Neoteny and Two-Way Sexual Selection in Human Evolution:

A Paleo-Anthropological Speculation on the Origins of Secondary-Sexual Traits, Male Nurturing and the Child as a Sexual Image

an article by David Brin, Ph.D.

Copyright David Brin. All rights reserved. No duplication or resale without permission.

Departing from the traditional view since Darwin, recent biological theory perceives evolution as a sequence of fairly rapid state changes that punctuate lengthy periods of relative equilibrium. There are several ways species can launch into rapid change:

• Geographic barriers separate sub-populations, isolating divergent gene pools. Long separations result in speciation. If groups are re-united before that point, a sudden influx of stockpiled genes from the isolated reservoir can speed change within the parent stock.⁹

• High attrition rates due to new environmental factors can speed adaptation. In particular, ever-changing suites of parasites seem to offer a badly-needed explanation for the existence of "heritable fitness," and even the existence of sex itself. (S.W. Gangestad, 1993.)

• Self-generated adaptation pressure occurs when a species opportunistically moves into a new niche, thereby encountering new life-threatening dangers. Thus, the novel opportunities offered by, say, taking to the trees, will be for naught unless the species soon evolves a healthy respect for arboreal snakes.

Another way evolution can accelerate is the major exception to natural selection admitted by Darwin, and the one way species can be said to design themselves: Sexual Selection. The bird of paradise and mandrill are vivid examples of what can happen when female choice of "quality" males becomes tied not just to the male's robustness or fidelity, but to some outward and apparently arbitrary physical display -- e.g., length of plumage or vividness of color.

We will not go into the details of this process, or the ongoing debate over whether or not such traits advertise health or heritable fitness (Thornhill & Gangestad, 1993). Sexual selection in humans is discussed by Gangestad (1993), where it applies to matters of simple, first-order self interest optimization by human females (the presumed choosers).

What has long escaped discussion are the second-order effects, where "runaway" sexual selection may have resulted in human traits that are as exaggerated as any bird's tail. Nor has there been much investigation of females as objects of sexual-selection, rather than simply as classical selectors.

In "runaway" sexual selection, the selected trait becomes more embedded and exaggerated with each passing generation, requiring the next wave of the selected sex (usually males) to compete from a new plateau, which amplifies the trait even more, and so on. Even if the degree of exaggeration threatens the viability of the species at large -- e.g., the titanic antlers of the extinct Irish elk -- this may not abate the driving competition among individuals for reproductive success.¹⁰ The models of R. A. Fisher (1958) long ago showed that evolution of a sexually selected trait, and the preference for it, can strongly correlate, with both accelerating in tandem.

Why is it nearly universally males of species who become burdened with huge antlers, giant tail feathers, or other garish exaggerations? A mistake of teleology might claim this is only fair, since females carry the major costs of reproduction, and a larger share of the risk. A more valid explanation lies in the fact that females in these species have a dictatorial veto over which males get to breed. Males wind up being selected to satisfy any criteria females get in the habit of using.

But now let us return to the situation among humans. We have seen that Homo sapiens has a queer arrangement in which both sexes must compete for partners, and both, in turn, must choose. The stage is set for trait-runaway by sexual selection to take place in an unusual two-way mode -- acting not only on males, but on females as well.

Human runaway sexual selection? At first glance we would seem too sensible a species for anything like that. We don't appear to have been saddled with burdensome exaggerations like antlers or bright tails. Or have we?

Consider the greatest exaggeration of them all... our powerful, out-sized brains. Not only do large infant craniums put human mothers in great stress while giving birth, the brains within strike some biologists as extremely perplexing. In their cups, philosophical anthropologists can sometimes be heard wondering why humans "overshot" the mental capacity we needed in order to become masters of the planet -- in other words, competent hunter-gatherers with stone tools and fire. That was enough to remove a lot of environmental stress, and should have led to a period of equilibrium. Instead, change only accelerated, until in short order we produced encephalization capable of conceiving mathematics, spacecraft design, and music more precise than any bird or whale could ever produce.

