Why are males and females 50%-50%? [View all]

[cthulu2016]

Last edited Sat Feb 18, 2012, 03:57 PM - Edit history (1)

Edit: I enjoy learning things from posting threads and considering replies. In this case, I'll update/clarify the OP on a couple of things. Some of the specific statement in the OP are quite flawed, and others true in their implication but so sloppy as to be misleading. First, all humans start out as female and the males must then make a very tricky developental switch that is often fatal. So we adjust (tending toward the 1:1 sex ratio) by producing many more XY (male) potential persons than XX (female). But since many of the males fail by birth the ratio is about 105:100 male. The birth defect rate of males is higher and I would bet total pre-pubescent mortality is higher for males, again trending toward the evolutionarily stable 1:1 ratio. An environment can favor one or the other and that will manifest itself, but all things being equal the logic of sexual reproduction provides constant pressure toward 1:1, even if in the face of an environment that says otherwise. Googling Fisher's law offers the original theortical model of hy we tend toward 1:1 and Dawkins THE ANCESTOR'S TALE has a lovely chapter on Fisher's Law.

The overall point, though, is that neither male or female human beings are more "valuable" in evolutionary terms, and that whatever perception to the contrary is an artifact of culture, not nature. (It's pro-woman argument -- an approach like China that artificially favors males will run into natural head-winds because eventually those with sons will become less and less likely to have grand-children.)

Oddly enough, there's a concept missing I would expect. Primary sex ratio is at conception. Secondary is at birth. Tertiary is ratio in the whole population. But since the real import of sex ration is reproductive I would expect tertiary sex ratio to be breeding age population, and that's the one that ought to matter most because breeding, and only breeding, can perpetuate change or stability in sex ratio. I do not know this, but it's a good guess... Our primary is 170:100. Our secondary is 105:100. And our tertiary is female-heavy because women live longer. At some point the ratio is 1:1 and I would guess that crossover occurs during prime mate-selection age, because that's the goal. If tigers just loved little boys to eat and didn't care for girls then w would produce more and more little boys to keep the breeding population at rough parity.
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We humans tend to think that having sons is more valuable and it may be in practice, but only due to artificial peculiarities of our economics.

Nature sees no difference in value, and it's interesting to see why. We create fetuses at a rate of 50% male, 50% female. That's our natural rate. And from that we know that in the evolutionary long haul there has been no advantage in having children of one sex or the other. (Meaning that any perceived advantage is social/cultural.)

The genetic math of males (at least among mammals) is that sons are high-stakes gambles and daughters are "sure things." A son might sire a thousand grandchildren but will often sire none. A daughter is fairly sure to give birth to as many off-spring as her body can handle, which is a smaller number.

An individual creature can easily have the trait of producing only males or only females and if there was any evolutionary advantage (meaning a higher number of grand children, great-grand children, etc.) then there would be a lot more males or females born, accordingly.

In a population of 90% females there will be almost no monogamy. Any son is likely to produce scads of grandchildren. Harems would be the norm. The value of sons (in genetic survival terms) would be sky high and anyone who produced more sons naturally would find their genetic material spreading through the population. Those likelier to produce daughters find their genes squeezed out. More and more males are born.

In a population of 90% males the average male will produce no off-spring. The average male will be killed in competition with other males, or driven from sociaty as a rogue male. The situation would be so extreme that cultures would start summarily "exposing" most male infants. The value of producing daughters would be immense, and anyone with a knack for producing daughters would find their genes spreading and more daughters would be born.

And in species after species the stable distribution is 50-50%. Any temporary swing one way or another is smoothed out naturally and other traits change to support the 50-50%. For instance, if females cannot collect enough food during pregnancy or early child-rearing then other traits rise in value, like monogamous families or cooperative tribal organizations.

In human society, however, we can create laws or customs that favor males economically and create situations where parents opt for sons, and we often have. But they are genetically unsustainable and even economically unsustainable in the long run. If China had 70% male children then the economy must change. Reverse dowries would develop where a man could only gain a wife by securing the financial security of her parents... the reason (security) people opt for sons would develop into a reason to have daughters. Women would become more valauble, in every way. And such a China would probably find itself moved by the invisible hand of population dynamics to find reasons for wars, decreasing the male population.

The system can be gamed for a genration or two so people will do so, but in a world where all parents chose the sex of all children we would soon settle on parents in aggregate choosing 50-50% as the social and economic value of the sexes changed to accomodate the practical numbers.

Here's a very cool side note: Humans relibaly produce a tiny number more females... 50.1% to 49.9%, something like that. One theory of the discrepency is that mitacondrial DNA may find a subtle way to try to favor spontaneous abortion of males because mitachondrial DND comes entirely from the mother and can only be passed along by daughters. I am not suggesting that is the reason, it's just a beautiful theory.