The Evolutionary Psychology of Human Sex and Gender

By Daniel Pouzzner, 2000-Apr-11/12


phase contrast micrograph of cyanobacteria
cyanobacteria: of asexual procreation
One jaguar, head first, attentive
jaguar: of sexual procreation

Sexuality is an indispensable mechanism whereby biological evolution bridged the immense gap between asexual/Lamarckian unicellular organisms, and the now plainly preeminent humans. For tens of millions of generations, the lineage leading to humanity has been formed and continued exclusively by sexual procreation. The precise microbiological mechanics of sexuality vary widely from species to species, while the functional consequences - strictly sexual procreation, in which each individual has precisely two parents, each contributing roughly half of the individual's genome - is absolutely conserved. This evidences the overwhelming evolutionary pressure in favor of the sexual mechanism.

In this essay, I give an overview of the differences between the male and female human phenotypes, concentrating on mental distinctions. For a broad treatment of human brain structures and mental phenomena, see The Symphonic Architecture of Mind, also by this author.

The only universal genetic difference between a male human and a female human is that the male has an additional chromosome, ``Y'', and it is by far the smallest of all. This tiny minority of genetic content is all that's necessary to toggle between a male and female phenotype - and in fact most of the male-specific blueprint is not on the Y chromosome at all, but only toggled by it.

Phenotypically, males and females share every organ - every muscle, every bone, every viscus, and indeed every suborgan of the brain - each serving the same function, interrelated to other organs in precisely the same manner - with qualitative gender dimorphism in only the tiny minority with exclusively procreative functions, and a small fraction of the machinery of the brain, particularly certain portions subserving emotion. Overall size - and certain proportions (particularly hip dimensions, but also nuances of facial features, relative neck size, and various other details) - are dimorphic, and males obviously have more hair (particularly on the face), but males and females otherwise look the same.

The basic evolutionary pressures on males and females are identical. Without regard for gender, genes tend to flourish which encode phenotypes that are successful in obtaining for themselves and their relatives (those who share their genes - particularly children, of course) healthy and regular supplies of water and foodstuffs, in procreating, and in protecting themselves and their relatives from the diseases and injuries that are caused by environmental elements, microbial insults, from social or predator attack, or from accidents and errors of action. This success has ample social dimensions, and because humans are highly social, there is a definite - if modest - gender dimorphism in the mechanical and mental aptitudes of males and females. Nonetheless, these dimorphisms are quantitative differences. Neither gender has anything approaching a monopoly on any qualitative capability aside from the basic mechanics of procreation themselves (impregnation vs. pregnancy and infant nutritional provision).

In particular, cognition and emotion are characterized by very little dimorphism. It is far easier to enumerate the differences than the similarities, and I will do this forthwith.

Cognitive and emotional gender dimorphism correspond directly to dimorphism in somatic anatomy and metabolism.

Female procreative organs implement the mechanics of pregnancy. The activation of this system is expensive metabolically, and grossly impedes the envelope of physical performance. This activation is, however, prerequisite to survival of the species. Female procreative organs are internal and substantially protected from the elements and from injury. Male procreative organs, in contrast, are exceedingly exposed, so that male fertility has an elevated sensitivity to physical injuries that are symptomatic of non-competitiveness. Male procreation does not usually involve any metabolic or physical taxation beyond that incurred by the sex act.

Sexually mature females have mammary glands that are deleterious to performance in locomotion, combat, and many other physical activities, but permit the provision of nutrition (and immune system information) to children, which is prerequisite to survival of the species.

On average, within any congenetic and ethnically homogeneous sexually mature human sample space:

On average, males are distinctly superior at hurling rocks and spears at targets. In fact, on average they are discernibly superior at the pursuit of goals through direct action without a social intermediary. A female on average has a corresponding advantage in using social mechanisms (particularly, language) to lead another person to act in pursuit of her goals. Though the dimorphism in tendency and aptitude is definite, of course males nonetheless pursue goals through social mechanisms, and females do so through direct action without a social intermediary.

Famously, males tend to navigate spatially, while females tend to navigate using landmarks. This has been accounted for by reference to the stereotypical role of the male as journeying hunter and the female as homebody managing the inventory of household implements, but of course this tendency is far from a hard and fast rule, and so does not in fact evidence a strict compartmentation of roles along those lines.

Males, because of their physical prowess and since they are not weighed down by pregnancy or children in tow, are clearly better suited to hunting journeys and to combat. To be effective, these enterprises require social organization and coordination, and in particular require that people be related to each other in terms of who coordinates whom - that is, be organized into social hierarchies. Males thus have a dimorphic phylogenetic emotional predisposition to accept and perform within these organizations, and part of this is a predisposition to obedience and leadership. One consequence of these predispositions is that, while a female tends to have an advantage in leading another individual - her mate, in particular - a male tends to have an advantage in leading many other individuals at once - a whole community, for example.

