From this curious unsatisfying essay by Philip Kitcher (and I wish I could remember who sent me there, but it was some time ago), which I shall respond to in detail at some point in the future (by which time he'll probably have forgotten he ever wrote it):
“A tale from the history of human biology brings out the point. John Arbuthnot, an eighteenth-century British physician, noted a fact that greatly surprised him. Studying the registry of births in London between 1629 and 1710, he found that all of the years he reviewed showed a preponderance of male births: in his terms, each year was a “male year.” If you were a mad devotee of mechanistic analysis, you might think of explaining this—“in principle”—by tracing the motions of individual cells, first sperm and eggs, then parts of growing embryos, and showing how the maleness of each year was produced. But there is a better explanation, one that shows the record to be no accident. Evolutionary theory predicts that for many, but not all, species, the equilibrium sex-ratio will be 1:1 at sexual maturity. If it deviates, natural selection will favor the underrepresented sex: if boys are less common, invest in sons and you are likely to have more grandchildren. This means that if one sex is more likely to die before reaching reproductive age, more of that sex will have to be produced to start with. Since human males are the weaker sex—that is, they are more likely to die between birth and puberty—reproduction is biased in their favor.”
This is the standard misunderstanding of how evolution works. It is very hard to avoid the language of teleology when talking of evolution. Even experts do it, and, although they at least only use the language, not the theoretical framework, it is very confusing to those who are trying to grasp the idea. The idea that ‘evolution’ holds a meeting with itself and says hold on chaps we need more boys in Bognor Regis this weekend, is nonsense, but it seems to be what people believe. In the ongoing debates between creationists and evolutionists that do so much to enliven the drudgery of existence in the southern USA, the evolutionist side is handicapped by the fact that most of its proponents are motivated by pure faith just as much as the creationists are. They ‘believe’ in evolution, but they don’t understand it.
Zebras did not evolve stripes to confuse lions. There are zebras because they evolved stripes which confuse lions. In the great game of life and evolution, being favoured means not dying young, and so getting the chance to reproduce. Less successful evolutionary pathways are truncated, not by some sentient ‘evolution’, but by the teeth of lions.
In most countries more boys are born than girls, but there is no obvious reason for this. The usual explanation is that, where there happen to be fewer males (for example) they will have a wider choice of females and so will produce more offspring (the males will have more offspring on average than the females). Thus those people who are genetically predisposed to have produce boys will have more grandchildren and the balance will be restored.
You may have noticed the hand-waving slipped in there. It assumes that there is such as thing as being ‘genetically predisposed to produce males’. This is not known to be true. It is known to be true that we are genetically predisposed to produce roughly the same number of boys as girls.
The real reason the balance is approximately maintained is that anything else, in a given population, would be unstable and would cause such a population to die out (or to seek to redress the imbalance socially, by mixing with neighbouring populations). This is observably true, because only approximately balanced populations exist, but we don’t know why it’s true. All we can do is wave out hands about and talk about restoring equilibrium. But a given population doesn’t restore its equilibrium. The unbalanced populations disappear. We only see the survivors.
We are, biologically, monogamous, and it is that that probably leads to there being an approximately equal number of males and females.