Evidence of subconscious gender choice on the part of mothers. Apparently, women who believe they have longer to live are more likely to produce sons. No one's quite sure why yet.

"It's not something I've ever come across in practice," says midwife Sue Jarman from South Norwood Medical Centre, London, but she says the idea that unborn sons are biologically more costly makes sense. Male babies are more likely to spontaneously abort than female babies, so women may need to be on top form to carry a son to term.

Once born, males may also require more parental investment than females. Women tend to breast-feed sons longer than daughters, says Jarman.

Girls are much more biologically valuable than boys, as well; a community with 1 man and 1000 women can survive to another generation, but a community with 1000 men and 1 woman will die out. Don't know how that translates to longevity-expectation per se, but it would make sense that a vague (or not-so-vague) concept of "threat" would tend to produce more girls.