**Women who believe in long life bear sons
**
Women who think they will live to a ripe old age are more likely to give birth to sons than daughters, reveals a study of UK women.
The new research suggests that even in affluent western countries, a woman’s health and living conditions influence the sex of her babies - a link that until now has only been found conclusively in developing nations.
Earlier studies have shown that women in developing countries who are poorly nourished are more likely to give birth to girls than boys. A popular theory suggests that this is an evolutionary adaptation that aims to deliver the greatest number of grandchildren.
The idea is that if parents can raise a strong and healthy, attractive son, he can father lots of children by many women. But boys demand more resources both in the womb and during their upbringing. So if parents have few resources to give, it may be better to play it safe and have daughters, who can produce at least some grandchildren.
How a woman’s body skews the sex ratio to order is not entirely clear, but one possibility is that fit women living in good environments have raised levels of testosterone, which in turn promotes the survival of male embryos in the womb.
**Fitness and environment **
Sarah Johns, a biological anthropologist at the University of Kent in Canterbury, UK, wondered whether the boy-girl ratio might also correlate with psychological factors, such as a woman’s perception of her own lifespan.
Earlier studies have shown that people can estimate their lifespan fairly accurately, presumably because they factor in their own fitness and environment quality, as well as perhaps the longevity of their parents or grandparents.
“People’s subjective life expectancy is actually quite an accurate prediction of when they will die,” says Johns. “They are continually reassessing what the future holds, so it’s a psychologically salient variable.”
Johns analysed questionnaires received from more that 600 women living in Gloucestershire who had given birth to their first child within the past four years. The women were asked the sex of their first child and the age they guessed they would be when they died. The women thought they would live anything from two to 106 more years.
Mysterious effect
Sure enough, the women who predicted long lives were more likely to have had a baby boy than those who envisioned an early death. The ratio of boys to girls for women who thought they had more than 60 years more to live was about 1.08, but for women who gave themselves less than 48 more years, the boy-girl ratio was only 0.78.
A woman who expects a long lifespan might be more likely to have a boy purely because her optimistic estimate reflects excellent fitness and a good environment. But it is possible that psychology is also somehow tied up in this mysterious effect.
Johns says one of the biggest surprises was that this effect occurs in an affluent western country, where malnourishment is rare. “Even here, women who are perceiving the environment as difficult and who have an increased sense of their own mortality are less likely to have sons,” she says.
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