Optimists more likely to give birth to boys
By Ian Sample
August 5, 2004
Women who believe they are going to live for a long time are more likely than less optimistic women to give birth to sons, a new study suggests.
Researchers reached the strange conclusion after completing a survey of British women who had recently become mothers. They found that for every extra year a woman thought she was going to live, the odds of her firstborn being a boy increased significantly.
Sarah Johns, a biologist at the University of Kent who led the study, said the effect indicated that some people's perception of their future wellbeing was influencing the proportion of the sexes in the population.
Researchers have previously discovered that women in good physical health, and those living in comfortable conditions with plentiful food, show a tendency to give birth to males, while women enduring more adverse conditions tend to produce more females.
In the latest study, reported in the journal Biology Letters, 609 new mothers in Gloucestershire, south-west England, were asked, among other questions, to what age they expected to live. Some of the women, who were mostly from lower-middle and working-class backgrounds, believed they would die as young as 40, while others believed they would still be alive at 130.
According to Dr Johns, the link between perceived life span and the sex of newborns could come down to the "selfish" behaviour of our genes.
Evolutionary biologists call this the Trivers-Willard hypothesis. It suggests that when a mother feels under pressure, it makes biological sense for her to give birth to a girl.