Where the Girls Are. More males are born for every female in temperate and subarctic latitudes (blue) than are born in the tropics (red and yellow).

Kristen Navara, The University of Georgia.

Tropical Parents More Likely to Hear, "It's a Girl!"

Wondering about the gender of your future offspring? Check your GPS. Girls are more likely to be born at tropical latitudes than in temperate or subarctic climes, according to new research. The study provides the first global look at human sex ratios and could shed light on how temperature and day length influence human reproduction.

Many animals adjust the sex of their offspring based on environmental cues such as temperature, but whether humans do the same is difficult to measure. On average, about 105 boys are born per every 100 girls, a ratio that varies by continent. Differences are difficult to nail down; however, due to insufficient data and stressful conditions like war that potentially affect rates of fertilization and miscarriage. Until now, the only studies done on the connection between human sex ratios and latitude--one in North America and one in Europe--were geographically limited and reached opposite conclusions.

To overcome these problems, reproductive endocrinologist Kristen Navara of the University of Georgia in Athens analyzed global sex ratios over 10 years, enough time to be sure that short-term social or economic crises weren't driving the results. Navara had previously found that Siberian hamsters raised with fewer hours of daylight produced more male offspring, and she wondered if the same might be true for humans. Using the CIA's The World Factbook and other government publications, she gathered sex ratio data for every nation with a decade of uninterrupted statistics. She then analyzed the figures from 202 countries based on latitude, average temperature, day length, and socioeconomic status.

The analysis, reported today in Biology Letters, found a clear pattern. Just like hamsters, humans living in areas with long, dark winters have more boys: 51.3% of babies born in temperate and subarctic regions are male, compared with 51.1% in the tropics. No one knows exactly when or how sex selection happens, but Navara says that some animals skew sex ratios as early as fertilization. "I suspect that all of this has something to do with melatonin," a hormone that regulates sleep-wake cycles and the production of female reproductive hormones, says Navara, noting that melatonin release varies in response to day length and season.

Reproductive biologist Thane Wibbels of the University of Alabama, Birmingham, says the results suggest the ability to match offspring gender to environment may be adaptive for humans, though no one knows why boys might have an advantage outside the tropics. "She's uncovering something that now we have to address physiologically as well as evolutionarily," he says. One key to understanding why sex ratios differ, suggested reproductive biologist Mary Mendonça of Auburn University in Alabama, might be to investigate why some countries like Venezuela buck the tropical girl trend.