colleagues from the university of London, United
Kingdom, explore the variables which make for marital
Sociobiology has been accused of sexism (as a matter of fact is has
been accused of every negative -ism which happened to be in
fashion). But that surely must be a profound and tragic
misunderstanding. Sexism is an ideology of discrimination on the
basis of sex/gender, implying devaluation or inferiority of one sex
vis-à-vis the other (for `male chauvinist pigs' it is women
who are considered the inferior sex; for particular breeds of militant
feminism it is men who are considered to be subhuman `scum' or
worse). But there is nothing in sociobiology to warrant such an
accusation. Superiority-inferiority considerations simply have no place
in any science worthy of the name. To attempt to uncover sex
differences and to try to explain these in terms of the different
strategies which have evolved for the two sexes in the context of a
long history of reproductive competition may be considered raving
madness or sheer nonsense, but it is not, and has
nothing to do with sexism. (Sometimes the accusation is arrived at
via a more convoluted route, which goes something like this:
Sociobiology denies the exclusively sociocultural or environmental
determination of sex differences and asymmetries, therefore it is
responsible for the continuation of the situation of inequality,
therefore it is conservative and/or reactionary, and therefore it is
sexist. Quod erat demonstrandum).
Sociobiologists are probably also more inclined than other social
scientists to predict conflict, deceit and manipulation on all levels of
existence: ethnic and other group conflict, male-female conflict, male-
male conflict, female-female conflict, parent-offspring conflict,
offspring-offspring conflict, even mother-embryo conflict and
intragenomic conflict. But that, too, is a far cry from sexism. In this
volume, Ullica Segerstråle, from the university of Chicago,
U.S.A. discusses the sometimes troubled and ambivalent relationship
of sociobiology and feminism.
Males are predicted to be sexually indiscriminate. Indeed, in many
species the males will try to copulate with anything that even
remotely resembles a female. Also the so-called `Coolidge-
phenomenon' (sexual habituation to the present mate, and strong
arousal in response to the presence or anticipation of a potentially
novel sex partner) points to the fact that males are selected to prefer
diversity and variety in their sexual diet. "Small wonder that
selection has favoured sex-crazed human males" (R.L. Smith,
1984). Such male indiscriminateness (simply because he can afford
to), together with the fact, as we saw, that "Nature makes more
errors in the male" may explain why vacuous, erratic,
extravagant, non-reproductive, promiscuous, wanton and orgiastic
sexual behaviour is virtually exclusively a male `prerogative'; and
why the three P's (prostitution, pornography and paraphilias) appeal
predominantly to men.
Females, in general, do not gain reproductively by such
extravaganzas, and they are predicted to be extremely reluctant, coy,
discriminate and selective in matters sexual, except in certain
circumstances where relative promiscuity, or sequential polygamy or
multiple matings may evolve as a strategy of female choice in the
context of sperm competition and infanticide (e.g. in chimpanzees and
barbary macaques (Macaca sylvana).
The power asymmetry between the sexes in all human societies we
know (though some are much more egalitarian than others) is the
most remarkable of all sex differences. Everywhere it is males trying
to control the sexual and reproductive potential of females, and not
the other way around. Women may exert considerable power behind
the scenes, but the gladiators and the clowns in the political arena are
males. Van den Berghe (1979) is undoubtedly the most acute analyst
of this phenomenon. His work is highly recommended also for the
reader who wants to make sense out of the wondrous world of matri-
and patrilineality, in- and outbreeding, avuncu-, neo-, uxori- and
virilocality, endo- and exogamy, dowry and brideprice, polygyny and
monogamy; and how these phenomena, that have vexed
anthropologists, can rather successfully be explained along
sociobiological lines. In this volume Bobbi Low, from the university
of Michigan, U.S.A, explores this power asymmetry in pre-industrial
societies, while Vincent Falger, from the university of Utrecht, the
Netherlands, explores the problem in modern, industrial
R.L. Smith (1984) argued (following others) that continuous sexual
receptivity in human females, cryptic or concealed
ovulation, and some other female anatomical peculiarities
and feminine characteristics (such as the hemispheric, pendulous
breasts, and menstrual synchronization) have evolved to obscure a
human female's current reproductive value and confuse males as a
countermeasure to male resource allocation and anticuckoldry
strategies. These female adaptations enhance opportunities for
facultative polyandry and thus promote human sperm
Most animals have regular patterns of mating (mating seasons) when
copulation can result in pregnancy. This periodicity is usually
synchronized between the sexes, and is mediated by hormones
released in response to a variety of environmental stimuli.
