Study Identifies Correlation Between Sex Ratios At Birth, Latitude

Main Category: Women's Health / Gynecology
Also Included In: Abortion;  Public Health;  Pediatrics / Children's Health
Article Date: 23 Apr 2009 - 5:00 PDT

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Although researchres have known for decades that more boys than girls are born throughout the world, a new study published online in Biology Letters shows that the closer a population lives to the equator, the smaller the difference becomes, the New York Times reports. Past studies have identified correlations between the different sex ratios at birth and a wide range of social, economic and biological factors, including selective abortion, infanticide, war and other variables. Because so many factors are involved, determining the impact of a single variable "has proved an almost infinitely complicated exercise," the Times reports.

To better determine which factors affect the sex ratio, study author Kristen Navara of the University of Georgia compared the latitudes of the capital cities of 202 countries with 10 years of data on sex ratios at birth. Navara used unemployment and gross national product statistics to estimate each country's socioeconomic status, and she also calculated a political instability index. The study found no relation between the number of boys born and socioeconomic and political factors. However, there was a significant correlation between sex ratios that were skewed toward boys and climate variables related to latitude. African countries had the lowest sex ratios, with 50.7% of births being boys, while European and Asian countries had the highest, with 51.4% of births being boys. According to the study, the effect of latitude was present across wide variations in lifestyle and socioeconomic status, with large differences in sex ratios between tropical regions closer to the equator and temperate regions farther from the equator. The correlation also remained after Navara excluded data from Asian and African countries that might have skewed sex ratios because of sex-preference practices, such abortion of female fetuses or killing of infant girls. Therefore, sex-selection practices did not explain the correlation, according to the Times.

The Times reports that there are a few possible explanations for the correlation but that none entirely explain the results. One explanation could be that there is a survival value in producing more girls in warmer regions. There could be genetic or racial differences that play a role, although the fact that the correlation persists across different populations make this possibility unlikely, the Times reports. Other theories include the quality of sperm at different temperatures causing variations, or "some event during gestation at warmer temperatures that causes more male fetuses, or fewer female fetuses, to spontaneously abort," according to the Times. Navara said the study suggests 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" (Bakalar, New York Times, 4/21).

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