Why Does War Breed More Boys?

[Beorn]

Why Does War Breed More Boys?



Surge of male babies in wartime is due to a male gene, says evolution researcher
By Laura Allen Posted 12.17.2008 at 3:41 pm 12 Comments

Boy, oh Boy: When men return from war, love surges—and so does the preponderance of boy babies. National Archives

A curious shift occurs during and right after a war: more boys tend to be born than girls. It’s been documented for decades in many nations, especially during long conflicts with many troops deployed. The cause of this boy boom has long flummoxed thinkers and scientists. Ideas have veered from the theological—a divine call for new men to replace those lost in battle—to the coital—returning soldiers have lots of sex, and so will be more likely to fertilize at a time in their ladies’ cycle that’s ripe for making boy babies. A new study in the journal Evolutionary Biology rejects them all. Instead, it pins the “returning soldier effect” on a gene expressed by men only. It also shows how researching your family tree can help you place bets on the sex of your next kid.
“I wasn't satisfied with the explanation that it was due to couples having more sex,” says Newcastle University’s Corry Gellatly, who did the work as part of his Ph.D. thesis. Gellatly was curious about studies of male shrimps, marine worms, and yes—human males—that showed that their likelihood of producing male offspring seemed to mimic that of their parents. In other words, males who have more brothers than sisters would in turn produce more sons than daughters. Thinking this may be the root of the wartime peaks, Gellatly sought to investigate the trend on a large scale. He sifted through 927 North American and European family trees posted on an online database by both amateur and professional genealogists, and tallied the sex ratios of siblings for each generation.
"The family tree study showed that whether you're likely to have a boy or a girl is inherited,” says Gellatly. “We now know that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters.” Women, however, did not have the same tendency.
Gellatly argues that a gene—which is carried by both men and women but only active in men—influences what proportion of a man’s sperm carry the X chromosome and how many carry the Y. The sperm’s X or Y status determines the sex of the baby upon meeting the egg, which only carries the X chromosome. More Y sperm=more XY (male) babies.
How might this gene tip the baby balance after a war? Consider the legacy of two hypothetical men—let’s call them John and Rich. John has three sons, all of whom go off to fight, and one daughter, who does not. Rich has the opposite family structure: three daughters and one son. John is more likely to see multiple sons return from war alive—and with Y-leaning sperm. So they’ll produce more sons of their own. Rich is likely to lose his only son, which if alive, would have fathered girls to even out the sex ratio. The mechanism, Gellatly’s genetic model shows, shifts the sex ratio back to normal as the dip in male mortality recovers.
The genetic explanation of the returning soldier effect is, for now, a thought experiment—the gene responsible has not been found.

Source.

[Æmeric]

Maybe it's an effect of a male hormone - present at higher levels in men returning from war/aggression - on the sperm that give the y-sperm an advantage in fertilizing the egg.:shrug

A majority of all babies born in all cultures are male. I think it is highest in Negroes & lowest in East Asians. I believe it is 105 males for every 100 females among Europids. Negroes have the highest levels of testosterone, Asian males the lowest (on average) maybe that accounts for it. Curious thing, male infants die at a higher rate then female infants in Western societies so the slight inbalance evens off by early adulthood. In other societies female babies die at a higher rate, from neglect or outright infanticide.

The example of one father with 3 sons & 1 daughter vs. the father with 1 son & three daughters - that is just random chance. It's not anything genetic. My maternal grandfather was one of 5 sons, no sisters. He fathered 5 daughters & 2 sons.