One possible solution to the problem of overshoot is that, quite simply, men and women might once have found the trait of intelligence "sexy" in each other. Brains, then, would be our equivalent of peacocks' tails... except in our case selectivity was shared by both sexes, and in turn both sexes shared in the amplified trait.

Are there other possible pivots of sexual selection? While watching out for cultural interference, consider, what do contemporary men and women say they want in the opposite sex? David Buss (1994), a University of Michigan psychologist, conducted a survey of 10,000 people in 37 cultures on six continents, concerning the traits people find attractive in the opposite sex. Oversimplifying somewhat the results of Buss and others¹¹ -- those attributes listed as most desirable fit the priorities discussed in section one. Women tend to rank first kindness, intelligence, and self-confidence. Also rated highly were accomplishment, reputation, health, vigor, reliability, and sense of humor. Physical handsomeness, while appreciated, is usually not among the highest mate-choice desiderata, except when the topic is extramarital affairs. Youth is not a major consideration.

Again oversimplifying, men seem to evaluate women in two stages. Stage one begins and ends with physical attractiveness--manifested in terms of health, youth, and secondary sexual characteristics. These alone are generally enough to give rise to at least mild sexual fantasization. A whole new domain opens however, when men contemplate marriage or committed alliance, at which point some men contemplate the very same traits listed in the previous paragraph -- the sorts of things that might help determine a woman's suitability as a long-term partner and ally. The mere existence of stage two in human males is actually quite remarkable, as mammals go -- a strong sign of men's movement toward a more monogamous reproductive strategy, in which his choice is as nearly important to him as a woman's is to her. As we have seen though, this applies only to some men.

But let's go back and consider a man's stage one. Health and youth, as prime triggers of initial male arousal, make Darwinian sense. For while sperm is cheap, it makes little sense to deposit anywhere it will do no good. On selfish-gene terms, a male will be attracted to copulate with a female young and healthy enough to be fecund.¹²

But what of those pronounced secondary female sexual characteristics which make up the third trigger of male arousal? Here we see strong indications that women have been competing with each other for quite some time, and a degree of "runaway" has indeed taken hold to dramatically alter their form and destiny.

One exaggerated female secondary sex characteristic is large breasts. Back in the 1960s, some anthropologists proposed that the "purpose" of these enlarged glands -- often far greater than needed to produce milk for nursing infants -- was to attract males to desire copulation. Some even suggested that breasts mimic the twin globes of the buttocks, and became adaptive as a way to entice males to mount face to face. This hypothesis has major flaws:

1. We now know a sub-species of chimpanzee, the bonobo, or pygmy chimp, routinely mates face to face without any such adaptation. So do orangutans.

2. Anyone thinking that a typical male, once already aroused, needs enticement to mount simply does not understand males, period.

Anyway, the problem for human females was never to get males to copulate, but to get them to desire more than just one copulation... to willingly offer partnership lasting beyond the blush of youthful fecundity. Exaggerated breasts do nothing to enhance this, at least not directly. I will soon propose that their evolution was much more convoluted than that.¹³

But first, let us return at last to another secondary characteristic, one with far more influence than large breasts, or even youth and beauty, over a man's willingness to consider a woman a possible mate. After all, tastes toward those attributes vary considerably, among societies and from era to era. Even hourglass figures, which Devendra Singh (1993) finds to be desired across cultural boundaries, only serve as an anchoring mean around which considerable variation in desirability is seen.

We have already mentioned this other trait, which is an obligate attraction-trigger, in that its absence can be a nearly universal turn-off of male desire. This trait is some degree of neoteny of physical appearance -- or paedomorphism. Consider the obvious. Failure to retain certain childlike body attributes can be extremely prejudicial to a woman's opportunity to breed. Give or take a shadow, here or there, we know that most human males simply will not be attracted to copulate with, or pair bond to, women possessing beards! Nor are bony eye ridges, thick necks, or basso voices considered feminine. In their presence, even monumental breasts or perfect hourglass figures will not compensate.

