Returning soldier effect

The returning soldier effect is a phenomenon which suggests that more boys are born immediately after wars, even before modern methods to know the sex of the baby before birth. ^{[1] [2]} This effect is one of the many factors influencing human sex ratio. It was especially noticeable worldwide during and right after both of the World Wars. ^[3]

Sex ratio without war

The normal ratio is estimated to be some 1.03 to 1.06 males per female ^[4] , which appears to compensate for the fact that child mortality rate among boys is slightly higher than among girls, and that adult men are more likely to die from an accident than women. ^[3]

Cases before Ultrasound

The phenomenon was first noticed in 1883 by Carl Düsing of the University of Jena, who suggested that it was a natural regulation of the status quo. Writing in 1899, an Australian physician, Arthur Davenport, used Düsing's findings to hypothesize that the cause was the difference between the comparative ill-health of the returning troops compared to the good health of their partners. ^[5]

Research published in 1954 by Brian MacMahon and Thomas F. Pugh showed that the sex ratio of white live births in the United States had shown a marked increase in favor of boys between 1945 and 1947, after World War II, with a peak in 1946. ^[6]

In 2007, Kanazawa Satoshi published a paper theorizing that the effect was due to "the fact that taller soldiers are more likely to survive battle and that taller parents are more likely to have sons". This was based on his research of British Army records from World War I, which showed that "surviving soldiers were on average more than one inch (3.33 cm) taller than fallen soldiers". ^[1] Other genetic explanations have been proposed. ^[7]

Valerie Grant attributed it to changing hormone levels of women during war, as they tended to "adopt more dominant roles". ^{[8] [9]}

William H. James writing in 2008 gave an increase in coital rates by returning soldiers as a possible cause.

Cases after Ultrasound

With the advent of Obstetric ultrasonography for Prenatal sex discernment, which was first proved in an academic setting in 1962, and became commercially commonplace at around 1985 with FDA approval, it became possible for parents to know the sex of offspring before birth, introducing the cause of Sex-selective abortions, significantly obscuring the original returning soldier effect.

William H James also noted that a fall in the ratio of male births had been recorded in Iran following the Iran–Iraq War, "explained by psychological stress causing pregnant women disproportionately to abort male fetuses". ^[10]

See also

• Killer ape theory
• Trivers–Willard hypothesis

References

1. Kanazawa, S. (27 September 2007). "Big and tall soldiers are more likely to survive battle: a possible explanation for the 'returning soldier effect' on the secondary sex ratio". Human Reproduction. Vol. 22, no. 11. pp. 3002–3008. doi:10.1093/humrep/dem239 . Retrieved 25 December 2020.
2. "Returning soldier effect". Jagiellonian University . Retrieved 25 December 2020.
3. Rutherford, Adam; Fry, Hannah (2 October 2023). "Introducing… Uncharted with Hannah Fry". The Curious Cases of Rutherford & Fry. BBC Radio 4 . Retrieved 24 February 2024.
4. Chao, Fengqing; Gerland, Patrick; Cook, Alex R.; Alkema, Leontine (7 May 2019). "Systematic assessment of the sex ratio at birth for all countries and estimation of national imbalances and regional reference levels". Proceedings of the National Academy of Sciences. 116 (19): 9303–9311. Bibcode:2019PNAS..116.9303C. doi: 10.1073/pnas.1812593116 . PMC   6511063 . PMID   30988199.
5. Davenport, Arthur Frederick (1901). "Notes on the Origin of Sex". Intercolonial Medical Congress of Australasia: Transactions of the Fifth Session, Held in Brisbane, Queensland, September 1899: 123–130. Retrieved 26 December 2020.
6. MacMahon, Brian; Pugh, Thomas F (June 1954). "Sex ratio of white births in the United States during the Second World War". American Journal of Human Genetics. 6 (2): 284–292. PMC   1716540 . PMID   13158334.
7. Allen, Laura (18 December 2008). "Why Does War Breed More Boys?". Popular Science. Retrieved 12 July 2023.
8. Grant, Valerie J (2008). "Sex-of-Offspring Differences between Mothers". Evolutionary Psychology. 6 (1). doi: 10.1177/147470490800600117 . S2CID   146289598.
9. Ridley, Matt (6 October 1994). The Red Queen: Sex and the Evolution of Human Nature. Penguin UK. ISBN   978-0-14-196545-1.
10. James, William H (March 2009). "The variations of human sex ratio at birth during and after wars, and their potential explanations". Journal of Theoretical Biology. 257 (1): 116–23. Bibcode:2009JThBi.257..116J. doi:10.1016/j.jtbi.2008.09.028. PMID   18952111.