On average, males have a higher density of neurons in cortex, roughly 35000 more per square centimeter. This, in combination with an average brain volume (for congenetic adults) 10-15% larger than the average female brain volume, endows males with a higher information storage and processing capacity. The pressures that led to this dimorphism are clearly complicated. Salient among them are the asymmetric advantage to males of detailed working knowledge of broad expanses of geography and its concomitant fauna, for the purposes of hunting and warfare. Also asymmetrically advantageous is detailed knowledge of the position, role, attributes, and personal connections of the great many other people in the community with whom hunting and warfare operations must be coordinated. The cognitive demands of courtship (discussed below) tend to be greater for males. Opposing the pressure for a larger more energetic brain is the pressure for a brain that is metabolically easier to maintain. It is selection for metabolic maintainability that results in females being smaller than males overall, their brains included.

Because the loss of a few males - even quite a few - often results in no significant reduction in the number of the next generation, and because for a male the procreative dividend of heroic excellence tends to be immense, on average males exhibit less strategic conservatism than females. If an able male were to engage in coldly rational risk analysis, this is precisely the strategy he would pursue: the dividend and expectation of success in a heroic pursuit is so huge that the penalty and expectation of failure is overwhelmed.

Because the number of the next generation is mostly insensitive to the number of males engaging directly in procreation in the current generation, males are in fact instinctually more free to dedicate time and energy to matters wholly divorced from their own direct procreative prospects. Males are simply under less evolutionary pressure to procreate successfully. A male is nearly as effective at propagating his genes indirectly, by attending to the logistical and strategic interests of his congenetic community, as by actually procreating directly. In some cases this may even be more effective than direct propagation. Upon close examination, it is my impression that male homosexuality is a bona fide physiological condition in most cases. This is quite congruous with the reduced evolutionary pressure on males to procreate directly. In fact, it can be argued that the incidence of male homosexuality is evidence of a selection for community members who contribute only indirectly to the evolving genetic corpus of the community.

This incidence is created in a particularly clever way. With each subsequent pregnancy with a male fetus, the likelihood of a homosexual phenotype increases by about a third. This may be because a woman tends to subject successive children to increasing concentrations of testosterone in utero, which in utero has feminizing effects (estrogen has masculinizing effects in utero). Or it may be an effect of the mother's immune system activity. Whatever the cause, with progressing probability, the male children of prodigious mothers have an attenuated or wholly lacking instinct to procreate (with a drive to engage in superficially similar acts remaining as a largely inconsequential residue, venereal disease notwithstanding). The particularly clever aspect of the mechanism is that it tends to offset the mother's genetic over-representation, caused by a plethora of male offspring, with effectively inhibiting distortion of the procreative instinct.

In contests to win the favor of females, males pit themselves against each other, and put on displays to demonstrate their fitness. This activity is often dangerous, and consumes a great deal of time and energy, which is therefore not available for directly constructive efforts of immediate benefit to the community. Thus, reduced participation among males in procreative competition, at the individual level (as suggested in the previous paragraph) and in terms of time invested (as discussed below, regarding menstrual synchrony) is a collective adaptation. Of course, monogamy is the major mechanism by which males and females alike avoid the perils of procreative competition, and the modern free market system is the major institution whereby competition among males is made to constructively benefit the community instead of detracting from it.

Construction and interpretation of sexually motivated behavioral messages is highly dimorphic. Typically, a female offers noncommittal clues, requiring careful decipherment, that lend themselves to easy and plausible denial if her interest dissolves during courtship. She requires that courtship endure for a lengthy period, sometimes many months. A male, in contrast, readily and early offers explicit and undeniable declarations and overtures, and prefers the shortest possible courtship. The manner of interpretation is a mirror image of this. The female ratifies only explicit and undeniable declarations and overtures, and then only hesitantly and with the proviso that economically expensive activity be displayed in clear support of the declarations. She is largely oblivious to noncommittal clues. The male is very sensitive to any conduct which might constitute a subtle or covert declaration or overture, and tends to interpret and welcome them as such, with little hesitancy or doubt. The female presents a bevy of puzzles for the male to identify and solve, demonstrating his prowess, whereas the female is loath to address any puzzle posed by the male, who does not tend to present them in any case. The rationale for these radically dimorphic adaptations is the radically dimorphic level of risk and investment represented by the sex act. The male stands to lose almost nothing by it, and so interprets even the subtlest and most ambiguous of gestures as an overture to engage in sex. He easily delivers dramatic and plain declarations and overtures, since so little is at stake. In contrast, the female potentially stands to lose everything, and so requires thorough, undeniable, and consistent declarations of interest and commitment by the male, clearly corroborated by his conduct. She delivers dramatic and plain declarations and overtures, if at all, only after a lengthy and exhaustive courtship has confirmed the viability of the male.

