Runaway Produces Large Sex Differences

Another problem with the runaway brain theory is that runaway is supposed to produce large sex differences in whatever trait is under sexual selection. Peacock tails are much larger than peahen tails. If the human brain tripled in size because of runaway sexual selection, we might expect that increase to be confined to males. Men would have three-pound brains, and women would still have one-pound brains like other apes. This has not happened. Male human brains average 1,440 grams, while female brains average 1,250 grams. If one measures brain size relative to body size, the sex difference in human brain size shrinks to 100 grams. This 8 percent difference is larger than would be predicted by a sexblind theory like E. O. Wilson's cultural feedback loop, or the

Machiavellian intelligence hypothesis. But it is much smaller than the runaway brain theory would predict.

Similarly if creative intelligence evolved through runaway sexual selection, we would expect men to have much higher IQs than women. There are some sex differences in particular cognitive abilities, mostly quite small, with some giving the male advantages, and some the female. However, there appears to be no sex difference whatsoever in the underlying "general intelligence" ability (technically called "the gfactor") that IQ tests aim to measure. The best analysis has been done by Arthur Jensen in his 1998 book The gfactor, and he concluded that "The sex difference in psychometric g is either totally nonexistent or is of uncertain direction and of inconsequential magnitude." Nor is any sex difference found in average performance on the most reliable IQ tests that tap most directly into the g factor, such as an abstract symbolic reasoning test called Raven's Standard Progressive Matrices. Men have a slightly greater variation in I Q , producing more geniuses as well as more idiots, but this greater variation in test scores does not appear to reflect a greater variation in the underlying g factor. This absence of a sex difference in general intelligence does not seem consistent with the runaway brain theory that sexual selection on males drove human intelligence.

Sex differences can occur on different levels, however. One could argue that runaway sexual selection did not favor brain size or intelligence directly, but the behavioral manifestations of high creative intelligence. On this view, perhaps runaway sexual selection accounts in part for the greater propensity of males to advertise their creative intelligence through trying to produce works of art, music, and literature, amassing wealth, and attaining political status. A strong version of this theory might suggest that human culture has been dominated by males because human culture is mostly courtship effort, and all male mammals invest more energy in courtship. Male humans paint more pictures, record more jazz albums, write more books, commit more murders, and perform more strange feats to enter the Guinness Book of Records. Demographic data shows not only a large sex difference in display rates for such behaviors, but male display rates for most activities peaking between the ages of 20 and 30, when sexual competition and courtship effort are most intense. This effect can be observed from any street corner in the world: if a vehicle approaches from which very loud music is pouring, chances are it is being driven by a young male, using the music as a sexual display.

Certainly mere may be many cultural reasons why men behave differently from women. If all sex differences in human behavior are due to sexist socialization, men it may be appropriate to dismiss all cultural and historical evidence concerning a greater male propensity to produce noisy, colorful, costly displays. The runaway brain theory simply suggests that evolved differences in reproductive strategies and display motivations may have been a factor in the historical prominence of male cultural production. Evolution is certainly not the only factor, because the last century has witnessed a rapid increase in women's cultural output, economic productivity, and political influence. Women's ongoing liberation from the nightmare of patriarchy has been due to cultural changes, not genetic evolution. Darwin would probably have been astounded by the political leadership ability of Margaret Thatcher and the musical genius of Tori Amos.

There is a serious problem of scientific method here. The runaway brain theory predicts greater male motivation to display creative intelligence in all sorts of ways, just as male birds are more motivated to sing. Human history reveals that cultural output across many societies was dominated by the behavior of males of reproductively active ages. Yet those societies, and the historical records themselves, were biased by many female-oppressing cultural traditions. (These traditions may have evolutionary roots in male propensities for oppressive mate-guarding, but such propensities would be distinct from any evolved male propensities for creative display.) I honestly do not know how much weight should be given to cultural records that reveal higher male rates of display, and which thereby seem to support the runaway brain theory. We clearly should not accept such records at face value as direct reflections of evolved sex differences. But if we dismiss such records completely, are we doing so because the records are utterly worthless as scientific evidence, or because we find the data politically unpalatable? Should we reject a theory of mental evolution that successfully predicts an observed sex difference, in favor of some other sex-blind theory that predicts a desired sexual equality in culture production that has not yet been observed in any human society?

Male nightingales sing more and male peacocks display more impressive visual ornaments. Male humans sing and talk more in public gatherings, and produce more paintings and architecture. Perhaps we should view the similarities between peacocks and men as a meaningless coincidence, due to sexual selection in the first case and a history of patriarchal oppression that just happened to mimic the effects of runaway sexual selection in the second case. This issue is so scientifically challenging and politically sensitive that it will only be resolved when evolutionary psychologists, cultural historians, and feminist scholars learn to collaborate with mutual respect and an open-minded dedication to seek the truth. Personally, I believe that the current evidence supports two provisional conclusions: sexual selection theory explains many human sex differences (including differences in the motivation to produce creative displays in public), and many pathological traditions have inhibited female creative displays in the last several thousand years. Some people view these two beliefs as mutually exclusive, but I cannot see why they should clash, except at the level of ideological fashion, in the same sense that lime green clashes with electric blue.

In summary, the overall evidence for sex differences is confusingly mixed. At the level of brain size and raw intelligence, human sex differences are too small for the runaway brain theory to work. Although brain size within each sex is correlated about 40 percent with general intelligence, the slightly larger brains of males do not yield a higher general intelligence than those of females. At the level of sexual behavior and cultural output, sex differences are enormous, but they are shrinking rapidly, and are conflated with patriarchal cultural traditions. Overall, this pattern of evidence does not support a strong version of the runaway brain theory, nor does it support any other theory in which male sexual competition through toolmaking, hunting, or group warfare was the driving force behind the human mind's evolution. If sexual selection was important in the mind's evolution, it could not have been a type of sexual selection that produces large sex differences in brain size or general intelligence. At this point, it may help to step back from the runaway brain theory and consider sex differences in a more general evolutionary framework.

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