Evolution, care of the young, and sex versus reproduction

It's no surprise that we seek out sex and enjoy it. It's no surprise that women as female mammals are strongly motivated to care for their offspring. Unlike most mammals, humans have evolved so that adult males provide resources (originally this was mostly food) for the children they have fathered, so they also care about the welfare of those children and invest a great deal in them. That is exactly what evolution by natural selection would predict.

Our desire for our children to thrive, and our willingness to invest heavily in them is so commonplace that we may lose track of how it's not obvious. Even selfish people who treat those outside their families very poorly most often leave all their assets to their children. We don't say, "Oh, he wasn't selfish -- he left money to his children!" In our minds, leaving money to our children *is* a form of selfishness. Given that we are all mortal, this is exactly what evolution would predict. Even selfish people with considerable money rarely spend all they have on hedonistic pursuits in their later years, but instead think it is right to leave something to their kids. They will typically do this even when their children are estranged, or hate them, or have adopted values very different from the parents' own.

At the heart of evolution by natural selection is leaving many descendants, and where this starts is being the father or mother to many children. The advent of reliable contraception in the past 70-odd years has created a major divergence between current selection pressure and that operating at any time in the past. Evolution did not require us to actively want to have children. All it required was the desire to have sex. Sex naturally caused pregnancy and in turn babies were born. Evolution has shaped us so that once they are born, it is a very high priority to care for them. But evolution never made us say, "I would like to have children." I think all human societies understand that sex causes pregnancies, and people in earlier times may have had the idea that they had enough children already. But this competed with the immediate urgent desire to have sex, and also a male's insistence on having sex with his mate even if she didn't want to. These factors are still in play in the large parts of the world that do not have access to reliable contraception.

But where contraception (and as a fallback, abortion) is freely available, birth rates have plummeted. We can fulfill our urges to have sex without having children. Of course many people do want children, but they typically want just one or two. Evolution's goal is to have many children. While medical advances have cut child mortality dramatically, three is still a minimum for a population to replace itself where a significant number of people choose to have no children at all. Why do people not want lots of children? For those of us who are fairly well off, there are opportunity costs -- lots of fun things we could do with our time instead of raising children. Also, we care about the welfare of our children, and realize that our existing one or two children will thrive better if they do not have to share our limited resources with further children.

The resulting dynamic seems to be that people in rich countries do not replace their numbers, but their place is taken by immigrants from regions where reliable contraception is not available. When those immigrants take on the values of the rich societies and have access to contraception, then they do not replace themselves either. But there are always more immigrants to come in, creating perhaps a dynamic equilibrium.

If this state of affairs persisted for thousands of years, evolution would strongly favor genes that tend to make people want to have a lot of children, not just a lot of sex.

Evolution would favor genes predisposing men and women to be willing to pay large sums to be sperm donors and egg donors. The fact that no such donors today pay for the privilege, and that egg donors in particular need to be paid for their services shows that evolution has made us want sex, but did not make us want to have offspring.