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This question already has an answer here:

What kind of evolutionary and/or environmental pressures could seriously alter/affect the sex ratio of human sub-species and/or humanoids? For instance, what kind of pressures would lead to male births becoming more common than female births even generations later, or female births more common than male births by a significant margin?

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marked as duplicate by Willk, sphennings, Logan R. Kearsley, kingledion, Andon Jan 22 '18 at 2:57

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    $\begingroup$ Male births actually are more common than female births -- currently about 106 to 100 in western countries, absent any artificial manipulation. Historically, rates as high as 115 to 100 have been recorded. This is a very significant margin -- it would allow a country, for example, to employ 6% of its males in the army (that's a large army) without creating a mass of unmarried women. $\endgroup$ – AlexP Jan 21 '18 at 22:22
  • $\begingroup$ I was asking about factors that could shift things even further. I knew that male births were slightly more common in humans, I was asking what could shift things so that, say female births were almost twice as common, or about 150 male births to 100 female ones, stuff like that. $\endgroup$ – Preg-Fan Jan 22 '18 at 1:05
  • $\begingroup$ How is this different from (your) question that Bashful Beluga listed as a duplicate? I don't see a difference. $\endgroup$ – kingledion Jan 22 '18 at 2:18
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There is some evidence to indicate that China's one child policy has already led to such a scenario. The rationale is that Chinese people still carry the tradition that the next generation looks after the previous one, and that if you can only have one child, then have a boy because boys will be better able to care for their parents later in life.

This did not lead to biological biases per se, but rather to higher infant mortality rates among female babies. That said, the impact is the same; a much higher number of males by proportion to females of a certain age.

If you have social pressures such as this that make a culture 'prefer' a specific gender, then prospective mothers will find other ways to make sure that they have a better chance of conceiving such a gender.

There are many herbal remedies already available that are believed to assist with gender selection; pH levels in the uterus are supposed to have an impact, and of course rhythm method can improve your odds if you want to conceive a specific gender (I won't go into the science or beliefs behind any of these as they are out of scope of the question).

Ultimately, while it's possible things as simple as diet could have a material impact on birth genders, the first pressure is likely to be social in nature and such pressure, combined with human ingenuity, will lead to a material outcome as has already been demonstrated in places like China.

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