If the lottery from sperm and egg to baby were as fair as a flipped coin, the number of girls and boys born would be identical. But a new study reveals that having a dad around the house may increase the likelihood of boys being born. This is the first clear evidence that living arrangements can affect sex ratios at birth, but questions remain as to how and why it happens.
Intrigued by anecdotal 19th century reports that children born out of wedlock were less often boys, human ecologist Karen Norberg of the National Bureau of Economic Research in Cambridge, Massachusetts, waded through data on 86,436 children born in the United States between 1959 and 1998. The data came from government surveys that were originally designed to look at anything from infectious diseases to income dynamics, but all included information on the mother's household immediately before the birth of the child. When Norberg compared the children born to single mothers with those born into a two-parent household, she found a slight, but highly significant, change in sex ratio: Solo mums had an average of 49.9% boys, whereas two-parent families averaged 51.5% boys.
Testing a subset of nearly 3000 mothers who had had children both when they were single and when they had a live-in partner, Norberg found an even more pronounced difference: In the presence of a dad, the baby was 14% more likely to be a boy, she reports online in the Proceedings of the Royal Society of London, Series B, on 21 October.The results could explain why the boy-girl ratio among newborns in developed countries seems to have dropped over the past 30 years, says Norberg. The underlying mechanisms are not yet known, but Norberg thinks mothers may be able to tweak their reproductive system to give Y-bearing sperm a nudge. As for the benefits of having fewer boys, one possibility is that boys are harder to raise for a single mother, says Norberg. They are "more prone to accidents, fights, and so on," she says, adding that there are many other hypotheses that need checking out.Other sex-ratio researchers are enthusiastic. Evolutionary biologist Steven Orzack of the Fresh Pond Research Institute in Cambridge, Massachusetts, says the new data are "very important" for the understanding of human sex ratios.