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This is a highly modified Trivers–Willard effect, known in the trade as a local-resource competition model.57 High rank leads to a sex bias in favour of the gender that does not leave at puberty. Could it possibly apply to human beings?

Do Dominant Women Have Sons?

Mankind is an ape. Of the five species of ape, three are social, and in two of those, chimpanzees and gorillas, it is the females that leave the home troop and the males that do not. In the chimpanzees of Gombe Stream in Tanzania studied by Jane Goodall, young males born to senior females tend to rise to the top faster than males born to junior females. Therefore female apes of high social status ‘should’ – according to the Trivers–Willard logic – have male young and those of low social status ‘should’ have female young.58

Now men are not excessively polygamous, so the rewards of large size to men are not great: big men do not necessarily win more wives and big boys do not necessarily become big men. But mankind is a highly social species and his society is nearly always stratified in some way. One of the prime, indeed, ubiquitous perquisites of high social status in men, as in male chimpanzees, is high reproductive success. Wherever you look, from tribal aborigines to Victorian Englishmen, high-status males have had more children than low-status ones. And men’s social status is very much inherited, or rather passed on from parent to child. Women generally leave home when they marry. I am not here implying that the tendency for a woman to travel to a man’s home when she marries is instinctive, natural, inevitable or even desirable, but I am noting that it has been general. Cultures in which the opposite happens are rare. So human society, like ape society but unlike most monkey society, is a female-exogamous patriarchy and sons inherit their father’s (or mother’s) status more than daughters inherit their parents’ status. Therefore, says Trivers–Willard, it would pay high-ranking fathers, or dominant women, or both, to have sons and subordinates to have daughters. Do they?

The short answer is that nobody knows. American presidents, European aristocrats, various royals and a few other élites have been suspected of having male-biased progeny at birth. In racist societies, subject races seem to be slightly more likely to have daughters than sons. But the topic is too fraught with potential complicating factors for any such statistics to be reliable. For example, merely by ceasing to breed once they have a boy – which those interested in dynastic succession might do – people would have male-biased sex ratios at birth. However, there certainly are no studies showing reliably unbiased sex ratios. And there is one tantalizing study from New Zealand that hints at what might be found if anthropologists and sociologists cared to look into the matter.59

As early as 1966, Valerie Grant, a psychiatrist at the University of Auckland in New Zealand, had noticed an apparent tendency for women who subsequently gave birth to boys to be more emotionally independent and dominating than women who gave birth to girls. She tested the personalities of eighty-five women in the first trimester of pregnancy using a standard test designed to distinguish ‘dominant’ from ‘subordinate’ personalities – whatever that may mean. Those who later gave birth to daughters averaged 1.35 on the dominance scale (from o to 6). Those who later gave birth to sons averaged 2.2.6, a highly significant difference. The interesting thing about her work is that she began before the Trivers–Willard theory was published, in the 1960s. ‘I arrived at the idea quite independently of any study in any of the areas in which such a notion might reasonably arise,’ she told me. ‘For me the idea arose out of an unwillingness to burden women with the responsibility for the “wrong” sex child.’60

Her work remains the only hint that maternal social rank affects the gender of children in the way that the Trivers–Willard-Symington theory would predict. If it proves to be more than