On average, the magnitude and incidence of jealousy is roughly similar in males and females. However, its causes are distinctly dimorphic. Male jealousy is usually prompted by a suspicion or determination that a female partner is engaging in coitus with someone else. Female jealousy usually follows from a suspicion or determination that a partner's emotional fidelity has been diluted or redirected by involvement with someone or something else - involvement which needn't even be sexual. The explanation for the dimorphism is clear: male procreative strategy centers on successful monopoly of a female's procreative organs, whereas female procreative strategy is the pursuit of a monopoly on the protective and productive attention of a male partner.

Males have a tendency to specialize - to construct intensely detailed cognitive models of, and direct intense attention at, a particular narrow field of activity. Specialization is itself a risky strategy, since a skillset can be rendered obsolete by cultural innovations, but competitive excellence can be attained only by specialization. (Importantly, an individual is not limited to a single area of specialization.) Specialization is not about adequacy, it is about excellence. A community without specialists is at a gross competitive disadvantage compared to one that has them - this is why division of labor is de rigueur in real world economic practice.

Indeed, competition is a far more natural theme for males than for females. Cooperation is clearly a natural theme for both genders.

Motherhood is not in fact a specialty, and females are predisposed to construct sweeping, generalistic, integrative world models, suitable for presentation (to children, but also to other community members) and enforcement (detecting when a rule has been broken, and directing corrective action). That there is a phylogenetic institution concerned with forming, relaying, and indeed imposing integrative world models, is vital to cultural continuity and consistency, and this continuity and consistency immensely benefits survivability.

The menstrual cycles of women in prolonged proximity synchronize, so that whole communities can achieve a synchrony in which procreation exerts a major influence on community activities for only a week or so each month. This adaptation has multiple dividends. The disruption and destruction of procreative competition is confined, allowing the community to concentrate on other matters most of the time. That competition is also concentrated, so that the procreative prospects of less fit males are made all the dimmer.

This synchrony also tends to inhibit the genetic uniformity and potential for incest that would result if a single dominant male were able to impregnate many or most women in his community by moving at a leisurely pace from one to another. With the traffic jam of fertility created by cyclical synchronization, he is simply unable to reach most women before they have already been impregnated by other men who are in regular and prolonged proximity to the women. Though it is hardly prohibitive, the synchronization phenomenon is a phylogenetic pressure toward monogamy or diversity, and against harem-like arrangements.

Human sperm is of a low quality - relative to many animals, including primates, a low proportion of the sperm is viable - and this encourages pair bonded sexual partnerships to the detriment of any arrangement of irregular and infrequent coitus, particularly harem-like arrangements and surreptitious affairs.

Women's visual and olfactory sexual preference is biased toward desirable partners during the period of fertility and toward less desirable ones the rest of the time. This is likely an adaptation running counter to monogamy, which tends to maintain the procreative availability of desirable partners and the pacification and utilitarian engagement (household protection and production) of undesirable ones. Underscoring this adaptation, incidence of orgasm in females is predominantly dictated by the perceived sexual (hereditary) desirability of the male, and household partnership or emotional involvement is not a predictor. This is significant because the vaginal and uterine contractions that accompany orgasm promote successful fertilization. Exerting an opposite influence on the probability of successful fertilization is ``flowback'', by which unwanted semen is expelled from the vagina.

Human ovulation is concealed, not advertised. This is unusual among extant species, and the development of this adaptation underscores how vital it is to the evolution of the species that women maintain substantial, practical, if largely unconscious control over their impregnation.

The temporal window during which fertilization is possible ends just 24 hours after ovulation. Only during this period, the uterine mucosa drop their immunity-enhancing viscosity sufficiently to allow penetration by sperm. Desired sperm that arrive prematurely are waylayed and suspended in pockets in the cervical wall, and are reanimated by a signalling mechanism when the fertilizable egg makes its entrance. The pistonlike thrusting of the penis during coitus, combined with the foreskin behind and under which debris is accumulated, constitutes a pump that works to remove seminal matter that might have been deposited by a sexual competitor. Additionally, the duration since the male last engaged in coitus with the instant partner modulates the volume of his ejaculation, to better the chance that a competitor's attempt at fertilization will be thwarted by his own.

Oral sex is a practical behavior. Through various sensory channels - but primarily, through olfaction and taste - the one playing the oral role is able to gain information revealing the health of a present or prospective partner, and often, whether he or she has recently engaged in sex with someone else.

Masturbation in males serves to purge the seminal tracts of sperm that is no longer viable, and is perhaps useful in purging the system of other perishable fluids. Male masturbation culminating in orgasm (and concomitant ejaculation) is only possible with direct physical manipulation. Masturbation in females serves no practical purpose, and orgasm can be attained completely in the absence of direct physical manipulation.

Female homosexuality is largely or entirely an unintended and inconsequential result of the sexual drive. Exclusive homosexuality (as distinguished from bisexuality) is pathological. Even among exclusively homosexual females, maternal instincts tend to be very much intact and overpowering. Insemination does not strictly require the cooperation of the female, or in general require any organized action by the female, so that the pressure to ingrain fixed action patterns of procreation initiation is reduced relative to that on males.