Primates deviate from this typical mammalian pattern in that males
occasionally copulate with infertile females. This behaviour is
widespread among the primates (e.g. Hrdy, 1981) and is most highly
developed in humans. Human females do not experience oestrus,
offer no conspicuous morphological or behavioural evidence of
ovulation, and are more or less continuously receptive to coitus from
the onset of puberty.
Cryptic (concealed) ovulation and perennial sexual receptivity in
humans has intrigued sociobiologists, and many have attempted to
identify the ultimate causation of these phenomena. Most theorists
have suggested that concealed ovulation and continuous female
receptivity facilitate monogamous pair-bonding through the
mechanism of permanent sexual attractiveness (cf. Alexander &
In contrast, Benshoof & Thornhill (1979) proposed that
concealed ovulation and continuous female receptivity evolved after
female monogamy because any group-living female with
inconspicuous oestrus in a monogamous mating system would be in a
better position to mate with a superior male without her being
detected by her primary mate. Benshoof & Thornhill further
suggest that ovulation is apparently concealed from the female
because some degree of self-deception facilitates the deceit of
Sexual selection depends on the success of certain
individuals over others of the same sex, in relation to propagation of
the species; whilst natural selection depends on the success of both
sexes at all ages, in relation to the general conditions of life. The
sexual struggle is of two kinds; in the one it is between individuals of
the same sex, generally the males, in order to drive away or kill their
rivals... whilst in the other, the struggle is likewise between
individuals of the same sex, in order to excite or charm those of the
opposite sex, generally the females, which no longer remain passive,
but select the more agreeable partners (Darwin, 1871, p.
This is how Darwin introduced the concept of sexual selection. The
first kind of the sexual struggle is now better known as male-male
and female-female competition. The second kind of the sexual
struggle envisaged by Darwin is now known as epigamic selection or,
simply, the principle of female choice. Darwin indicates the sexually
dimorphic secondary sexual characteristics of many species, including
humans, as the result of sexual selection. The gorgeous and exorbitant
plumage of male birds-of-paradise, for example, is the result of such
a kind of runaway sexual selection, based on the attractiveness to the
females of the most exuberant-looking males. When sexual selection
operates among males, adult males tend to become larger, heavier,
showier, more competitive and better armed, and their behaviour
patterns and ecological requirements tend to diverge from those of the
females. This is one of the reasons why E.O. Wilson (1975) calls sex
"an antisocial force in evolution": it generates and
multiplies conflicts of interests.
A whole array of traits is associated with the greater sexual
competitiveness of males in a wide range of species. These include
not only greater size and gaudiness, but also the price males have to
pay for this: greater vulnerability and frailty in development, and
shorter lifespans due to senescence, high risk-taking and mortality
The ultimate basis of sexual selection is greater variance in mating
success within one sex. In humans, for example, some powerful men
may have many wives and children, while many poor, low-status men
have neither. This differential in reproductive success underlies the
formulation of Parental Investment Theory, developed by Trivers
(1972), as outlined in the chapters to follow.
As may be seen from the bibliography at the end of this introduction
(which covers only monographs and readers - excluding the many
thousands of articles on the subject - and does not claim to be
exhaustive), sex differences have been documented primarily by
psychologists, psychoanalysts, anthropologists and feminist writers,
long before sociobiology even existed. They are therefore not evil
inventions by sociobiologists contrived in order to subordinate
women, as I heard arguing recently. Sociobiologists have mainly
added a new interpretation, a new framework for the understanding of
The conventional interpretation of these sex differences was in terms
of differential learning, differential reinforcement, differential
socialization, culturally-imposed sex role requirements and restraints,
sociocultural division of labour, and so on. But, as was already
recognized by Archer (1976), such an interpretation does not address
the following question: "The first is whether the nervous system
of the developing child or of the adult, by being male or female,
facilitates the learning of certain aspects of the appropriate sex role.
The second related question concerns the evolutionary origin of the
sex differences, whether they are the indirect consequence of
adaptively significant biological sex differences of ancient
origin". (In 1976 this must have sounded like heresy). And a
third question might be added: "Whether or not sex differences
in behaviour are already apparent prenatally, and after birth are
responded to differentially by the parents".
Let us see what exactly was at stake, then and now. The following
were - and still are - considered `established' psychological sex
- Women show lower sensory detection thresholds than men for
touch, pain, hearing, taste, smell, and scotopic (dark) vision, whereas
men show lower thresholds for photopic (daytime) vision. Weiert
Velle, from the university of Oslo, Norway, discusses these sex
differences in sensory modalities in great detail. Hal Daniel, from the
university of East Carolina, presents an experiment on differential
perception of vocal sexiness.
