Sexual selection should produce sexually dimorphic traits, idiosyncratic with respect to environments. Natural selection should produce sexually monomorphic traits, correlated with environments. The idea that minds evolved to mate is clever. The idea that minds evolved to compete fits the evidence better.
2. The Mating Mind starts with an insightful history-of-science contrast between Darwin - who published his short sketch on the Descent of Man along with his long list of examples in Selection According to Sex, and Wallace - who downplayed both sexual selection and brain evolution. It adds an up-to-date review of theories of female choice for fitness indicators, like parasite resistance or low mutation loads. And it ends with broad-stroked, considerate, literate, and clever chapters on the evolution of art, morality, language, and creativity. What if curiosity evolved through mate choice "as an anti-boredom device" (p. 412)? What if Cyrano and Scheherazade got the girl and boy they were after because language evolved through "runaway sexual selection, mental biases in favor of well-articulated thoughts, and fitness indicator effects" (p. 353)? What if "generosity works as a Zahavian handicap that displays fitness,"and the Rockefeller Foundation was John D.'s "peacock's tail" (pp. 307, 327)? What if canvas painters' motivations were like satin bowerbirds' - who, if they could express themselves in print, might write: "It is a happy coincidence that females sometimes come to my gallery openings and appreciate my work, but it would be an insult to suggest that I create in order to procreate. We live in a post-Freudian, post-modernist era in which crude meta-narratives are no longer credible as explanations of our artistic impulses" (p. 270)? All of this is interesting, but I'm not sure all of it's right. Two things bother me most.
3. One is that sexual selection should produce sexual dimorphism. Whether females pick males because they're showing off genes that survive well in rapidly changing environments, or genes that carry few deleterious mutations, or genes that are simply trendy, females should be reluctant to show off themselves. Males should be flashy; females should be plain. If wit, verbal ability, virtue, and beauty are effects of sexual selection, then women should be dim-witted, dumb, nasty, and dull. Miller asks, "is this a fatal problem?" (p. 375). I think it is. As Miller points out, sex differences in general intelligence (the g factor) are virtually nonexistent. People's brains are not very sexually dimorphic. And the reason is that women have more on their minds than sperm. Miller is satisfied that "single mothers may have been the norm during most of human evolution" (p. 189). Not after their brains got large. Big-brained, late-developing babies got help from more than one parent, or didn't make it. That's true now from primitive Bolivia to modern Canada, at least. The children of single mothers are more often aborted, killed at birth, stressed or abused as they grow up. For that reason, evolving Homo sapiens females - like pair-bonded passerine birds and some cichlid fish females - almost certainly chose males for more than good genes. They chose good providers. Humans have been, historically, the most polygynous species on earth. Not because men varied conspicuously, genetically; but because men varied conspicuously, materially. For instance.
4. The fourth Roman emperor, Claudius, who lost his dad as a baby, "was so troubled by various diseases that he grew dull-witted and had little physical strength" (Suetonius, Claudius, 2). But he had access to hundreds of slave women, and had other girls procured by slave men- who were helped out by Messalina, Claudius' first wife. 1400 years earlier Akhenaten, the 18th-dynasty Egyptian who pushed monotheism, was like other pharaohs the product of royal incest. He was a collection of bad mutations. Reliefs accordingly show him with narrow shoulders, a potbelly, huge thighs, and spindly legs. But, like other pharaohs, Akhenaten rounded up women from all over Africa and Asia as tribute. They probably weren't all reluctant to come. Women in harems may not always have got sperm from healthy men, but they almost always slept in good beds. And left their children with more than just genetic inheritances.
5. The other "problem" is an alternative explanation. Maybe sexually monomorphic brains are products of natural selection. Brains don't come cheap. As Miller knows, ours make up just about 2% of our body weight, but eat up 15% of our oxygen, 25% of our metabolic energy, and 40% of our blood glucose. That's a huge cost. A cost unlikely to have been recouped by choosy women - even if they produced wiser, kinder, chattier and more aesthetic sons. And a cost unlikely to have been offset by promiscuous men - who had many opportunities in history, but a lot fewer in prehistory, when societies were small, and hierarchies were relatively level. Instead, the costs of big brains might have been compensated - in both men and women - by social competition. As Miller is aware, "adaptations that have large survival benefits typically evolve many times in many different lineages, in a process called convergent evolution" (p. 19). Among hominids, and among primates, group size and neocortex size correspond. Apes for instance in small groups (like orangs and gibbons) have smaller neocortexes; and apes in big groups (like chimps and humans) have bigger ones. Across taxonomic groups, big brains tend to accompany social life. Brain sizes of 1300 c.c.'s are rare, but high brain to body mass ratios are common to social ungulates (like elephants), social cetaceans (like dolphins), social canids (like dogs), and social primates (like humans). This isn't my idea - it is, in various versions, Nick Humphrey's, Richard Byrne's, Andrew Whiten's, and Robin Dunbar's. But it's a good one. Sexual selection should produce sexually dimorphic traits, idiosyncratic with respect to environments. Natural selection should produce sexually monomorphic traits, correlated with environments. We need more data. Given what little we have, I favor the Byrne/Whiten/Dunbar/Humphrey theory. But love to read Geoff Miller's pretty, polite, prolix and unpredictable - in a word, protean - prose. If he were rich, I might ring him up.
Miller,G.F. (2000) The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature. Heinemann
Miller,G.F. (2001) Precis of: The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature. PSYCOLOQUY 12(008) http://www.cogsci.soton.ac.uk/cgi/psyc/newpsy?12.008