Rape is clearly disfavored. The positioning of the male genitalia so that they are vulnerable to blows and injury clearly tends to inhibit rape, though I think it unlikely that an adaptive selection against rape accounts for the arrangement since the arrangement is much older than bipedal mammals. The positioning of the female genitalia so that insemination is impossible without separation of the legs is also an arrangement that disfavors rape and is probably driven partly by that dividend (lower animals have a similar arrangement by way of a tail and the act of sitting). Flowback and concealed ovulation, and some of the related mechanisms discussed above, also strongly disfavor rape as a procreative strategy, and almost certainly did develop precisely to this end. Rape is disfavored because the strategic judgement of the female regarding when and with whom to procreate has major immediate and long-term survival benefits.

Effective female procreative strategy is very different from effective male strategy. There is a definite and not particularly prodigious limit to the number of children a single female can produce. Much more important to this analysis is the extreme cost and risk involved in human pregnancy and childbirth (especially in the pre-technological era during which humans evolved to their present state), and the extreme cost of rearing a human - a cost that tends to be asymmetrically borne by the birthing female. Females, particularly those who are pregnant or have immature children, are thus extremely risk-averse. For most, their inherent preference is a very orderly and stable community that exchanges freedom for security, and less reliable private resources for more reliable collective resources. Moreover, this tendency to prefer security over freedom tends to be proportional to the sexual attractiveness of the female, since the risk of rape is otherwise similarly proportional. This theme culminates in what Lionel Tiger (professor of anthropology at Rutgers) terms ``bureaugamy'', in which women embrace the state as protector, provider, and head of household. Bureaugamy is a hybrid of three components: the insatiable female appetite for security, the institution of unrestrained universal democracy, and Hegel's vaunted bureaucracy, whose agents - in order to obtain for themselves resources and importance - orchestrate the extortive predation of resources from producers, the bribing of constituencies with those resources, and the protection of constituencies from predation.

Female procreative strategy is like that of single-celled organisms - steady, inexorable multiplication - and this phylogenetic procreative strategy greatly colors their total phylogenetically predetermined ethos. Evolution has not yet had an opportunity to embed in the genome awareness that this strategy is disastrous when pursued by a technological species. It is an inherent conflict in the species that this strategy is phylogenetically predominant for females, while the characteristics that lead to technological innovation are phylogenetically predominant for males.

Consistent with this strategy of inexorable multiplication, females prefer to procreate with males who are important, leading to the dimorphic tendency of males to pursue importance. Importance comes in two flavors: influence or control over many other people (elevation in the social hierarchy), and predication of the welfare of many other people on the success of the male at issue in his chosen vocation. In the former case, the advantage is that the male is positioned to exploit the resources of the community to increase procreative success, and that the male bears genes that are conducive to elevation in the social hierarchy. In the latter case, two distinct pressures are at work. The first is similar to that which accounts for the appeal of influential males: the family of a male on whom the community relies can expect extraordinary graces and protection from the community. From the point of view of the female, the male's vocation is just a roundabout route to influence. The second pressure is quite different. The perpetuation of genes that produce people with aptitude for activities on which community prosperity is predicated is vital to the community (in the next paragraph, a special case of this dynamic is treated), and so is selected for - by tailoring of female human nature - prima facie. To a certain degree, this pressure is at work even in the female preference for leadership per se, since a community organized coherently by an effective leader is more competitive than one that is socially disorganized or incoherent.

In a seeming paradox, females often choose to procreate with males who have a phylogenetic predisposition to wreaking havoc on order and stability - who are the sort one might expect to choose and succeed in heroic pursuit. The explanation for this instinct is that, without the perpetuation of genes that lead to such tendencies, the community loses its capacity to accommodate crises (disorder and instability - particularly, warfare and natural disasters) and so is at a gross disadvantage relative to a population that has retained this capacity. In fact, without this capacity, humanity might have already passed into extinction. The iconoclast is despised for the disruption he threatens or causes, until the day he is revered as a hero for saving society from far graver disruption.

In a mirror to the seeming paradox of the previous paragraph, males - many of whom have a phylogenetic predisposition to shattering order and stability - often choose to procreate with females who are risk-averse and hostile toward things that threaten order and stability. The explanation is also a mirror - stabilizing and preservative influences have great survival value - but more obviously, these traits are precisely those that are conducive to successful rearing of children.

It is well to mention some basic attributes, some dimorphic some not, that play prominently in the perception of sexual desirability. The waist to hip ratio preferred by average males in females is .65, whereas that by average females in males is .85. These are the average ratios seen in the population. Females prefer males with proportionately larger necks, and males prefer females with proportionately more slender necks. This is also observed in the population. Other typical markers for the estrogen suffusion that accompanies normal female puberty are a small chin, full lips, amply proportioned breasts, and relatively prominent eyes, and these traits are attractive to average males, though their adaptive value in and of themselves is otherwise dubious. In particular, humans are unique in that the female's breasts are voluminuous even when there is no child to nurse. This unique adaptation has been explained by human bipedality and frontal mating, unique among mammals, making it advantageous to echo the age-old appeal of full buttocks with full breasts. At puberty, the relative leg length of the female increases, and this too is attractive to average males, though it is also strictly adaptive.

