Scientists have known for a long time that an increased number of boys are born during and after major wars. The phenomenon was first noticed in 1954 with regard to white children born during World War II in the United States. It has since been replicated for most of the belligerent nations in both World Wars.

The phenomenon has been dubbed the “returning soldier effect.” There is no doubt that the phenomenon is real, but nobody has been able to explain it. Why are soldiers who return from wars more likely to father sons than other men?

There is now evidence that, at least among the British soldiers who fought in World War I, those who survive battle and return home to be reunited with their wives are taller than those who die (and therefore never have another chance to have a child). A comparison of the physical characteristics of the British soldiers who survived or died in World War I shows that surviving soldiers are on average nearly one inch taller than fallen soldiers. The average height of the surviving soldiers is 66.4 inches, while that of the fallen soldiers is 65.5 inches. Even in the small sample that is examined, this one-inch difference is highly statistically significant.

As we explain in Chapter 5 of Why Beautiful People Have More Daughters, taller parents are more likely to have sons than shorter parents are. So the excess boys born during and immediately after the World Wars might be a consequence of the fact that taller soldiers, who are more likely to have sons to begin with, are more likely to survive the war and return home, whereas shorter soldiers, who are more likely to have daughters, are less likely to survive the war and return home to have daughters.

Now you may be asking yourself, “What difference can such a small height advantage — slightly less than one inch — possibly make?” Detailed calculations show that the one-inch difference is more than twice as sufficient to account for all the excess boys born in the United Kingdom during and immediately after World War I. True, a one-inch increase in height only increases the odds of having a son by 5 percent. However, because so many men (nearly one-third of those between the ages of 15 and 40 in the UK) were mobilized during World War I, the 5 percent increase in the odds of having a son for the taller surviving soldiers translates into millions of excess boys. It is more than sufficient to account for the entire “returning solder effect” in the UK after World War I.

This explanation for why more boys than usual are born during and after wars raises a further question: Why are taller soldiers more likely to survive wars? I will speculate on this question in my next post.