Why are more boys born after wars?
Why the ‘returning soldier effect’ might be down to more than divine intervention

There is a widely known phenomenon, dubbed the ‘returning soldier effect’, where the proportion of male babies born increases following large-scale wars. While this bizarre effect appears to be nature’s way of equilibrating the gender ratio to compensate for the death of many men in these wars, it seems there’s also some science at play.
This phenomenon was first documented in the 1950s by researchers MacMahon and Pugh. They observed that the US sex ratio increased marginally in favour of boys in the years immediately following World War Two. The increase from the baseline percentage of male births from 51.406% to 51.48% was only small, but nonetheless statistically significant. This was mirrored in Europe, where most nations also saw a rise in male birth rates following 1945. It is interesting that the pre-war gender ratios already favour male births, which seems to compensate for the higher levels of boys dying in infancy.
Whilst being widely recognised, the reasons behind the ‘returning soldier effect’ remain a point of contention. Attempts to rationalise this effect have led to a range of theories. When it was first observed, the accepted view was that this must be a heavenly interference. However, new theories have since emerged focusing on different aspects of the reproductive process.
“One theory of interest claimed that taller soldiers were more likely to survive war, and that taller parents were more likely to have sons”
One theory of interest claimed that taller soldiers were more likely to survive war, and that taller parents were more likely to have sons. Whilst this might seem improbable, both facts have been proven to be statistically true, and when used together do provide a potential explanation for this phenomenon. A study analysing the heights of surviving soldiers in World War 1 compared to those that did not survive concluded that the mean height of surviving soldiers (168.63 cm) was higher than those that died (166.26 cm). Similarly, the same study used data from the National Longitudinal Study of Adolescent Health to demonstrate that height of parents had a statistically significant positive effect on the probability of having a son. However, this hypothesis, whilst providing statistical evidence as to why this effect might occur, fails to provide any biological explanation as to why taller men might be more likely to survive battle, or as to why they might then be more likely to conceive sons.
An alternative theory is the ‘maternal dominance hypothesis’. This explanation for the increased male birth proportions centres on women, who remained at home and adopted roles that were generally regarded as being ‘masculine’. These jobs might include driving vehicles, working in factories, or even serving in local resistance armies. The theory suggests that adopting these more masculine roles led to increased female testosterone levels, which supposedly favoured male pregnancies. However, this theory also lacks any biological basis (both that testosterone levels rise, and that this might increase male conception rates) and therefore is also unlikely.
“The theory suggests that adopting these more masculine roles led to increased female testosterone levels, which supposedly favoured male pregnancies”
The most likely mechanism focuses on the stage of the menstrual cycle at which fertilisation occurs. This hypothesis assumes that the frequency of intercourse increases immediately after a war; a reasonable assumption given the ‘baby booms’ that were observed following both World Wars. Therefore, if people are having more sex, they are more likely to conceive a child earlier on in the menstrual cycle. In the earlier stages of the cycle, female oestrogen and gonadotrophin levels are higher, in addition to a slightly elevated cervical pH. These hormonal and pH conditions might marginally favour a male conception.
It is worth noting that the increased chance of conceiving a male earlier on in the menstrual cycle is negligible on an individual level, and the effects can only be seen on a population-wide scale. It will be interesting to see how the gender ratios will change in the future. There is some evidence to suggest that people are having less sex nowadays than previously, so it remains to be seen whether this will translate into more female babies.
Want to share your thoughts on this article? Send us a letter to letters@varsity.co.uk or by using this form.
Features / The forgotten family member: Oxbridge College sisterhood
12 February 2025Comment / Why Oxbridge’s offers day matters
10 February 2025News / Pembroke’s TikTok famous Director of Music steps down from role
12 February 2025Comment / There is a hypocrisy of tolerance here at Cambridge
14 February 2025News / News in brief: Rowing, research, and a royal documentary
11 February 2025