Panos

THAT mother knows best is no secret. That her reproductive organs also know best may come as more of a surprise. But that is what two evolutionary biologists, Robert Trivers and Dan Willard, hypothesised nearly four decades ago. Boys, they reasoned, will thrive reproductively when they have grown big and strong in resource-rich environments. Otherwise, they will do badly. Girls, by contrast, will do reasonably well across the board and thus have a comparative advantage over their brothers in poorer situations. Parents, meanwhile, have a genetic incentive to see their progeny do well. Give a mother abundant resources, then, and her body should favour sons. Place her in difficult conditions and she should have more daughters.

The Trivers-Willard theory has been tested with success in several species of wild animal. Showing it to be true in people, however, has proved difficult. But a paper just published in Biology Letters by Thomas Pollet of the University of Groningen, in the Netherlands, and his colleagues makes a brave attempt to do so. Dr Pollet tests it by studying polygamous households. As wives are added to such a household, its resources will necessarily be split more ways. Even if they are shared equally, the first wife will have had a head-start on the others—and, life being what it is, she may retain a dominant position.

Much of the world has given up open polygamy, of course (though the discreet sort remains common everywhere). It is, however, still practised in parts of Africa. Dr Pollet and his colleagues therefore turned to Rwanda, and used data gathered in a census of that country taken in 2002.

They found 96,880 married women who reported having children. They classified the marriages in question as either monogamous or polygamous. The wives in polygamous marriages were further classified as either “first”, “second” or “third or lower order”. As the researchers suspected, when all other things were equal mothers in monogamous marriages had most sons: 101 for every 100 daughters. Those who were the first wives of polygamists scored similarly. Wives who were “third or lower order”, though, had only 94 sons for every 100 daughters.

Of course, all other things are rarely equal. The team also found and corrected for the facts that a mother's age and which province she lived in affected the sex ratio of her children. Her level of education, whether she lived in a town or the countryside, and whether she owned the house she lived in did not, however, seem to make any difference (not that a lower-order wife is ever likely to own a house).

What the researchers did not correct for was the father's age. And previous studies have shown that older fathers are relatively more likely to beget daughters. So part of the effect Dr Pollet has discovered might be caused by the fact that, obviously, a man is older when he takes a second or subsequent wife than when he marries for the first time. This age effect, too, could be ascribed to the Trivers-Willard hypothesis if, say, sperm quality falls with age. But it would be a separate phenomenon from what is going on in the mother.

If that confounding variable can be taken care of, though, and the effect persists, the next obvious step is to determine what mechanism within the mother's body causes the discrepancy. The Y-chromosome, which carries the genes for maleness, comes from the father, so it cannot be the case that the mother is failing to ovulate “male” eggs. It seems most likely that potential sons are failing to implant in the wall of the womb, and so never get the chance to develop.