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Imagine a species that evolved almost identical to our own, except for one difference. A child's sex is not decided by the presence of a Y chromosome, and all embryos have the potential to evolve into either sex. The mother can decide the sex of the child she carries through regulation of hormones released during early development, which she has at least partially-conscious control over.

I'm wondering rather we would have the same sex distribution as we have now? Keep in mind that in the wild a sex distribution of 50/50 would be expected due to fisher's principle. However, technology tends to modify the way humans view the world. I suspect primitives humans would have the same 1:1 ratio, but would modern humans with modern technology and culture still maintain this ratio? what factors may cause a change in this ratio.

P.S. Keep in mind the evolutionary psychology. They would have evolved to have an instinctual understanding of fisher's principle which would likely limit extremes shifts either way.

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  • $\begingroup$ Surely this would depend on the culture, so without knowing more about that how can we answer? $\endgroup$
    – Tim B
    Aug 19, 2015 at 9:57
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    $\begingroup$ I could imagine large trends one way or the other over a period of a few years leading to a greater age range of a culturally acceptable match. $\endgroup$
    – Phil
    Jan 27, 2016 at 18:02
  • $\begingroup$ do you count only XY and XX ? cause there are like 8 sex combinations like XXYY-XYY-XYYY-XXYYY-XXY-X0 and probably even more.... $\endgroup$
    – Charon
    Aug 7, 2016 at 1:18

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I suspect that you would get boom and bust cycles. Girls would be more valuable when they became scarce and people would have more. Boys would be more valuable when girls were too abundant and people would have more. But group think in relation to current conditions would probably prevent a stable equilibrium without some sort of legal requirement.

In the same vein, at least until the New Deal, boom and bust cycles in agricultural commodities were commonplace due to the same sort of mechanism each season until a certain degree of governmental guidance was inserted into the process, because farmers responded to very limited information and decision making was very chunky (you only get to decide what to plant once a season) rather than a continuous feedback process.

A side effect of that is that social mores would oscillate a lot. This is because a lot of quite complex dating/marriage/gender relations/gender power structure/etiquette turns out to be powerfully influenced by the male-female ratio. This is a quite general phenomena. For example, the way young men and women interact in small town residential colleges where women significantly outnumber men (e.g. at historically women's liberal arts colleges that went co-ed) differs greatly from how they interact when the proportions are reversed (e.g. at military and engineering schools). It happens at the level of whole societies as well.

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The exact ratios will depend on cultural influences in any society though large deviations from 50/50 will naturally be self limiting over time.

Given the prevalence of patriarchal societies throughout human history, a strong preference for male children would probably prevail because males are stronger and can work in the fields. Males also make good soldiers. Parents want wives for their sons thus making daughters valuable too though they will be under the same pressures for males.

For example, the One-Child policy in China showed that culture's preference for males. India also has strong preferences for male children.

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    $\begingroup$ Without any weapons males are stronger than females. Not that much when we have hot weapons and robots that are stronger than both males and females. Also development of machines reduce men's advantage in work as well. This might be a main reason for feminism. You can argue that men prefer sons to spread their Y-Chromosome. But we can equally argue that women should prefer daughters to spread their mitochondrial DNA. I do not believe that large deviations from 50/50 will necessarily naturally be self-limiting. It is likely but not necessarily has to be true. Societies frequently go haywire. $\endgroup$
    – Ying Zhou
    Aug 19, 2015 at 14:12
  • $\begingroup$ (continued) Runaway things are not rare in societies, though globalization both reduce the possibility of some erratic society existing and increase bad effects on humanity when most of us deviate in the same direction. $\endgroup$
    – Ying Zhou
    Aug 19, 2015 at 14:14
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    $\begingroup$ To expand on the India and China angle: Sons (especially the first) stay with the parents and care for them in old age, as well as inheriting the household. Daughters move out to care for their husbands' parents. A couple with only daughters would have no one to care for them in this tradition. This make sons more valuable than daughters to them. $\endgroup$
    – Cyrus
    Nov 4, 2015 at 8:39
  • $\begingroup$ Men wanting to spread their Y chromosome makes no cause they have an X chromosome too and other 22 chromosomes that get mixed up too... in both cases genes from mother and father are spread equally regardless of what sex a children has... with your Logic people would want their sons to be intersex cause XYYY has more chromosomes Y than just XY... $\endgroup$
    – Charon
    Aug 7, 2016 at 1:15
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Boys would likely outnumber girls.

A modern society doesn't last long because the rate of technological change is so rapid. Cultural behaviors remain long after technology makes them obsolete.

If we look back at our stone age ancestors, the obvious best way for a male to pass on his genes is to avoid wars where he might get killed. However for the group to survive, the males must attack other groups (to keep them from becoming more powerful) and defend against other attacking groups. Failure to produce a new group of warriors with each generation leads to the culture of the group being destroyed. Thus most societies teach a warrior culture to boys so they are willing to sacrifice themselves for the good of the group.

If parents can choose the sex of their child, this warrior ethos will be expanded to include parents so that parents will have more boys to satisfy the needs of the culture. While this will limit the natural growth rate of the tribe (fewer girls to have babies), it will be made up for by capturing girls from other tribes to serve as child-bearers of the next generation. Obviously the parents wouldn't be making the best individual choice, but the culture would compel them and the culture would survive.

This could lead to an interesting situation where there are subordinate girl cultures and more powerful boy cultures. One powerful tribe in the area would birth an excess supply boys, and the other subordinate tribes would all birth an excess supply of girls. In this case the numbers might reverse and result in a larger number of girls being born.

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