There is a term—my in-laws introduced me to it—for children born into a life of inherited wealth and unearned leisure. They are “fortunate sperm.” A newly published study gives that term new meaning. We all assume that the gender of a child is basically random—the result of a ferocious, wriggly race between tens of millions of sperm to get to and burrow into the waiting egg. But the new research suggests that mothers in a wide and diverse array of mammal species are able, perhaps unconsciously, to choose the gender of their offspring. Somehow the moms are rigging the game.
The idea that this might be the case was most famously laid out in a 1973 paper (PDF) by the evolutionary biologist Robert Trivers and the computer scientist Dan Willard. The central insight was that gender has a lot to do with one’s reproductive success. In many mammalian species, being male was either very good or very bad, reproduction-wise. Among lions, an alpha male, with an entire pride’s worth of females at his disposal, will have lots of offspring. But most male lions are not alphas, and they won’t have any offspring at all. Being female, on the other hand, is a safer, if less potentially lucrative, genetic bet. Since access to wombs is what all those male lions and bighorn sheep and elephant seals are competing over, most females will have at least some offspring.
For the rest of the story: http://www.businessweek.com/articles/2013-07-15/what-your-babys-gender-could-say-about-your-genes#r=rss