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The genius of Robert L. Trivers
It is commonly believed that whether parents conceive a boy or a girl is entirely up to chance. Close enough, but not quite. It is largely up to chance, but there are factors that very subtly influence the sex of an offspring. It is also commonly believed that exactly half the babies born are boys and the other half are girls. Close enough, but not quite. The normal sex ratio at birth is .5122 – 105 boys for every 100 girls. But the sex ratio varies slightly in different circumstances and for different couples. So what factors affect the sex of the child?
Any discussion of sex ratio at birth must begin with the work of Robert L. Trivers, who is one of the greatest evolutionary biologists of his generation. In 1973, Trivers teamed up with a mathematician, Dan E. Willard, to formulate one of the most celebrated principles in evolutionary biology, called the Trivers-Willard hypothesis. The hypothesis states that wealthy parents of high status have more sons, while poor parents of low status have more daughters. This is because children generally inherit the wealth and social status of their parents. Sons from wealthy families, who themselves become wealthy, have, throughout most of evolutionary history, been able to expect to have a large number of wives, mistresses, and concubines, and produce dozens or hundreds of children, whereas their equally wealthy sisters can have only so many children. So wealthy parents should “bet” on sons rather than daughters.
Conversely, poor sons can expect to be completely excluded from the reproductive game, because no women would choose them as their mates. But their equally poor sisters can still expect to have some children if they are young and beautiful. The “fitness ceiling” – the best one can do reproductively – is much higher for men than for women, but the “fitness floor” – the worst one can do reproductively – is much lower for men. So poor parents should “bet” on daughters rather than sons. Natural selection designs parents to have a biased sex ratio at birth depending upon their economic circumstances – more boys if they are wealthy, more girls if they are poor.
There is evidence for this hypothesis throughout human societies. American Presidents, Vice Presidents, and cabinet secretaries have more sons than daughters (although only during the first half of its history). Poor Mukogodo herders in East Africa have more daughters than sons, both at birth and in the 0-4 age group. Church parish records from the 17th and 18th centuries in Germany show that wealthy landowners in Leezen, Schleswig-Holstein, had more sons than daughters, while farm laborers and tradesmen without property had more daughters than sons. Among the Cheyenne Indians on the American Plains, prestigious, high-status “peace chiefs” have more sons than daughters, while poor and marginal “war chiefs” have more daughters than sons in the 0-4 age group. In the contemporary United States and Germany, the elite – judged by the listing in their respective country’s Who’s Who – have a greater proportion of sons among their offspring than does the population in general. In an international survey of a large number of respondents from 46 different nations, more wealthy individuals are more likely to indicate a preference for sons if they could only have one child, whereas less wealthy individuals are more likely to indicate a preference for daughters. While there is some counterevidence, most evidence is in support of the Trivers-Willard hypothesis.
In my next post, I will discuss how Trivers’s genius in the Trivers-Willard hypothesis has been extended to new directions.