- Males are generally reported to show more `aggression' than
females, for measures of aggression taken during school life, and in
laboratory measures of aggression taken in adulthood. Tore Bjerke,
from the university of Trondheim, Norway, and Johan van der
Dennen, from the university of Groningen, the Netherlands, will
explore this crucial topic further in this volume.
- Men show faster reaction times when a simple motor task is
involved, are more accurate in target-directed motor skills, and are
faster on rotary-pursuit tasks.
- Women perform better on tasks requiring more discrete or finely
controlled motor responses and precision manual tasks, such as typing
and other so-called `clerical skills', and outperform men on
- Women show greater linguistic abilities (especially verbal fluency)
from infancy onwards, and perform better on verbal IQ tests.
- Men perform better in tests requiring judgment and manipulation
of spatial relationships.
- Men perform better on tests of mathematical reasoning ability,
while women outperform men on tests of mathematical
- Women are more highly attentive to sensory input and respond
both to its emotional and socially meaningful properties.
- Men are, in general, more object-oriented and achievement-
oriented, while women are more socially-oriented, less competitive,
and more nurturant.
- Even when we cross the boundaries of what is considered normal,
the sexes do so in different ways. Sex differences are striking in a
number of psychopathological conditions. In childhood conduct
disorders involving antisocial or destructive behaviour boys
outnumber girls by far. Anxiety disorders and phobias are
predominantly female, and animal phobias are almost exclusively
female. Psychopathy (sociopathy) is predominantly a male domain. In
drug addiction and alcoholism males greatly outnumber females, etc.
To these may be added the following cross-cultural observations
(Rosenblatt & Cunningham, 1976), also quite well-known at that
- Gender seems to be a basic distinction in many languages and
possibly a basic distinction of social organization in all societies. In
every society there is some division of labour by sex.
- One task which may be at the root of many other sex differences
in behaviour is childcare. Childcare is everywhere the duty of the
child's mother. In some societies, older children, men and older
people of both sexes may also have childcare duties.
- In the search for calories, women in societies around the world
bring in an average of roughly 30-40% of the caloric intake.
- Warfare seems to be primarily an activity of men. No major
society dominated by female warriors has been documented in the
- Cross-sex violence seems frequent during courtship in societies
with relatively great freedom of choice of spouse.
- Across the world it appears that men have higher public status than
women. Men typically are the formally-recognized heads of domestic
- Women have power over men through their control of sexual
relations with a mate and through the influence they have with their
children and their natal kingroup.
- As might be expected, there are also cross-cultural differences in
the ways boys and girls are socialized.
(These are all, of course, general statistical statements which establish
means and variances - with sometimes considerable distributional
overlap - not statements about individuals, who may widely deviate
from the mean, such that some men are more feminine in e.g.
physical appearance than the average woman, and some women more
masculine than the average man).
The point I want to make is that the various sex differences, from
subtle perceptual ones to worldwide sociopolitical asymmetries,
in toto make sense only in an evolutionary context,
i.e., as a result of different selection pressures operating during the
millions of years our ancestors lived in small kin-groups as
scavengers-gatherers-hunters; fierce mating competition; sexual
selection; and, very probably, a mutual `arms race' of deceit- and
An evolutionary view of sex differences in behaviour is not a relapse
into the obsolete and acrimonious nature-versus-nurture debate.
Rather, it provides a general framework for the attempt to understand
them as consequences of different reproductive and parental
investment strategies. How the sex differences arise and operate -
whether genetically, congenitally, hormonally, socially or culturally
acquired - and how saliently they are expressed, is rather immaterial
from the point of view of the ultimate purpose for which they were
designed by natural selection.
It is important to understand that an evolutionary vision of the human
condition, as outlined here, does not deny the importance of cultural
and socialization factors. There is no `innate' rigidity or `genetic
determinism' implied. Firstly, because every trait is always expressed
as an inextricable product of genotype and environment interaction.