Though large differences in heritable quality between two females both of childbearing age easily overwhelm it, males clearly prefer females who exhibit the physical signs of youth. These signs are present from adolescence to roughly age 27. The physical robustness of youth is directly and dramatically associated with the physical capability to succeed in pregnancy, birthing, and rearing of young, and this wholly explains the male's preference. Younger females are less likely to have sustained injury, or to have contracted an infection or disease.

Sexually mature females preserve the characteristics of children to a far greater degree than do sexually mature males. The chief advantage of this adaptation was briefly mentioned above: the potential for expressiveness is heightened, as is the prospect that others will act toward them protectively (because treating them as children). Some of these childlike characteristics attenuate resistance to injury, but (as explored above in detail) they tend to be beneficial overall. The male's sexual preference for young females (who superficially resemble children) is sometimes distorted into a pathological attraction to or preference for children as objects of sexual attention.

Typical markers for the testosterone suffusion that accompanies normal male puberty are a large chin, a prominent brow, relatively sunken eyes, bushy eyebrows, dense facial hair, deepened and strengthened voice, and amply proportioned musculature, and these traits are attractive to average females. The adaptive value of these traits is clear: they promote pugilistic, industrial, and social robustness and effectiveness, and general resistance to injury. Facial hair moreover tends to conceal injuries - an asset in and of itself.

An intense preference for facial and body symmetry is common to both genders. Somatic and cosmetic asymmetries record failures of an organism to adhere to its genetically encoded body plan under environmental stresses, and so tend to evidence counter-adaptive genes.

Females, in contrast to males, have no particular age preference per se. Instead, they prefer males who have demonstrated exceptional leadership, survival, and productive capacities, and these attributes tend to be evident in older males. Young males still exert a particular attraction for females, however, because they have the youthful physiological vigor with which to provide for the female's physical requirements, and a psychological immaturity that facilitates the bending of his will to her purposes.

Males are sexually stimulated by images of female genitalia, but females are relatively unresponsive to images of male genitalia. This dimorphism is explained by the differing roles of the two genders in procreation. As noted above, mechanically the male plays the active role, and the deliberation and cooperation of the female are not necessary, only facilitative. Thus, whereas in males specific immediate action prompted by the sight of the female genitalia is decisively adaptive, in females it is not nearly so consequential, and so a similar instinct does not develop clearly. Females are, however, sexually stimulated by images of the faces of desirable males. This response adaptively interlocks with the various mechanisms whereby the female modulates her fertility (discussed above).

In the industrial and technological era, the non-dimorphic preference for an athletic build and sun-tanned skin is enhanced because these traits have become luxuries whose attainment is a time-consuming distraction from the formulae of economy, and thus whose display advertises the excess fitness of the individual. As technology makes the supply of food regular and dependable for the prosperous, the survival advantage of excess fat vanishes, so that trimness becomes an advertisement of prosperity. With ``anorexia nervosa'' and ``bulimia nervosa'' this trend is carried to the point of pathology.

*

Contraceptive technologies radically alter the cost analysis of, and distort the social dynamics associated with, sexual conduct. In general, the balance of power is upset, tilting dramatically back toward the female. The female's preference for security-enhancing social institutions is reduced, since rape does not lead to pregnancy. Notwithstanding the relative rarity of the measure, personal weapons (particularly, handguns) are the preeminent contraceptive technology, and prevent not only the pregnancy, but the rape act itself, reducing the preference for security-enhancing (and freedom-impairing) social institutions to the maximum degree.

The statistics of firearms ownership and usage bear out the protective role envisioned by women who arm themselves, and also attest to the profound predisposition of females, noted above, to act through social intermediaries (or, at least, in a socially constrained way) rather than directly. The Gallup organization, in a poll conducted in 2005, found that 47% of men report gun ownership, compared with 13% of women. Of those who did report gun ownership, 74% of women reported self-defense as a reason, compared with 63% of men, and women reported hunting and target shooting as reasons at lower rates than did men. Inge Anna Larish, writing in the University of Illinois Law Review (“Why Annie Can't Get Her Gun: A Feminist Perspective on the Second Amendment”, 1996), notes that “While violent crime against young males decreased from 1974 to 1987, violent crime against women increased nearly fifty percent. Ninety percent of the 4,693 women murdered in 1991 were slain by males (only one out of eight males murdered in 1991 were slain by females) and every fifteen seconds a woman is battered.” That women have more to gain from gun ownership than do men is clear, and that gun ownership is much less prevalent among women than men is equally clear. This game-theoretic disparity is probably explained at least in part by phylogenetic predisposition, and the inability of phylogeny to accommodate the rapid, fundamental changes of circumstance brought about by technological innovation.

