The phenomenon known as the “returning soldier effect” has intrigued researchers for decades. After major wars, particularly the World Wars, demographers observed something strange. That is a notable increase in the number of boys born compared to girls. This unusual demographic pattern was named the “returning soldier effect”. However, it has sparked a wide range of studies aimed at explaining the underlying reasons. But is there enough data to prove this effect, or is it merely a coincidence?
Typically, the global sex ratio at birth favors boys, with approximately 105 males born for every 100 females. However, this ratio can increase after major conflicts, where a noticeable spike in male births occurs. The first major observation of this phenomenon was made after World War I and World War II. The birth records showed an increase in the proportion of male children. Especially, in the UK and the US, male births surged in the years following these wars, and similar patterns were observed in other countries involved in the conflict.Let us explore the evidence behind the returning soldier effect, with facts and data to assess the validity of this claim.
What is the Returning Soldier Effect?
The “returning soldier effect” refers to the higher-than-average birth rate of boys observed after men return from war. The phenomenon was first noticed after both World War I and World War II. The countries involved in these conflicts saw a considerable spike in male births in the years immediately after the wars ended.

What Does the Data Tell Us?
One of the most striking examples comes from post-World War I and World War II data. According to statistics provided by the journal Human Reproduction, the birth sex ratio, which normally sits around 105 male births for every 100 female births, shifted significantly upward during these post-war periods. For example, in Germany between 1946 and 1947, the ratio reached 108 male births per 100 female births, a statistically significant increase.

Data from the United States provides further support. Trend analysis of the sex ratio at birth from 1940 -2002 published by the CDC found that the sex ratio peaked in 1946. It rose to a ratio of 1059 male births to 1000 female births, immediately following WWII.
In the UK normal sex ratio in peacetime is 51.2%. However, this changed to 51.365 during wartime (1914–1920, 1939–1948).
Theories Behind the Returning Soldier Effect
Why does this increase in male births happen? Several theories have been proposed, and scientists have debated the causes for years. One widely accepted explanation is related to the selective pressures of war. Men are disproportionately affected by the casualties of war. Therefore, an increase in male births could be nature’s way of balancing the population.
Hormonal Changes: One common theory is that stress and trauma experienced by soldiers during wartime may lead to hormonal changes upon their return. This influences the likelihood of conceiving male offspring. Elevated testosterone levels, for example, have been linked to a higher chance of male conception.
Selective Fertility: Another theory suggests that healthier or more robust men, who are more likely to survive the physical and emotional toll of war, may have a higher tendency to father boys. Studies have shown that male sperm is more likely to produce male offspring when the father is in peak physical condition.
Natural Population Balancing: Some scientists argue that the returning soldier effect is a natural mechanism of population balancing. After wars, when a significant portion of the male population has been lost, nature might increase the male birth rate to help restore population balance.
According to the observations of statistician Bill James, the chances of a woman having a boy or a girl might vary very slightly as per her menstrual cycle. If the woman gets pregnant earlier on, then she’s more likely to give birth to a male child. This has to do with the changing hormone levels or acidity levels at different points of the menstruation cycle.
Taller Soldiers Returning from War and Other Hypotheses
Evolutionary Psychologist. Satoshi Kanazawa says this could be due to taller soldiers returning from war and fathering sons.
“I suggest that the higher sex, ratios during and immediately after wars might be a byproduct of the fact that taller soldiers are more likely to survive the battle and that taller parents are more likely to have sons.”
Satoshi Kanazawa, Evolutionary Psychologist
Kanazawa’s research indicated that surviving soldiers had a higher mean height. Therefore he posits that taller men who survive war might account for the fathering of more sons.
Several other hypotheses include the ‘maternal dominance hypothesis’ which contends, first, that the mother, not the father, determines the sex of the offspring. Secondly, that dominant, ‘tough’ women high in testosterone are more likely to have sons. Concerning the returning soldier effect, V.J. Grant argues that women become ‘tougher’ during wars. Because they have to take over some of the traditionally male roles in society in the absence of men. Hence, they as a result have more boys presumably because their testosterone levels increase during wars.
W.H. James contends that during wars, couples are reunited only during short leaves from the armed services. They are expected to have frequent intercourse during such short leaves. Hence result, are more likely to conceive during the early phase of the cycle, when the estrogen/gonadotrophin ratio in women is high. Mammalian (including human) sex ratio is higher when the maternal estrogen level is higher at the time of conception. Hence, couples who have high frequencies of intercourse (such as briefly reunited soldiers on short leaves and their wives) are more likely to conceive sons. This “frequency of coitus hypothesis” however lacks empirical evidence as per Kanazawa.
The Role of Socioeconomic Conditions
While biology plays a critical role, socioeconomic conditions are also believed to influence the returning soldier effect. Societies recovering from war often undergo significant social and economic changes, which could affect family planning and birth patterns. In Japan, for example, following World War II, a combination of post-war reconstruction and government policies encouraging childbirth contributed to the rise in male births

Criticism and Skepticism
While there is substantial evidence supporting the returning soldier effect, some researchers argue that the phenomenon is not as straightforward as it appears. Dr. Simon Cox, a demographer at the University of Edinburgh, highlights that while the effect is evident in large-scale conflicts, it does not occur uniformly across all wars or regions.
“In some post-war periods, we do not observe a statistically significant change in birth sex ratios. Other factors, such as local socioeconomic conditions, could also play a role,”
Dr. Simon Cox, University of Edinburgh
Moreover, critics also point out that data inconsistencies may arise due to the limited availability of reliable demographic records from wartime and post-war periods. This issue is particularly evident in less developed regions where record-keeping is less precise. A study published in Demography shows that the returning soldier effect was less pronounced in countries like Italy and Japan compared to Germany and the U.S.
Conclusion
The returning soldier effect remains one of the more fascinating and puzzling demographic phenomena observed in modern times. While the exact biological and social mechanisms behind the increased birth rate of boys remain debated, there is substantial evidence to support its existence. Data from post-war periods, such as after World War I and II, provide compelling examples of this trend, with researchers pointing to a combination of evolutionary pressures, hormonal changes, and socioeconomic factors as potential causes.
Although some skepticism remains, particularly regarding the universality of the effect, the bulk of the evidence points to a genuine, recurring demographic shift in favor of male births after large-scale wars. The returning soldier effect, while complex, appears to be a real and measurable phenomenon that continues to intrigue scientists and historians alike.
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