An interesting study claims that latitude influences sex ratio. The study looked at sex ratios of boys vs. girls at birth in the capital cities of "202 countries." The study found that "countries at tropical latitudes produced significantly fewer boys (51.1% males) annually than those at temperate and subarctic latitudes (51.3%)." I don't know if those numbers are significant differences, especially when so many other factors could influence sex ratio. The New York Times reports on the study.
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I used to live in Nepal many years ago and I noticed that a few of families I knew had an enormously disproportionate number of daughters compared to sons … between 3:1 and 6:1. It may have been a statistical fluke, but it seemed so mathematically anomalous that I wondered if there might be other causes in play.
When I mentioned all this to a visiting doctor I knew, she hypothesized that the figures might result, not from pure chance, but from the pattern of STD’s in the society. Sperm for males and females, she said, have different characteristics, and one of these is a differential in ability to ascend fallopian tubes that have STD-caused scarring. The result could conceivably be a marked sex ratio imbalance at birth in individuals whose tubes were scarred significantly.
While the effect may operate primarily on an individual level, disease patterns are wider than that, and possibly there may be some overall societal effect on sex-rations that depends on the epidemiology of STD’s in the society?
Two additional factors that tend to support this hypothesis circumstantially are that the ethnic group in question, Tamangs, tended to value female offspring relatively highly , and at the time there was no female infanticide or selective abortion that I ever heard of. I had originally tentatively wondered about the possibility of selective male infanticide, but as I came to know the people in question better, it became obvious that this was not valid idea at all. The second factor was that Tamang mores in the 1970s and 1980’s, were not exactly puritanical (though far from loose), and while one doesn’t have any data on prevailing STD rates, um…. that they might have been present to sufficient degree in the society to influence sex ratios, if this effect really occurs, is hardly a totally preposterous assertion (which would apply most places, one way or another, I would imagine.)
Insofar as to some degree the level of economic development, and attendant on this, the level of medical care, European-style education, use of STD preventive measures, etc., used to correlate with latitude, perhaps the real variable causing the hypothesized effect is not latitude per se, but something medical that correlates with it.
Would anyone who is better qualified in this area than I am care to comment on this line of speculation? I myself don’t have the background to know whether what the doctor suggested to me is likely to be true or not.