With medical contraception (through pharmaceuticals, mechanical apparatus, surgical procedures creating temporary sterility, and abortion), the female is able to deftly manipulate her fertility, so that she chooses the occasion of impregnation (if any), and the father of her children, with great precision and with no necessary regard for her cohabitative partner. This is clearly important, though it is simply a quantitative enhancement of an important phylogenetic process. Medical contraception, because it is predicated on the continued availability of complex technological products and services, reduces the female's preference for security-enhancing social institutions to a far lesser degree than do personal weapons.

All contraceptive technologies have a similar impact on the procreative dynamic. Consider this parallel: if an appointment by a person (in a bureaucracy) is ``sticky'' - can't be revoked absent impeachable conduct by the appointee - then the appointer will make his choice carefully and thoughtfully (at least comparatively). If the appointer can revoke an appointment at any time, for any reason or no reason, he will tend to make sloppy appointments, under the ``let's give it a shot'' principle. In the former case, there is a recognition of investment and concomitant careful attention, while in the latter there is no sense of investment. The trouble is that the latter system promotes neglect. Since repair requires action - the revocation of an appointment - the tendency is to under-repair, and so the bureaucracy steadily decays as it is populated by a steady stream of slipshod appointments.

The parallel of the above to the instant biological dynamic may be obvious. If a woman has no sense that a sexual partnership is likely to constitute a procreative investment, she will be less discriminating in choosing her partners. However, the very acts at issue produce psychological attachments that tend to lead to actual procreation, as the female deliberately removes the contraceptive technology (typically after the couple has wedded). But the relatively unfit partner has snuck past the female's discriminatory sentry. In a natural setting, he never would have been allowed to be intimate in the first place, and so the attachment, wedding, and procreation, would never have occured. The forward-going result is a tendency toward lowered fitness.

The female is able to use sex as bait, with no significant metabolic consequence to herself (except that abortion, if this is the chosen means of contraception, is moderately risky and injurious). Using this tactic, females can create Skinner boxes that reward compliant behavior in a lengthy campaign. By introducing noise into the system (imperfect consistency in the reward signal) the indoctrination of the male is made most effective.

STDs exert a pressure on the system approximately opposite that of contraceptive technologies. A contraceptive technique or suite of techniques that is both covert and provides confident protection against STDs maximizes the influence of the phenomena described above.

Absent immunity to STDs, sexual activity is associated with important costs and risks for the female, remotely similar to but hardly interchangeable with the investment dynamic of procreation itself. The discriminatory principles at work are quite different.

With investment by procreation, the forward-going fitness of the partner is a paramount determiner, both because this fitness will tend to be propagated to the children, and because this fitness will allow the partner to effectively provide for the needs of the female and her children.

Absent procreation, but present the risk of STD infection, the discrimination includes no such longitudinal strategy. Determining whether a prospective partner is infected is itself a cost, and the requirement definitely tends to make the female less profligate, but the selection of partners will nonetheless be less sensical then in a completely non-technological setting.

*

Mothers inherently tend more toward other-centeredness (tailoring behavior for the benefit of another) than do males, or females who are not mothers. The latter evidently tend toward self-centeredness. Self-centeredness is so strategically valuable that it is often not abandoned unless and until other-centeredness is a clear imperative - that is, until a child's survival is predicated directly on the other-centeredness of its mother. Also consider that pregnancy itself is an arrangement in which the female is parasitized by a separate, if congenetic, organism. Also consider the modern institutions of adoption and surrogate motherhood, in which the instincts formed to benefit congenetic children are co-opted. The phylogenetic predisposition toward potential other-centeredness is clearly greater in females.

Note carefully that, in the final analysis, the other-centeredness of mothers is just as self-interested as the self-centeredness of others. It is simply that, by the action of certain brain organs, the reward for (pleasure from) the other-centered behavior of provision and protection, and the punishment for (pain from) failing in this behavior, overwhelms other reward and punishment signals.

For the most part, males are subject to no emotions of protective attachment with the robustness and intensity of a female's for her children. In the environment in which human nature was sculpted by evolution, there was no reliable method by which a male could determine that a child is his own, while a female could not help but know with certainty. In particular, there was a good chance that a male acting to protect a child would reinforce genes that originated with another male, actually inhibiting his own genes. Thus, the male's protective instincts toward a child have far less intensity than the female's - though of course they are still present, both because the child might be his or at least carry some of his genes, and because protecting the child protects the mother and gains her allegiance. In a clear adaptation to encourage protective involvement of the father, infant children bear a marked resemblance to the father - and not to the mother - regardless of the child's gender. An involved father is clearly a great strategic asset.

Human females, in an arrangement humans share with few other species (with certain higher primates and whales, and perhaps a few other mammals), often (and in modern times, almost invariably) live many years into menopause. Because rearing of human children is a lengthy and intensive process, childbirth made even more perilous by advanced age imminently risks leaving existing children motherless and so likely doomed. Moreover, procreation by a mother so advanced in age that she is unlikely to survive long enough to rear the new child, is futile.

