Voici un article de Jerry Coyne dont nous avons extrait trois passages, sur le dimorphisme sexuel et la différence entre les hommes et les femmes :
"a post one week ago, “The ideological opposition to biological truth,” I argued that sexual dimorphism for body size (difference between men and women) in humans is most likely explained by sexual selection, and that it also reflects behavioral differences between males and females: males compete for females, and greater size and strength give males an advantage. That competition results from females—in many species, not just ours—being a “scarce” resource for males, since the number of males capable of breeding far exceeds the number of females who cannot breed because they’re tending offspring or in gestation. This disparity can be categorized in two ways:
- The behavioral operational sex ratio: the ratio of sexually active males to fertilizable females at a given time. This is about 11.7 in humans!
- The physiological operational sex ratio, the same ratio but for all individuals capable of reproducing (rather than those actually engaged in mate-hunting). This is about 8.6 in humans.
The ratios are greater in some primates (gorillas have values of about 84!), but if they’re greater than 1, there’s room for sexual selection, since there are more males seeking females than there are females available as mates. This itself is one bit of evidence for the operation of sexual selection in humans.
Now how the sexual selection actually operated in our ancestors is not perfectly clear. Some of it, as the data suggest, involves male-male competition: fights between males to control females, as we witness in gorillas, deer, and elephant seals. Females are more or less constrained to mate with the winning males. Or females may prefer to mate with the biggest and strongest males, for those males may protect their offspring—and hence the female’s genes—better than do smaller, weaker males. (This gives an evolutionary advantage to those females who can discern and choose the best males.)
Both of these factors can, of course, work at the same time, and there are other more arcane forms of sexual selection I won’t mention, including other signs in males of “good genes”. But any sexual-selection scenario goes along with a difference in sexual behavior, explaining why, even today, males are more promiscuous and willing to mate than are the choosier females.
A further possibility is that there could be an ecological distinction between males and females, with males hunting, and thus needing size and strength, while females do gathering (presumably females don’t have time to hunt because they’re rearing children). That doesn’t involve sexual selection, but it also fails to explain all the data, like the correlation between sexual dimorphism and polygyny within humans, and the fact that in our primate relatives there’s not only the same correlation among species, but no palpable division of labor among males and females. It also doesn’t explain the existence of traits like beards, lower voices, or same-sex aggression among human males but not females. Nevertheless, there’s no reason why several forces couldn’t work together to cause men to have evolved larger body size and increased musculature (as well as other features) in our ancestors. But surely sexual selection is one, for the evidence below fits no other hypothesis."
- In human societies studied by Richard Alexander, those societies that are more polygynous (in which males compete more intensively for females) show greater sexual size dimorphism than societies that are more monogamous. This was a prediction made before the data were acquired—a prediction derived from sexual selection theory. And it was fulfilled. UPDATE: I see now that Alexander’s finding wasn’t reproduced in another experiment, so consider this conclusion questionable.
- Among species of primates, there’s a good correlation between the polygyny of a species and sexual dimorphism: those species in which males have a higher variance in offspring number, and in which males thus compete more intensely for females, also show a greater ratio of male/female body size, even when corrected for phylogeny. (Too, in primate species in which males fight each other over females, the relative size of the canine teeth, used in battle, is larger than in species showing less direct male-male competition.)
- In humans, as in many other species in which males compete for females, the sex ratio at birth favors males. They then die off at a higher rate due to higher risk-taking and exploratory behavior, and also senesce faster, which is why among older humans there are so many more females than males. (Check out any Gray Line tourbus.) This is predicted by sexual selction theory.
- In line with the above, in humans and other primates, males show from the outset great exploratory and risk-taking behaviors, and as adults show many other behaviors that differ from those of females, such as greater dispersal. Is this due to the Primate Patriarchy? Probably not, given that these differences in behavior are shown in many species besides ours and make evolutionary sense.
- In other sexually dimorphic primates, including chimpanzees and gorillas, direct contests between males can be observed, and probably existed in our ancestors since paleoanthropological data show that many more males were killed by violence than females, possibly reflecting inter-group battles, which in modern hunter-gatherer societies are often over females. Many societies also show “bride theft”, capture of females by bands of males—common in Amazonian hunter-gatherer societies.
- Male humans have more robust skulls than do females, including mandibles and brow ridges. This may reflect evolution to withstand blows to the head. (Males also have a higher tolerance for pain.)
