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.
- A 2008 study published in the Proceedings of the National Academy of Sciences found that Scandinavian birth cohorts that were exposed to extreme cold resulted in fewer male fetuses being born. However, because the weak male fetuses were culled in utero, only the strong ones survived, leading to men who had longer life spans compared to males in other cohorts.
- Chernobyl did more than lay waste to the Ukrainian countryside and its people. It also had the effect of decreasing the number of males born in surrounding countries. Furthermore, it appears that the decrease was tied to how much radiation exposure mothers faced, i.e., a lower proportion of males was born in areas experiencing higher levels of radiation post-disaster.
- September 11th had an enormous impact on the United States and researchers are still teasing out its effects on the health effects. Researchers from the University of California, Berkeley, looked at birth data from California and New York City following 9/11 and saw a drop in the number of males born three months after, what would be expected from miscarriages of weak male fetuses. Although there was a decrease in male births in both California and New York City, NYC had a greater drop, indicating a dose-like response to stress.
Why do stressors shift the sex ratio? The authors of the PNAS paper write that “natural selection has conserved mechanisms by which women subjected to environmental stressors abort male fetuses least likely to yield grandchildren. These mechanisms purportedly cull males because sons are more likely to die before reproductive age than are daughters, despite receiving greater maternal investment.”
This means that natural selection weeds out the fetuses that it deems unlikely to survive and reproduce, leading to a more robust population, one that is likely to survive whatever stressor might be around during that time. The fragility of the male fetus isn’t a problem during the good times and for whatever reason, the birth ratio is skewed towards males. As more instances of altered birth ratios come to light, we might realize that choosing a blue or pink color scheme is less like a coin toss and more a reflection of our world around us.