A more complex dividend of menopause is the expected benefit of cultural continuity imposed by these females on their family and community, which at a certain age outweighs the expected benefit of additional direct procreation. Of course, the communitarian and domestic labor potential of the female is a simpler but similar dividend, with a similar evolution in relative weight.

People undergo progressive stages of mental petrification. The first stage accompanies the arrival of sexual maturity and is not very constraining. In the female, a second stage accompanies the onset of motherhood - this stage is due entirely to relative invigoration of the amygdala and closely related basal forebrain structures, raising the proportion of thought and behavior due to its actions. A third stage accompanies the onset of menopause. The value system of a female at the onset of menopause is almost certainly the value system the female will have until the arrival of death. The menopausal female is essentially incapable of changing major established values. The memory coordination system (c.f. The Symphonic Architecture of Mind) no longer coordinates broad persistent changes to plastic neural substrate, so that even when a persistent change of mind is consciously and clearly intended, the change tends not to occur. Through psychopharmacology, this petrification could probably be at least partially dissolved. Comparison between male and female mental petrification is difficult, because specialization - a male tendency - is a form of mental petrification that sets in early, but is not a neurophysiological constraint (changes of specialty are neurophysiologically feasible).

Among the above catalogue of mental dimorphisms a few have neurophysiological correlates in portions of the brain that are not dedicated to the mediation and projection of emotion. In particular, the traditionally recognized male aptitude for spatial cognition, and female aptitude for symbolic cognition - if indeed real at all (I am personally skeptical) - would have correlates in such non-emotional regions. The dimorphism in the form of aptitude (with vs. without a social intermediary) for pursuit of goals also must have such correlates. The dimorphism in the tendency toward specialization also must have such correlates. The staged petrification of the female mind is also a phenomenon implicating non-emotional regions of the brain. However, the great bulk of the dimorphism is embodied by the emotional organs of the basal forebrain - mostly, the amygdala. (In this paper, for the sake of cogent brevity I have omitted the correlations between behavioral dimorphisms and the nuclei of the hypothalamus.)

The amygdala is about the size of an almond, and is positioned just inside the brain's front surface, low within the anterior part of the temporal lobe. It is a heuristic engine that, largely by hereditary predisposition, models natural physical principles, particularly in terms of competition and cooperation, and generally by creating pressure to behave in a manner the amygdala adjudicates is conducive to long term (hereditary) biological survival.

The amygdala is the tyrant dictating the emotions described above. A brief examination of the overall pattern of this tyranny reveals that it is dimorphic in its overall influence. Female cognition, at least after attainment of sexual maturity, is more tightly constrained by amygdalar tyranny, than is male cognition. A very precise and conservatively planned behavioral dance is required of females, if the community is to increase (or at least maintain) its numbers. The amygdala patiently and relentlessly walks the female through these steps.

Males are subject to less mental predestination in general, and less amygdalar tyranny in particular, precisely because there is such an advantage in a thorough exploration of the range of possible individuals (and concomitant fitness) made feasible by the greatly reduced sensitivity of the population to failure by males. The mammalian brain outside the basal forebrain has such plasticity that the same genotype can lead, through divergent experience, to personalities (goals and values, cognitive aptitudes, habits, etc.) that are radically divergent. This free formation of personality is a gamble, one that - viewed from the perspective of game theory and system dynamics - makes far more sense for males than for females. Evolutionary pressure thus allows the female mind less freedom.

It is important to recognize that the amygdala's actions (and those of the hypothalamus) are not strictly reasonable, and are certainly not the product of the sort of explicit reasoning consciousness performs. All of the basal forebrain's actions can in principle be explained, of course, but they are driven by crude phylogenetic heuristic models that are prone to important errors of detail. Mature females, in whom the balance of power is on average tilted toward the basal forebrain (and away from the potentially conscious reasoning mind) more than it is in males, are likely thus more prone to carrying into action certain errant commands of the basal forebrain. Mature females are also less able than males to detect that a thought or preference is amygdalar in origin, since the amygdala is more entrenched by influential connectivity with the machinery of literate cognition (in females only, significantly lateralized to the dominant hemisphere).

I must stress this closing point: I know of no qualitative dimorphism of the human machinery of cognition. Despite the array of mental constraints enumerated above, a given male or female can, through mental discipline, perceive, cogitate, and act, quite independent of these constraints. That said, any person, male or female, will with some frequency encounter circumstances in which these contraints influence or dictate perception, cogitation, and action. Through discipline, an individual can make this the exception, rather than the norm.

By Rachel Emma Silverman

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Fathers often talk the talk about sharing parenting duties with mothers when it comes to a newborn. But a new study finds that couples who profess to believe in equally-shared parenting rarely do so in practice.

The researchers surveyed 181 married, heterosexual, tenure track professors with children under age two. All of the professors had access to paid parental leave. Each survey participant was asked how their handling of about 25 child-care tasks compared with their spouse's handling of the same tasks. Among the tasks: changing child's diapers; taking child to doctor; feeding the child; staying home from work to care for the child; giving child a bath. (See the full list on page 21 of the study.)

