Returning soldier effect

{{Short description|Phenomenon in human reproduction}}

The returning soldier effect is a phenomenon which suggests that more boys are born immediately after wars.{{Cite news |last=Kanazawa |first=S. |date=2007-09-27 |title=Big and tall soldiers are more likely to survive battle: a possible explanation for the 'returning soldier effect' on the secondary sex ratio |language=en |volume=22 |pages=3002–3008 |journal=Human Reproduction |issue=11 |url=https://academic.oup.com/humrep/article-lookup/doi/10.1093/humrep/dem239 |access-date=2020-12-25 |doi=10.1093/humrep/dem239 }}{{Cite news|title=Returning soldier effect|publisher=Jagiellonian University|url=https://en.uj.edu.pl/en_GB/ju-research/-/journal_content/56_INSTANCE_2XezEHy2NT5h/81541894/143008520 |access-date=2020-12-25}} 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.

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.{{cite journal |last1=Davenport |first1=Arthur Frederick |date=1901 |title=Notes on the Origin of Sex |url=https://archive.org/details/b28083593/page/122/mode/2up |journal=Intercolonial Medical Congress of Australasia: Transactions of the Fifth Session, Held in Brisbane, Queensland, September 1899 |pages=123–130 |access-date=26 December 2020}}

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.{{cite journal |last1=MacMahon |first1=Brian |last2=Pugh |first2=Thomas F |date=June 1954 |title=Sex ratio of white births in the United States during the Second World War |journal=American Journal of Human Genetics |volume=6 |issue=2 |pages=284–292 |pmid=13158334 |pmc=1716540 }}

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". Other genetic explanations have been proposed.{{Cite web |last=Allen |first=Laura |date=2008-12-18 |title=Why Does War Breed More Boys? |url=https://www.popsci.com/scitech/article/2008-12/why-does-war-breed-more-boys/ |access-date=2023-07-12 |website=Popular Science |language=en-US}}

Valerie Grant attributed it to changing hormone levels of women during war, as they tended to "adopt more dominant roles".{{cite journal |last1=Grant |first1=Valerie J |date=2008 |title=Sex-of-Offspring Differences between Mothers |journal=Evolutionary Psychology |volume=6 |issue=1 |doi=10.1177/147470490800600117 |s2cid=146289598 |doi-access=free}}{{Cite book|last=Ridley|first=Matt|url=https://books.google.com/books?id=CWza0RyG_MwC&q=%22returning+soldier+effect%22&pg=PT566|title=The Red Queen: Sex and the Evolution of Human Nature|date=1994-10-06|publisher=Penguin UK|isbn=978-0-14-196545-1|language=en}}

William H. James writing in 2008 gave an increase in coital rates by returning soldiers as a possible cause. He 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".{{cite journal |last1=James |first1=William H |date=March 2009 |title=The variations of human sex ratio at birth during and after wars, and their potential explanations |url=https://pubmed.ncbi.nlm.nih.gov/18952111/ |journal=Journal of Theoretical Biology |volume=257 |issue=1 |pages=116–23 |doi=10.1016/j.jtbi.2008.09.028 |pmid=18952111 |bibcode=2009JThBi.257..116J |access-date=26 December 2020}}

The normal ratio is estimated to be some 1.03 to 1.06 males per female,{{cite journal |last1=Chao |first1=Fengqing |last2=Gerland |first2=Patrick |last3=Cook |first3=Alex R. |last4=Alkema |first4=Leontine |title=Systematic assessment of the sex ratio at birth for all countries and estimation of national imbalances and regional reference levels |journal=Proceedings of the National Academy of Sciences |date=7 May 2019 |volume=116 |issue=19 |pages=9303–9311 |doi=10.1073/pnas.1812593116 |pmid=30988199 |pmc=6511063 |bibcode=2019PNAS..116.9303C |doi-access=free }} 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.