If any trait is a likely candidate to have "run away" with women, as they competed and were chosen by men, it is very likely to be outward physical neoteny. There are several reasons why this makes evolutionary sense.

1. We were already headed in that direction. As stated earlier, humankind needed to become neotenous in order to retain into adulthood our child-like, flexible brains and personalities. This was especially crucial for the acquisition of language. With juvenilization already under way in some areas -- in neural wiring and behavior -- it is reasonable to suggest the trend might become the focus of sexual selection, taken in additional directions by one sex, under strong selection pressure from the other.

2. Neoteny is a general fall-back variable. We are not the only species to go neotenous. Under our selective influence, most breeds of dogs now show substantial neoteny over a wide range of independent attributes, from physical form to behavior. In fact, juvenilization may be looked at as nature's way of allowing a species to back out of an evolutionary corner and try again, starting with a fresher, more plastic set of traits.¹⁴

3. Neoteny is directly correlated with the very trait human females needed to attract in males. Consider the strange situation... human females were in competition with each other for mating, so they started developing external traits to attract males. But the problem was not simply to attract a male to desire copulation (which is trivial) but to attract the right type of male. In other words, the type of male given to protective or nurturing impulses.

Men are not langur monkeys. But even if infanticide played a role in our past behavior, there was also the countervailing tendency of tenderness to children. Studies by Robinson, Lockard and Adams (1979) showed that an infant's face -- especially smiling -- causes pleasure response at an involuntary level in many adult men, as well as large majorities of women. Countless tales of heroism by firemen and others who have risked their lives for the children of strangers show that this trait is well advanced, if not universally distributed among human males.

It is not at all preposterous, then, to suppose that when runaway sexual selection occurred in human females, it took off down a path that caused the external juvenilization of women... and that this was adaptive because it helped engender feelings of tenderness and protectiveness in some males. Tenderness which, in turn, might have been reinforced by female choosiness, so that trait was genetically rewarded in males.

The result may have been a cycle which continued round and round, accelerating with every loop... producing with each new generation females who were marginally more neotenous and choosy, as well as males who were marginally more likely to care what happens to their lovers and offspring. Such a cycle would have been self-feeding, self-reinforcing, and exceedingly powerful.¹⁵

Not only are human females compelled to compete for mates, something uncommon in most species, they appear also to have been the sex most changed (at least in outward appearance) by runaway sexual selection in Homo sapiens, making women the least primate-looking of all higher primates. In comparison, males have been left relatively untouched.¹⁶

Moreover, there is a predictable and tragic consequence to the development of neoteny as an emblem of adult female attractiveness. Consider it this way. Sexual selection requires two partners in order to work, first the sex under competitive stress (normally males, but in this case women) among whom a certain fraction are "chosen." If some trait has a high correlation with reproductive success, the prevalence of that trait will increase in the next generation. And true runaway will accelerate even faster if the choosers change as well -- becoming ever more critical and demanding of that trait. So if paedomorphism was women's runaway trait, there's every reason to picture men growing ever more attracted to paedomorphism in women, at a matching pace. Obvious enough, so far.

But paedomorphism means resemblance to children! Consider the bizarre dilemma, then. In order to attract quality mates -- protector types -- women began taking on the external features of the objects of the protective impulse -- children. This was rewarded, presumably, with reproductive success. But it also meant that men began associating with sexual desirability the very outward traits which are most directly associated with childhood!