The majority of professors – both male and female, particularly the women – held the view that men and women should share child care duties. But only three of 109 male faculty members surveyed reported that they did half or more of the care, while 70 of 73 women reported doing at least half–even when both spouses worked full time.

The study found that female professors who take paid maternity leave spent most of their time off to focus on infant care, including breastfeeding. Male professors, on the other hand, used their paid paternity leaves to focus on things other than infant care, such as research and publishing papers.

The study also found that women enjoyed doing child care work more than men.

“Our results suggest that one reason why female professors do more child care may be that they like it more than men do,” the researchers wrote in the study. “This conclusion is possible even though the vast majority of female respondents and a clear majority of male respondents believe that husbands and wives should share child care equally. Gender ideology about care may be less important than feelings on these matters.”

The study, in the January issue of the Journal of Social, Evolutionary, and Cultural Psychology, was conducted by Steven Rhoads, a political scientist at the University of Virginia and his son, Christopher Rhoads, an assistant professor of education at the University of Connecticut. (One caveat: the survey of the professors was done in 2001 — and ideas about gender and parenting may have changed over the last decade.) (Our fellow WSJ blog, Ideas Market, also has a take on the research here.)

The researchers say that offering paid paternity leave might actually serve to advance men more than women because men tend to use the time for professional work. While only about 12% of men currently utilize their post-birth leave option, the study finds, “if men should begin to take leave in much larger numbers, far from leveling the playing field, gender-neutral, post-birth leaves are likely to tilt the field further in favor of men.”

One could also argue, though, that paternity leave offers men the opportunity to learn to do more at-home and child-care tasks — and true equality in the workplace will never occur unless there is home equality as well.

Readers, what’s your take: Is paternity leave just a free ticket for dads to advance their careers? Male readers, how did you spend your time if you took paternity leave? Check out this list of child-care tasks on p. 21 of the study. Who does more of the tasks in your family? And who enjoys them more?

from the Wall Street Journal, 2012-Nov-16, by Matt Ridley:

Does Survival of the Sexiest Explain Civilization?

Evolution by sexual selection is an idea that goes back to Charles Darwin. He had little doubt that it explained much about human beings, and modern biologists generally agree. One of them has even put a figure on it, concluding that some 54.8% of selection in human beings is effectively caused by reproduction of the sexiest rather than survival of the fittest.

Some years ago, the evolutionary psychologist Geoffrey Miller in his book "The Mating Mind" explored the notion that since human males woo their mates with art, poetry, music and humor, as well as with brawn, much of the expansion of our brain may have been sexually selected.

Recently Jason Collins and two colleagues at the University of Western Australia, in a discussion paper posted on the Web, have made the case that sexual selection explains civilization itself. They mathematically explored the possibility that "as females prefer males who conspicuously consume, an increasing proportion of males engage in innovation, labor and other productive activities in order to engage in conspicuous consumption. These activities contribute to technological progress and economic growth."

Psychological evidence points the same way. In one experiment, men who were shown pictures of women promptly expressed more extravagant desires for expensive luxuries, whereas women showed no such effect after seeing pictures of men. There's historical evidence, too. As Aristotle Onassis is supposed to have said, "If women did not exist, all the money in the world would have no meaning."

Moreover, Michael Shermer, in his book "The Mind of the Market," argues that you can trace anticapitalist egalitarianism to sexual selection. Back in the hunter-gatherer Paleolithic, inequality had reproductive consequences. The successful hunter, providing valuable protein for females, got a lot more mating opportunities than the unsuccessful. So it's possible that men still walk around with a relatively simple equation in their brains, namely that relative success at obtaining assets results in more sexual adventures and more grandchildren.

If so, this might explain why it is relative, rather than absolute, inequality that matters so much to people today. In modern Western society, when even relatively poor people have access to transport, refrigeration, entertainment, shoes and plentiful food, you might expect that inequality would be less resented than a century ago—when none of those things might come within the reach of a poor person. What does it matter if there are people who can afford private jets and designer dresses?

But clearly that isn't how people think. They resent inequality in luxuries just as much if not more than inequality in necessities. They dislike (and envy) conspicuous consumption, even if it impinges on them not at all. What hurts is not that somebody is rich, but that he is richer.

This is a classic statement of sexual selection. It isn't the peacock with the big-enough tail that gets to mate; it's the peacock with the biggest tail. If this sounds old-fashioned in an age of working women, gender equality and relative sexual continence, then open your eyes and look around you: The man with the most money or power still gets more sexual opportunities than the man with the least. Ask David Petraeus.

In human beings, females compete for males as well as vice versa. In many species, sexual selection is a force that acts on only one sex, usually the male. Peahens, which can share the best males and don't require them to be diligent parents after mating, do not grow colorful tails. But in other species, notably some seabirds and parrots, where males and females share parenting duties equally, both sexes are equally colorful—a result of competition by both sexes to attract the best mates.

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