And secondly, our capacities for culture, language and learning (with
all its inherent constraints) are themselves products of
After the contributions in this volume were written, in 1988 for the
Oslo meeting of the European Sociobiological Society, a number of
important books and articles appeared among which I recommend
especially Lee Ellis: Theories of Rape: Inquiries into the
Causes of Sexual Aggression (New York: Hemisphere,
1989), which presents an evolutionary theory of rape and discusses
the (alleged) pornography-sexual violence relationship in meticulous
detail. Martin Daly & Margo Wilson's
Homicide (Hawthorne: Aldine de Gruyter, 1988) is
a valuable source of information on the role of sexual competition
and jealousy in male-male and male-female murder and criminal
violence in general. Laura Betzig, Monique Borgerhoff Mulder
& Paul Turke's (Eds.) Human Reproductive Behaviour:
A Darwinian Perspective (Cambridge: Cambridge University
Press, 1988) touches upon many of the subjects treated in this
volume, as does Anne Rasa, Christian Vogel & Eckhart Voland
(Eds.) The Sociobiology of Sexual and Reproductive
Strategies (London: Chapman & Hall, 1989). One
might also consult the research work by David Buss on conflict
between the sexes a.o. in the Journal of Personality and
Social Psychology. Christopher Badcock's Oedipus
in Evolution: A New Theory of Sex (Oxford: Blackwell,
1989) presents a valuable synthesis of evolutionary and Freudian
ideas on human sexuality.
This volume is not going to make this a better world, or bring joy to
mankind, but perhaps it might help in understanding each other a
little better. Brillat-Savarin, famous gastronome, gourmet and
connoisseur, said that the only enduring contribution to the happiness
of mankind would be a delicious new recipe. Unfortunately, the
contributors to this volume stumbled upon a career in
academicis and not in the lucullian arts. I, for one, sincerely
and humbly apologize for that.
1. The first problem with the traditional explanation in terms of group
selection is, Stearns (1988) explains, that group selection - or in this
case, selection of the attributes of species - does not
work under most circumstances. The reasons for this are as
(1) Selection pressure on a trait is proportional to the amount of
variation in reproductive success among individuals that can be
accounted for (a) by variation in the trait in question, divided (b) by
the generation time (In this case, the variation would be between
sexual and asexual individuals).
(1a) Generation times of individuals are much shorter than lifetimes
of species - by a factor of 10,000 to 100,000 for most eukaryotes.
Thus selection pressures on individuals are correspondingly much
larger than selection pressures on species.
(1b) The response to selection depends on the amount of heritable
variation available among the units selected. There is much more
variation available for selection among individuals within a sexually
reproducing species than there is among species within a lineage.
Therefore the response to selection is much faster and larger for
individuals than for species.
(2) Thus selfish individuals can always outcompete individuals that
sacrifice their own interests to those of the species. If asexuality is an
advantage to an individual, and sexuality an advantage to the species,
then we should find that most organisms are asexual, for only rarely
would the advantages of sexuality be so strong that species selection
would overcome individual selection.
The second problem with the traditional view is that sex does seem to
cost a lot. It should pay the individual to be asexual. For bacteria and
single-celled organisms, the principal cost of sex is probably the time
it takes to carry out recombination. This can lengthen the normal cell
division time by a factor of two or more. All organisms encounter
another cost of sex not related to anisogamy: recombination
rearranges genotypes that have high fitness. Whenever selection is
strong, this cost will be high, for by the fact of their survival all
parents indicate their fitness in the local environment. In anisogamous
species - all `higher' plants and animals - the female provides
cytoplasm to support the male genome. This results in the twofold
cost of males, or cost of genome dilution.
Compare a sexually reproducing female with an asexually
reproducing female. Suppose that all offspring cost the same amount
in both cases. The sexually reproducing female has offspring that are
half male, half female. The asexually reproducing female has
offspring that are all female, each of which also reproduces asexually
and replicates her entire genome. Because she produces twice as
many female offspring, after, for example, five generations she
should have 32 times as many female descendants, each of which
will also contain a complete copy of her genome. Not only does the
sexually reproducing female make only half as many female offspring
per generation; each of them contains only half of her genome.
It was this cost that first convinced Williams (1975) that the
prevalence of sexual reproduction poses a serious problem for
evolutionary theory. The advantages of sex to the individual have to
be very large if sex is to be maintained by natural selection in any
population in which parthenogenesis can arise.
In practice, the advantage of being asexual is rarely twofold. In most
animals the transition to asexuality is difficult. In small organisms the
actual costs of sex are reduced by intermittent sexuality - a series of
asexual generations followed by an occasional sexual generation. In
large organisms, such as mammals and birds, the realized costs of sex
are quite small because sexuality is fixed in these lineages. Because
the asexual option is simply unavailable, mutant asexual competitors
The major contemporary hypotheses for the maintenance of
The `Tangled Bank' hypothesis: A whole family of
models embodies the notion that the production of genetically diverse
offspring is advantageous in an environment that is saturated,
heterogeneous, or both. Ghiselin (1974) appears to have priority for
this idea, which was further developed by Williams, Maynard Smith
and Bell. Bell named it the Tangled Bank hypothesis, after the
closing paragraph in Darwin's Origin.
