The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (13 page)

chimpanzees"). A third species might develop a runaway preference for creative intelligence, and turn into us.

Depending on your philosophy of science, runaway's unpredictability could be seen as a strength or a weakness. It is a strength if you are looking for an evolutionary process that can explain why two closely related species take dramatically different evolutionary routes. It is a weakness if you expect evolution to be predictable and deterministic, able to explain exactly why one ape species evolved creative intelligence while another did not. Of course, if you think that our mental evolution was driven entirely by natural selection for survival abilities, a fairly deterministic attitude is appropriate. But if you accept that mental evolution could have been influenced by runaway sexual selection, which produces unpredictable divergence, then you can't expect it to be predictable or deterministic.

If our evolution was driven by an unpredictable process like runaway, we should not expect a precise answer to questions like "Why did we, rather than chimpanzees, evolve creative intelligence and language?", or "Why are we the first articulately conscious species on Earth?" It would be like a lottery winner asking why she won. However, we can still ask, "What are the adaptive functions of human creative intelligence, language, and morality?", and "Did these capacities evolve through survival selection, sexual selection, or something else?" Given an adaptation, we can still try to explain why it evolved to have the features and functions it does. We just might not be able to explain why it evolved exactly when and where it did, in the lineage that it did, rather than in other lineages.

The Problems with Runaway in
Explaining the Human Mind's Evolution

At first glance, runaway's speed and creative power sound like just what we need to explain the human mind's evolution. Brain size in our lineage tripled in just two million years. From a macro-evolutionary viewpoint, that is very fast—much faster than any brain size increase in any other known lineage. Music, art,

language, humor, and intelligence all evolved at some time during that explosive growth. On the geological timescale, the human mind's evolution looks faster than the flash from a nuclear strike does on the human timescale.

But evolutionary speed is relative. The human mind's evolution was actually much too slow to be explained by a single runaway event. Two million years is still a pretty long time— about a hundred thousand generations even for a slow-breeding ape like us. During that time, we added two pounds of brain matter—about a hundredth of a gram of brain per generation. A sustained runaway process would have been much more potent. Assuming a modest heritability and a modest amount of variation in brain size, I estimate that runaway could increase brain size by at least one gram per generation. That rough estimate assumes a sexual selection pressure on the low end of pressures that have been measured in other species in the wild. If this estimate is right, a single sustained runaway event would have been at least a hundred times too fast to explain human brain evolution. Brain size would have tripled in 20,000 years, not 2 million years.

Like a ramjet, runaway sexual selection has more of a minimum speed than a maximum speed. It just can't go slow. This is one reason why the simple runaway story makes a poor explanation for human brain evolution. Compared with runaway's hypersonic speed, human brain evolution was like a stroll through the park on a Sunday afternoon. Yet, if this speed objection seems to undermine the runaway brain theory, it undermines every other positive-feedback theory as well. The other processes proposed by E. O. Wilson, Richard Dawkins, Nicholas Humphrey, Andy Whiten, and Richard Alexander would also have run too fast.

This speed problem might be solved by supposing that human brain evolution, like the evolution of almost everything, happened

in fits and starts. There were short periods of relatively fast evolution when selection pressures were pushing in some direction, and long periods of stasis when selection just maintained the status quo against mutation. Fossil evidence suggests that brain size

increased quickly in a few dramatic bursts. The transition from 450-gram Australopithecine brains to 600-gram
Homo habilis
brains was one such burst (though
Homo habilis
is no longer thought to be our direct ancestor). Another burst produced the early 800-gram
Homo erectus
brain 1.7 million years ago. There were probably several more bursts during the evolution of
Homo erectus
over the next million years. Another burst produced the 1,200-gram archaic
Homo sapiens
brain. A final burst produced the 1,300-gram modern human brain about 100,000 years ago. Each burst looks short in terms of geological time, but lasted for hundreds or thousands of generations, plenty of time for standard selection pressures to mold traits. We do not yet have sufficient fossil evidence to tell whether each burst was driven by a very fast process like runaway or a slower process like ordinary survival selection.

So, where does this leave us? A single runaway event cannot explain two million years of human brain evolution because it would have been too fast and too transient. Instead, we could propose a multi-step runaway process, where each burst in brain size was driven by a separate runaway event. But that would beg the question of why all the runaway events increased rather than decreased brain size. In principle, a species could stumble into runaway sexual selection for the dumbest possible behavior produced by the smallest possible brains. A species of bumbling incompetents could evolve, despite the survival costs of their stupidity, as long as stupidity remained sexually attractive. Runaway is not supposed to be biased in any evolutionary direction, so it should be as likely to decrease a trait's size as to increase it. This makes it a poor candidate for explaining multi-step progressive trends.

Another possible answer to the speed quandary is to forget about fossil brains, and focus on human mental abilities. We do not know when language, art, and creativity evolved. Perhaps they all evolved together when modern
Homo sapiens
emerged about 100,000 years ago. Some archeologists even think that these capacities all evolved in a single burst 35,000 years ago, in an event

they call the "Upper Paleolithic revolution." Such rapid evolution might reflect a single runaway process operating over a few thousand generations in a single population, transforming a large-brained but unintelligent hominid into an intelligent, talkative human. The earlier brain-size bursts may have occurred for some other reason. Perhaps the key transition to the human mind was a brain reorganization rather man a simple brain size increase. The reorganization may not be evident in the record of fossil skulls, but may be more psychologically significant than earlier size increases. It may have been driven by a burst of runaway sexual selection relatively late in human evolution.

However, this theory fails to explain why brain size increased in all those bursts before our species evolved. It seems to me that the multi-burst trend toward larger brains should be explained rather than ignored. Pure runaway cannot explain it, because runaway does not have any intrinsic bias toward larger ornament size, higher ornament cost, or greater ornament complexity. The problem with runaway is not just its rocket-like speed. Its more fundamental problem is its neutrality, which makes it weak at explaining multi-step trends that last millions of years. The next chapter examines another sexual selection process that is much better at driving sustained progress in one direction.

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 sex-
blind
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
g
factor") that IQ tests aim to measure. The best analysis has been done by Arthur Jensen in his 1998 book
The g factor,
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.