Sex on the Brain: The Biological Differences Between Men and Women
By Deborah Blum
Viking Press; 352 pages; $24.95
Imagine a tropical island on which some of the girls, at adolescence, magically turn into men. Think of the scientific possibilities! Finally, we could tease apart nature and nurture and see whether men and women differed because of how they were brought up as children. As the twig is bent, we say, so grows the branch; we expect these teens to have girls' minds in boys' bodies and to suffer from a painful confusion of gender roles.
As it happens, this is not a thought experiment. In a few Dominican villages, some families carry a gene that leaves newborn boys with undescended testicles and a stunted penis resembling a clitoris. They are raised as girls until puberty, when the new rush of androgens gives them normal male genitals and a masculine body, complete with facial hair. The villagers call them guevedoces: "eggs [or balls]-at-12." The child switches genders, wears male clothing, begins to date, and turns into a normal man, without fuss or trauma. So much for bending the twig. Gender identity comes either from the effects of hormones on the brain or from the way people are treated as adults, or both; childhood nurture makes little difference.
Balls-at-12 is just one of the fascinating discoveries brought to light in Deborah Blum's excellent book Sex on the Brain: The Biological Differences Between Men and Women. This is the real Everything You Always Wanted to Know About Sex (or The Sexes). Why are there sexes? To change our biochemical locks every generation and keep a step ahead of the rapidly evolving pathogens that try to pick them. How different are men's and women's brain structures? Not very. Do raging hormones turn men into testosterone-poisoned rapists and women into weepy premenstrual husband-stabbers? No both times. Are men and women biologically different in ways other than the obvious anatomical ones? Yes--men are shorter-lived, more cerebrally lopsided, more violent, better at some spatial abilities, worse at verbal abilities, more competitive but more forgiving of their competitors, more sexually jealous, more socially obtuse, and more promiscuous (at least, they'd like to be).
Not only are we learning more about sex differences, but we also have an elegant theory to explain them. In the 1970s the biologist Robert Trivers showed how all the major differences between the sexes in the animal kingdom flow from a difference in the size of their investment in offspring. The female begins with the bigger ante--an egg that is far bigger than a sperm--and usually commits herself to even more, such as yolk; or, in mammals, blood and milk. The male contributes a few seconds of copulation and a teaspoon of semen. The number of offspring in each generation is limited by the female's contribution: one for each egg she produces and nurtures.
That has two momentous consequences. First, a single male can fertilize several females, forcing other males to go mateless. Males must compete for access to females by beating each other up, cornering the resources necessary to mate, or persuading a female to choose them. Second, a male's reproductive success depends on how many females he mates with, but not vice versa; for a female, one mating per pregnancy is enough. That makes females more discriminating in their choice of sexual partners.
Humans have added some twists to the mammalian pattern. Men generally invest in their children by providing food, protection, and care. So females also compete for mates, though they look for the ones most willing and able to invest, not the ones most willing to copulate (those are never in short supply). Females, like males, may be tempted by infidelity, though their genetic motive is quality rather than quantity. A discreet adulteress can get the genes of the fittest male and the investment of the most generous male. An easily cuckolded male would devote his efforts to the genes of a competitor, which is Darwinian suicide; hence men's intense sexual jealousy.
Blum is a superb science reporter who presents just the right amount of complexity, tries to explain findings rather than just report them, and writes in a consistently clear and pleasant style. Sex on the Brain is such a good window on the state of the art that its only flaws are the flaws of the researchers themselves.
Unlike Robert Wright and Matt Ridley, who have also written excellent recent books on the biology of sex, Blum does not ground her own story in rigorous evolutionary biology, but rather lets the laboratory scientists speak for themselves. Unfortunately, many good bench scientists are mediocre theorists, often by choice. "Why" questions are thought to be an indulgence, appropriate only for musings over beer at the end of the day. Blum reports (and occasionally echoes) some sloppy evolutionary "explanations," including casual analogies between arbitrary species and Homo sapiens, the equation of evolution with progress, the idea that contemporary changes in Western society are the vanguard of future evolution, and repeatedly, the error that our adaptations are for the good of the species.
A daptations are for the good of the genes that implement them, and one of the best demonstrations is right in Blum's territory: the 50-50 ratio of males to females. If organisms were designed to benefit the species, they would not waste half the available food on sons, who can't directly replenish the species with babies. Any necessary genetic variation could easily be supplied by a few studs. Organisms pump out sons because whenever females are more plentiful, the genes of mothers and fathers who bear sons have a reproductive field day, and the mixture settles at 50-50. If the species suffers, that's just too bad.
Blum not only fails to share these explanations, but also sometimes repeats ones that are downright wrong--such as that men die young because the species needs them less. A better explanation is that males' reproductive fate depends more strongly than females' on competing when they are young. So any gene that builds a man with a strong young body at the cost of a weak old body will prosper.