Dienekes points me to a paper with a straightforward title: “Humans at tropical latitudes produce more females”.
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.
Well, it’s not a big difference, but it takes tens of thousands of missing boys a year to make up those two tenths of a percent. Humans have a high rate of pregnancy loss. A relatively small increase in losses of males in tropical populations (or small decrease in losses of females) would make the difference. I bet it’s disease affecting one sex more than the other. A second hypothesis is X-linked incompatibilities in high-latitude populations. Either is testable.