Returning Soldier Effect

The returning soldier effect describes an observed rise in the proportion of male births following wartime periods, most notably around the two World Wars. Baseline human sex ratio at birth is about 1.03-1.06 males per female, and researchers have documented spikes in favor of boys after large conflicts (a pronounced peak in the United States occurred in 1946). The phenomenon was first identified in 1883 by Carl Düsing; Arthur Davenport in 1899 attributed it to differences in health between returning soldiers and their partners. Subsequent studies and theories include MacMahon and Pugh’s 1954 documentation of the post‑World War II increase, Kanazawa’s 2007 proposal that taller soldiers survived combat and taller parents are likelier to have sons, Valerie Grant’s hormonal/behavioral explanation tied to women adopting dominant roles during war, and William H. James’s suggestion that increased coital rates among returning troops could raise male births.

The rise in prenatal sex determination from the 1960s (and widespread commercial ultrasound by the mid‑1980s) introduced sex-selective abortion and has obscured the historical pattern in some regions. William H. James also recorded a post‑war decline in male births in Iran after the Iran-Iraq War, attributing it to stress‑induced selective termination of male fetuses. The returning soldier effect is therefore one of several biological, behavioral and sociocultural hypotheses invoked to explain variations in human sex ratios.