More sex = more boys, more stress = more girls

Science behind babies: The more sex you have the more likely you’ll have a boy and more girls are born in times of stress.

More males are born after wars
Examining the ratio of boys to girls born from 1838 to the modern day reveals some fascinating fluctuations.

In particular, spikes in 1919 and 1944 show that more males are born at the end of wars. This trend has been found in many other countries.

While there are those who believe a greater force must be at work ‘replacing’ the fallen with baby boys, there has been extensive research into this strange phenomenon and a number of more scientific explanations have been suggested.

One is that evolution has enabled women to have more boys in times of great loss of men as they will do better in a society short of males.

For me, the most coherent explanation is that the sex of the foetus is influenced by the hormone levels of the parents at conception, with more boys being conceived earlier on in the cycle.

The peak fertile time for a woman is around two days before ovulation but if couples have a lot of sex they are more likely to conceive before the woman reaches this peak, at an earlier point in her cycle.

Obviously during, and just after, major wars sex has to be crammed into brief periods of leave.

So a couple will maximise opportunities for making love with intense bouts of activity, with less consideration to where a woman is in her cycle.

So more conceptions are likely to occur earlier in the fertile period and therefore give a higher chance of having a boy.

This theory is backed up by another peak for boys in the UK in 1973. While there was no war on, this is when the average age of women at marriage was at its lowest — 21 — and there was a surge of teenage pregnancies.

It was a time of intense sexual activity in the young. Clearly frantic fornication produces more boys.

 … and more girls in times of stress

While more boys are born at the end of wars, relatively more girls are born in times of prolonged parental stress — for example, as happened after the 1995 Japanese earthquake and in New York in the wake of the destruction of the Twin Towers in 2001.

Prolonged financial hardship also produces a similar trend.

Researchers investigating miscarriages in California found that there were more miscarriages of male babies when the unemployment rates were higher between 1989 and 2001.

So the most likely explanation for this pattern is that stress during pregnancy leads to more miscarriages of male than female foetuses.

Additionally, the flip side of the argument that more sex produces more boys is that less sex results in more girls — and stress, of course, is a well-known passion killer.

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