Common sense dictates that a woman’s chance of having either a boy or a girl is about equal—fifty/fifty, like tossing a coin. Yet this isn’t always what happens. Although there are slightly more women on this planet than men, more males are born than females—but not always. Accumulating research indicates that environmental stressors—from climate change and wars to latitude and disasters—can lead to shifting sex ratios, usually meaning fewer male babies than female babies. On an individual basis, these effects are hard to measure, but at the population level, it’s becoming clear that natural selection begins working even while we’re in the womb.
Why Girls Rule
Roughly 51 percent of all children born in this world are male, meaning about 105 boys are born for every 100 girls. Theories seeking to explain this disproportion abound. Sperm that will produce male offspring travel faster. Viruses, such as Hepatitis B, can skew the sex ratio heavily male-dominant. Yet one hundred years after this ratio was acknowledged as fact, there remains no consensus and old theories continue to gestate, and new ones continue to emerge.
It appears that this 51 percent doesn’t always hold, however. Factors varying from the mundane (think location) to the unpredictable (think disasters) can alter it. In general, under times of stress, fewer males are born. That’s because women will spontaneously abort weak male fetuses, leading to an increase in the proportion of females being born. Because females are more likely to make it to reproductive age, more females mean a higher likelihood of progeny. Researchers know that males are more vulnerable to stressors not only in utero, but throughout their entire lifespan. Why women are usually more resilient to stress isn’t completely understood.
Getting Rid of Weak Links
Numerous factors can shift the birth ratio away from the male advantage. Besides research indicating that smoking (surprise, surprise) skews the sex ratios so that fewer males are born relative to girls, there are new insights into varying sex ratios:
- Hot-off-the-academic-presses, a paper from University of Georgia researchers states that more females are born in tropical climates than in temperate ones. Regardless of differing cultural norms, elevation, or socioeconomic status, the relationship holds: mothers living in tropical latitudes will give birth to roughly a quarter percent more females than those giving birth elsewhere.