- Men are not only taller and heavier than women, but are stronger, particularly in the upper body. While size differences are about 8%, and body mass about 15-20%, women’s bodies have a higher percentage of fat, so that when you look at fat-free body mass, men are 40% heavier, have 60% more lean muscle mass, 80% greater arm muscle mass, 75% more upper-body muscle mass, and 50% more lower body mass. This difference in relative amount of muscle mass cannot be explained by Dunsworth’s theory, which is purely about growth, but is explained by male-male competition under sexual selection—and perhaps by female preference as well. This is reflected in differential athletic performance, and is why men and women usually compete separately in athletics. Even for men and women of equal sizes, men are far stronger; as Hill et al. note, “the average man is stronger than 99.9% of women (some of this, of course, may be because men work out; I haven’t checked the references.)
- In every society studied, men are physically more aggressive than women, both in play as kids and as adults. The vast majority of murderers are men, and this aggressive activity peaks during men’s peak reproductive years, when they would be competing for mates most strongly. These data do not include killings in war.
- Traits like beards and lower voices in men (men’s vocal folds are 60% longer than women’s, giving them lower voices) have been shown to act as indicators of dominance; both are evolved morphological traits. (The evidence supporting all these claims can be found in the papers cited below.) Women also prefer larger men and deeper voices, so there may have been an element of female choice in sexual selection, though of course the observations we make are on modern rather than ancient hominins.
- Sexual dimorphism is also seen in our ancestors like Australopithecus and H. erectus, implying that it’s been acting on our lineage a long time. But there’s also some evidence, cited by Plavcan, that the degree of sexual dimorphism has waxed and waned as females got either bigger or smaller over time, implying that there may have been some separate natural selection in females that could increase or decrease sexual dimorphism (but never effaced it).
- Finally, Buss’s article and others not cited outline the psychological and behavioral differences between males and females that make sense under sexual selection. These not only include the greater promiscuity of males than females, but also the greater sexual jealousy of males toward women than vice versa (our male ancestors weren’t always sure who the father of their mate’s children was, while women were far more certain). There is also a big difference between males and females in their attitudes towards casual sexual experiences (guess in which direction), and in how exacting their standards are for a short-term mate (guess again). Men have lower psychological thresholds for risk-taking. And so on. As Buss wrote, “Large sex differences appear reliably for precisely the aspects of sexuality and mating predicted by evolutionary theories of sexual strategies.”
On peut faire le lien avec la polémique récente sur le fait que les hommes serait responsable de la plus petite taille des femmes dont une réponse a cette théorie féministe a été publié sur le figaro dont nous avons extrait quelques passages :
"Il est plus commun de nier l'existence de différences psychologiques entre les sexes, mais celles-ci sont pourtant également bien établies, notamment sur le plan des traits de personnalité. Mais l'existence de différences psychologiques entre hommes et femmes ne nous dit rien sur leur origine. Elles pourraient être le produit de différences génétiques entre hommes et femmes, de facteurs culturels ayant pour conséquence que les garçons et les filles ne sont pas traités de la même façon, d'une combinaison des deux et/ou d'une interaction subtile entre facteurs naturels et culturels."
"Les féministes ont tendance à nier l'importance des facteurs naturels et à affirmer que, dans la mesure où il existe des différences psychologiques entre hommes et femmes, elles sont le produit de l'ensemble de forces sociales qu'ils appellent le patriarcat. (Peu importe qu'il semble que, dans les pays où l'égalité entre les sexes est plus importante, les différences psychologiques entre les sexes sont également plus marquées.) Mais c'est beaucoup plus difficile dans le cas du dimorphisme sexuel, qui est généralement présumé être entièrement le fruit de forces naturelles."
"Dans tous les cas, pas le moindre doute n'est émis quant à la validité de cette théorie, qui est le plus souvent présentée comme un fait établi. Le moins que l'on puisse dire est que, pour des gens qui nous rebattent les oreilles à longueur de journées au sujet des méfaits supposés du patriarcat, les féministes ont plutôt bonne presse! C'est d'autant plus remarquable que la théorie de Touraille est très loin d'être un fait scientifique établi et que les raisons de douter de sa validité ne manquent pas. Je n'en évoquerai ici que quelques unes, sans prétendre clore le débat, qui de toute façon ne peut pas être réglé dans les pages du Figaro."
Un autre lien intéressant :