The calamitous sickness of sex with pre-pubescents is one of the nastier features of our species. It is denounced by the majority, yet persists at low levels in all cultures, posing a dilemma for those contemplating a better tomorrow for our descendants. But now we might suggest one possible explanation of the origin of this dysfunction. It may derive, at least in part, as an aberrant offshoot from the two-way cycle of runaway sexual selection just described. If ever there was proof that evolution is not planned, this is it. An undergraduate could have predicted the tragic consequences.

Fecundity, health, neoteny... these are superficial signs which human males came to associate with feminine sexual attractiveness. Unfortunately, this boat-load of attributes was without a tiller, headed on a collision course with the best interests of the very children the whole game is about in the first place! What was needed was an emergency adaptation to help sane human males tell children apart from adult, fecund women. It is at this point, I contend, that human females developed a secondary set of exaggerated physical traits, not to elicit sex from males but in order to help high-end males across this tragic trap. One of the most pronounced of these secondary traits was the ballooning of women's breasts.

A male does not need the stimulation of breasts simply to desire copulation. But amplified breasts, along with the waist-pelvis flair, do add to a suite of characteristics which give a normal adult male permission to admit his arousal, even to himself. They are not so much signals to indicate fecund femaleness as indicators of female adulthood. Healthy men are probably protected from sexual impulses toward children by a set of interrupt switches. These switches are by now so well developed that most men are scarcely aware of the beginnings of arousal by borderline pubescents... and feel shame when that arousal even momentarily reaches consciousness.¹⁷ It is the unfortunate failure of such switches that provokes some to behave aberrantly. Likewise, it was the successful introduction of those switches that caused the majority of men to need more than just health and neoteny to experience the full flux of desire.

So women were off again, down the steep slope of competition. All else being equal, the desirable males were more attuned to women with fully developed breasts than to those whose glands were sufficient for nursing, but compact as a chimp's. Today, in order to help win the desperate game, North American women spend the annual budget of some small nations "correcting" a physical condition which has nothing to do with any illness or incapacity to nourish their young. Breast augmentation joins other types of cosmetic surgery -- procedures designed to restore the appearance of health and neoteny. It is a sign not only of the stresses faced by women naturally, but how these have been amplified and exaggerated by contemporary society.

In this context, one might be tempted by an unusual interpretation of the "Venus-Ishtar" stone figurines found in neolithic sites -- depicting female forms with exaggerated breasts and hourglass figures. Perhaps they were portable and artificial permission cues, serving much the same function as the milder forms of pornography do today, allowing solitary males to release pent-up physiological tension that accumulates behind a dam of inhibitions that pre-date any religious stricture.

Notwithstanding such speculative asides, we are drawn to one unmistakable conclusion. In the strange story of humanity, it is the female who has been forced to wear the equivalent of bright, garish plumage, even though she is still the one with the most exhausting task in reproduction, and still the one with the most to lose. This combination is without known parallel in nature.

We are a strange clan, indeed.

Next... shall it be more 'storks' and fewer 'reindeer' or a female-only future?

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⁹Prof. William Calvin of the University of Washington contends that the ice ages acted on humans as a genetic "pump." During boom times, populations of competent, versatile northerners would have expanded in relative isolation. When the ice returned, these groups only partly died back, but also would migrate south, infusing new traits into the (larger) parent population. (back to where you left off)

¹⁰ An analogy would be if human females found the "wild, romantic drifter" type of male irresistible, despite the harm such types do to society's ability to maintain subtle networks of interdependence. Naturally, this could never happen; it is just a hypothetical situation. (back to where you left off)

¹¹ This generalization appears to hold particularly well for women who report a low frequency of sexual relations, especially extra-marital affairs. Gangestad (1993) accounts for this in an interesting way, presenting a theory of tradeoffs between a male's "investment potential" (IP) and his "heritable fitness" (HF). When women are not looking for a permanent mate, or have complete independence from any need for outside resource assistance, there appears to be a tendency to seek males on the basis of outward physical traits associated with genetic superiority (HF). In other words, when women need men only for seed, their attitude may swing toward that of elephant seal females. Throughout most of human history, however, a life-or-death need for loyal help (IP) probably helped drive the more prevalent attitudes reported by Buss. (back to where you left off)

