The mystery of genetics and war: why do more boys tend to be born after major conflicts?

Throughout human history, war has transformed society in both expected and unexpected ways. Economies shift, political landscapes evolve, and cultural norms are rewritten. Yet behind these vast changes lies an unusual biological trend that seems difficult to explain. Following major conflicts, there is a barely noticeable, but statistically significant increase in the number of male births — a phenomenon known as the "returning soldier effect," as noted by Varsity.

Some attribute this surge in births to fate or divine intervention, but science offers a multitude of intriguing explanations. This phenomenon was first documented in the 1950s by researchers McMahon and Pew, but it became particularly pronounced after World War II, when the percentage of male births in the U.S. rose from 51.406% to 51.48%. Although the increase was marginal, it was observed in many countries, suggesting the presence of a broader biological mechanism underlying this phenomenon.

Considering that the natural sex ratio at birth favors males, likely as compensation for higher male infant mortality rates, this unexpected surge in male births after war remains an interesting topic for researchers. Efforts to explain this effect have led to competing theories.

One hypothesis suggests that taller soldiers had a better chance of surviving combat and that taller parents are statistically more likely to have sons. A study published in the journal Human Reproduction, focusing on World War I soldiers, indicated that the average height of survivors was slightly greater (168.63 cm) than that of those who perished (166.26 cm). Additionally, data from the National Longitudinal Study of Adolescent Health revealed a correlation between parental height and a higher likelihood of male offspring. While this theory is intriguing, it lacks substantial biological and genetic justification for why height might influence both survival and sex selection.

Another theory, known as the "maternal dominance hypothesis," shifts the focus to women left behind during wartime. When men went off to war, women often took on traditionally male roles, engaging in physically demanding work in factories or even participating in resistance efforts. Some believe that such changes could have led to increased testosterone levels in their bodies, potentially affecting the likelihood of conceiving a male child. However, scientific support for the idea that such lifestyle changes can significantly alter hormonal balance or affect sex ratios at conception is limited.

The most plausible explanation relates to the timing of conception in relation to the menstrual cycle. The post-war increase in sexual activity, historically documented and referred to as the "baby boom," may imply that more pregnancies occur early in the cycle. At this stage, higher levels of estrogen and gonadotropins, along with a slightly elevated pH of the cervical mucus, could slightly favor male sperm. While this effect may be minimal for individual couples, at the population level, it could account for the observed shift in birth ratios.

As modern society experiences declining birth rates and changing reproductive patterns, it remains unclear whether the "returning soldier effect" will persist in future conflicts. Given that evidence suggests contemporary individuals lead less active sexual lives than previous generations, the biological forces that once subtly regulated sex ratios may no longer function as they did before. Whether this will result in long-term changes in the dynamics of the Earth's population is an open question, and only time and ongoing research will provide an answer.