Women who believe they are going to live for a long time are more likely to give birth to sons than less optimistic women, 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 increasedsignificantly.
Sarah Johns, a biologist at the University of Kent who led the study, said the effect indicated that - at least in some regions - 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 today in the journal Biology Letters, 609 new mothers in Gloucestershire 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 lifespan 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, from anything ranging from poor health and living conditions to scarce food, it makes biological sense for her to give birth to a girl rather than a boy.
"Having females is a safe investment, in that their reproductive success is consistent," Dr Johns said. "Boys are more risky. They are more likely to die young, and if they don't, they have to compete for females. They could potentially provide you with lots of grandchildren, but unless you can invest in them, you run the risk of having a 'dud' that doesn't produce any at all."
The finding did not mean that women could influence the sex of their unborn children by changing their outlook, Dr Johns said. "But it does indicate that sex ratio is being influenced by poor maternal conditions. It is striking that you can see this effect in a contemporary population - where there's an idea that everyone's equal, but they simply do not perceive it that way."
Robin Dunbar, an evolutionary psychologist at Liverpool University, said while the effect was interesting, biologists did not fully understand why it might happen.
Some have suggested that hormones could play a big role.
"It implies that the machinery of the body is subtly taking into account these kinds of contextual differences," he said.
