The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [61]
But we are not birds. The only way to be certain of rearing a boy is to kill a girl child at birth and start again, or to use amniocentesis to identify the gender of the foetus and then abort it if it’s a girl. And these repugnant practices are undoubtedly on offer in various parts of the world. The Chinese, deprived of the chance to have more than one child, killed more than 250,000 girls after birth between 1979 and 1984.65 In some age groups in China, there are 122 boys for every 100 girls. In one recent study of clinics in Bombay, of 8,000 abortions, 7,997 were of female foetuses.66
It is possible that selective spontaneous abortion also explains much of the animal data. In the case of the coypu, studied by Morris Gosling of the University of East Anglia, females in good condition miscarry whole litters if they are too female-biased, and start again. Magnus Nordborg of Stanford University, who has studied the implications of sex-selective infanticide in China, believes that such biased miscarriage could explain the baboon data. But it seems a wasteful way to proceed.67
There are many well-established natural factors that bias the sex ratio of human offspring, proving that it is at least possible. The most famous is the returning-soldier effect. During and immediately after major wars, more sons are born than usual in the belligerent countries as if to replace the men that died (this would make little sense: the men born after wars will mate with their contemporaries, not with those widowed by the war). Older fathers are more likely to have girls, but older mothers are more likely to have boys. Women with infectious hepatitis or schizophrenia have slightly more daughters than sons. So do women who smoke or drink. So did women who gave birth after the thick London smog of 1952. So do the wives of test pilots, abalone divers, vicars and anaesthetists. In parts of Australia that depend on rainfall for drinking water, there is a clear fall in the proportion of sons born three hundred and twenty days after a heavy storm fills the dams and churns up the mud. Women with multiple sclerosis have more sons as do women who consume small amounts of arsenic.68
Finding the logic in this plethora of statistics is beyond most scientists at this stage. Bill James of the Medical Research Council in London has for some years been elaborating a hypothesis that hormones can influence the relative success of X and Y sperm. There is a good deal of circumstantial evidence that high levels of the hormone gonadotrophin in the mother can increase the proportion of daughters, and that testosterone in the father can increase the proportion of sons.69
Indeed, Valerie Grant’s theory suggests a hormonal explanation for the returning-soldier effect: that during wars women adopt more dominant roles, which affects their hormone levels and their tendency to have sons. Hormones and social status are closely related in many species; and so, as we have seen, are social status and the sex ratio of offspring. How the hormones work nobody knows, but it is possible that they change the consistency of the mucus on the cervix, or even that they alter the acidity of the vagina. Putting baking soda in the vagina of a rabbit was proved to affect the sex ratio of its babies as early as 1932.70
Moreover, a hormone theory would tackle one of the most persistent objections to the Trivers–Willard theory: that there seems to be no genetic control of the sex ratio. The failure of animal breeders to produce a strain that can bias the gender of its offspring is glaring. It is not for want of trying. As Richard Dawkins put it:
Cattle breeders have had no trouble in breeding for high milk yield, high beef production, large size, small size, hornlessness, resistance to various diseases, and fearlessness in fighting bulls. It would obviously be of immense interest to the dairy industry