¹² A woman's fertile period is much narrower than a man's time as a potential father. This biological fact bodes poorly for those hoping propaganda alone might end the youth-fetishism of American males. While "good" men who have bonded to their wives can love them and continue to find them arousing until senescence -- and "age-ism" can, indeed, be ameliorated by good upbringing and consciousness-raising -- it is nevertheless almost certainly wired-in for the outward emblems of fecundity -- youth and health -- to be arousing to men. Like it or not, this is part of the foundation from which all future attempts at improved socialization and mitigation must commence. (back to where you left off)

¹³ I choose to concentrate on breasts here because the case is clearer. Regarding other pronounced female traits -- e.g., broadened pelvises and narrow waists--runaway sexual selection may have been a contributing factor. For instance, Devendra Singh (1993) finds that a ratio between waist and hip circumference of 3:4 is seen as attractive in women of almost any culture, despite wide variability in taste concerning overall "plumpness." Thornhill and Gangestad (1993) contend that estrogen causes exaggerated fat deposits in the gluteal-femoral region (thighs and buttocks) while testosterone causes deposit in the abdominal region. This ties in with their theory that human sexual selection is based on choosing mates whose appearance shows averageness plus symmetry, modified by those features exaggerated by testosterone in men and estrogen in women. Those hormones, in turn, are involved in hypo-active immune systems, so such exaggerations would advertise that here is a healthy individual who has parasite resistance to spare. Alas, by that standard, beer bellies and male pattern baldness should also be deemed attractive in males, as well as violent, irrational behavior. In any event, sexual selection cannot have driven the widening of the female pelvic girdle any faster than the punishment of horrible death inflicted during a difficult childbirth, as the cranial size of human infants expanded prodigiously. With all of these complications in mind, the decision to focus on breasts seems clear-cut. (back to where you left off)

¹⁴ Consider a leap of speculation. It might be proposed that, since it is males who are the usual crucibles of sexual selection, it is the male in most species who also starts out with an intrinsically broader range of variability... or ways to misread the blueprint. It is, after all, upon minor excursions from the old floor plan that sexual selection must act. While this variability guarantees a higher failure rate, and even occasional monsters, it also offers great rewards to the successful sport or variant. Thus a modest degree of instability may be inbuilt in males. On the other hand, the female reproductive pattern in most species is conservative, no female is likely to profit enough from wild excursions from the norm to make the risk worthwhile. But what if, suddenly, it is females who must compete, subjected to the tyranny of external choice? Then their very stability in following the species plan may turn into a disadvantage, robbing individuals of the sort of variability upon which a successful runaway leader depends. But there is always neoteny. Given that women were doomed to be swept into a (more typically male) runaway race of change and adaptation against each other, neoteny may have been the easiest path to take. This is, of course, an extremely tentative extrapolation. (back to where you left off)

¹⁵ Indeed, the drive toward female paedomorphism may have added synergistically to the brain-behavior neoteny trend discussed before... possibly helping to make us the mental giants of Earth. (back to where you left off)

¹⁶ Note that this dichotomy is particularly pronounced in the so-called Caucasoid races. In some other groups, males seem to differ less from females in hirsuteness and exaggeration of female breasts is less pronounced. Nevertheless, women of all races appear visually less apelike than their brothers. (back to where you left off)

¹⁷ Evolution is not planned. In order for my scenario to take place, we need a plausible cost to males, of not following the road I describe. Clearly sex with children is unprofitable, a waste of sperm and distraction. Moreover, once human life span lengthened enough for parents to expand their supervisory role, there would have been a drive for moms and dads to forbid other males to sexually assault their children. If this were enforced severely, one could imagine men developing a hesitancy about mating with any female lacking adulthood cues. (back to text)