Tropical latitude = more girls

The New York Times has a story about a new paper, Humans at tropical latitudes produce more females:

Skews in the human sex ratio at birth have captivated scientists for over a century. The accepted average human natal sex ratio is slightly male biased, at 106 males per 100 females or 51.5 per cent males. Studies conducted on a localized scale show that sex ratios deviate from this average in response to a staggering number of social, economical and physiological variables. However, these patterns often prove inconsistent when expanded to other human populations, perhaps because the nature of the influences themselves exhibit substantial cultural variation. Here, data collected from 202 countries over a decade show that latitude is a primary factor influencing the ratio of males and females produced at birth; countries at tropical latitudes produced significantly fewer boys (51.1% males) annually than those at temperate and subarctic latitudes (51.3%). This pattern remained strong despite enormous continental variation in lifestyle and socio-economic status, suggesting that latitudinal variables may act as overarching cues on which sex ratio variation in humans is based.

The piece in the times hits on the major issues here:

"There is no question that the vast majority of people in the tropics live in relatively poor and stressful societies," said Ralph Catalano, a professor of public health at the University of California, Berkeley. "If you control for the stresses of poverty, who have you got left that the test turns on?"

Dr. Navara defended her analysis. "Statistical analyses involving human populations are always tricky," she said, "but the analyses used here are robust and did not eliminate any variation that we would see in these populations."

There are some possible explanations, but none entirely satisfactory. It could be that there is some survival value in producing more girls in warmer regions, but it is unclear what this might be. There may be genetic or racial differences that could explain it, but the correlation persists over so many varied populations that this seems unlikely. Hamsters, mice and meadow voles also produce more male offspring during shorter days or colder weather, but the reasons in these animals are just as mysterious as they are in humans.

No one even knows whether human sex ratios are skewed before or after conception. Could the quality of sperm at different temperatures cause the variation at the moment of conception? Or is there some event during gestation at warmer temperatures that causes more male fetuses, or fewer female, to spontaneously abort?

"There's a possibility that humans might be responding to factors they were programmed to respond to a long time ago -- not cultural or socioeconomic, but climate and things like latitude," Dr. Navara said. "What's interesting is that we may be seeing something that attests to our animal ancestry."

The sex ratio difference was pretty obvious if you just inspected the data. Obviously I didn't control for the variables as above, but people shouldn't be too surprised. So what's going on? Remember the effect diet can have on fraternal twinning rates? I wouldn't be surprised if it's something to do with the environment; I've heard that agricultural scientists talk about how to modify the sex ratio outcome through weird techniques like mixing the semen with a sugar solution. If the ratio outcomes are hardwired, there are plenty of reciprocal transplant "natural experiments." Look at whites in Brazil and blacks in the United States.