The `Red Queen' hypothesis or Coevolutionary Arms
Race (So called after Lewis Carroll's Alice in
Wonderland, in which the Red Queen has to run very fast in
order to stay in the same place): The idea that recombination is an
advantage in a coevolutionary race against competitors, predators,
parasites and disease organisms has been brought up repeatedly.
Disease organisms and parasites have a fundamental advantage in an
evolutionary arms race. They can adapt themselves quickly to a
specific host genotype. This brings the host population under strong
frequency-dependent selection, for is pays to have a rare genotype
during an epidemic.
DNA Repair Mechanism: The advocates of this
hypothesis postulate that recombination evolved as a mechanism by
which damage in one chromosome could be repaired by information
in the homologous chromosome. This advantage was later followed
by biochemical complementation between the homologous
chromosomes that exploited the redundant information available in
the diploid genome.
2. Crow (1988) summarizes the costs and disadvantages of
recombination as follows:
1. Sexual reproduction is not very efficient qua reproduction. The
time and energy required for meiosis and syngamy are substantial. As
a means of multiplication, many asexual systems are more effective
and less error-prone.
2. With anisogamy and separate sexes there is the cost of males. A
female that could produce female progeny asexually with the same
efficiency as by fertilization would have a twofold advantage.
3. With sexual reproduction selection acts on the genic or additive
component of the genetic variance, whereas selection among asexual
individuals acts on the genotypic or total genetic variance. If the
genetic variances are the same, an asexual species can respond more
rapidly to selection - at least for a limited time.
4. If dominance and epistasis are present, there may be segregation
and recombination loads.
5. Free recombination does not provide a way for two or more rare
genes that are individually deleterious, but collectively beneficial, to
spread through a population.
6. There are well-established examples of situations in which there is
a clear advantage in holding certain genes together. Sexual
reproduction regularly breaks up favourable gene combinations.
3. The modes of sexual selection
I. Epigamic Selection
A. Based on choices made among courting partners
1. The choice among the different types of suitors is dependent
their relative frequencies
2. The choice is not frequency-dependent
B. Based on differences in breeding time: superior suitors offer to
breed more at certain times than at others
II. Intrasexual Selection
C. Precopulatory competition
1. Differential ability in finding mates
2. Territorial exclusion
3. Dominance within permanent social groups
4. Dominance during group courtship displays
D. Postcopulatory competition
1. Sperm displacement
2. Induced abortion and reinsemination by the winning
3. Infanticide of loser's offspring and reinsemination by the
4. Mating plugs and repellents
5. Prolonged copulation
6. In `passive phase' of courtship, suitor remains attached to
during a period before or after copulation
7. Suitor guards partner but without physical contact
8. Mated pair leaves vicinity of competing suitors
Table after E.O. Wilson (1975)
Sex Differences: Readers and Monographs
Ashmore, R.D. and F.K. Del Boca (Eds.) (1986) The Social Psychology of Female-
Male Relations: A Critical Analysis of Central Concepts. Orlando: Academic
Badcock, C. (1990) Oedipus in Evolution: A New Theory of Sex. Oxford:
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Bardwick, J.M. (1971) Psychology of Women: A Study of Bio-Cultural
Conflicts. New York: Harper & Row.
Bardwick, J.M. (Ed.) (1972) Readings in the Psychology of Women. New
York: Harper & Row.
Bell, A.P.; M.S. Weinberg and S.K. Hammersmith (Eds.) (1981) Sexual Preference:
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Bernard, J. (1981) The Female World. New York: Free Press.
Blum, H. (Ed.) (1977) Female Psychology: Contemporary Psychoanalytic
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Bonaparte, M. (1953) Female Sexuality. New York: International
Campbell, B. (Ed.) (1972) Sexual Selection and the Descent of Man 1871-
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Chodorow, N. (1978) The Reproduction of Mothering: Psychoanalysis and the
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Cox, S. (Ed.) (1976) Female Psychology: The Emerging Self. Chicago:
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Dahlberg, F. (Ed.) (1981) Woman the Gatherer. New Haven: Yale University
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This is the introductory chapter of The Nature of the Sexes: The
Sociobiology of Sex Differences and the `Battle of the Sexes' (Groningen: Origin
Press, 1992). There are still some copies available at a reduced price.