Women who want to increase their chances of giving birth to a girl should live closer to the equator, according to a University of Georgia researcher.
Most animals' nominal birth ratios are 50:50 male/female, but the proportion can be influenced by environmental factors, said Kristen Navara, a reproductive endocrinologist in UGA's College of Agricultural and Environmental Sciences. Her research with hamsters and mice has shown that more males are born in winter, when days are shorter, while longer days favor females.
Spurred by her findings, Navara wanted to learn whether the same phenomenon holds true in humans. She dug through the CIA's World Factbook and other government publications, gathering data on 202 countries with a decade or more of uninterrupted birth statistics. She then analyzed the figures based on latitude, average temperature, day length, and socioeconomic status.
Her study, the first ever of global birth-sex ratios, showed that people living in areas with longer and darker winters-such as those in temperate and subarctic regions-have more baby boys, which accounted for 51.3 percent of total births. In the tropics, only 51.1 percent of births were boys. Navara reported her findings in the April 2009 issue of Biology Letters.
Such a difference may seem small, but it can translate into significant numbers. In the Central African Republic, for example, 51 percent of births are to girls, making it the only country in the world to produce more girls than boys. In 2006, that percentage translated into 1,400 fewer boys than if the birth ratio had been 50:50. On average worldwide, 105 boys are born for every 100 girls (51.2 percent boys).
I'm still not sure which cue affects the rates," Navara said, "but based on previous studies in mammals, daylength and related hormonal changes could play a role."
She noted that people who live in temperate climates get fewer than eight hours of light during winter. As a result, their bodies produce more melatonin for longer durations of time than those who live in the tropics. She adds that this idea has not been tested and there are other factors that also vary with latitude that may contribute. Other tropical countries with lower male birth rates included Grenada (50.2 percent), Mauritius (50.3 percent), and the Bahamas (50.5 percent).
"Of the 20 countries with the lowest ratios, 18 were at tropical latitudes," said Navara. "This overarching pattern remained despite their enormous socioeconomic variations."
The biological trend was also unaffected by cultural factors. For example, because baby boys are favored over girls in some Asian and African cultures, the high rates of selective abortion and infanticide there have skewed the overall sex ratio toward males. "We eliminated Asian and African countries in a second round of analyses to eliminate sex-specific abortion factors," said Navara. "But even then, the trend of women who live nearer the equator giving birth to more girls was still significant."