I suspect a ton of the psychological illnesses/problems we see today are the result of radical divergence from the behaviors we had in our ancestral evolutionary environments. It would be remarkable if the brain didn't make a bunch of assumptions on the structure of our environment when tuning stimulus sensitivity, neurotransmitter production, etc.
Some examples that seem obviously concerning to me are changes in child-rearing patterns (later and fewer), changes in diet (less fat and protein, more stuff that didn't exist in the evolutionary environment at all), changes in stress exposure (mental, physical, and immune [especially parasitic]).
> I keep wondering how many of the psychological problems arise from people having children much much later in their lives than used to be normal
This is also something I have been thinking about for a long time now. The human body, instinct and mind seems to be made to raise children. As a new parent, you have zero experience in handling a child. Yet after just a few minutes, your instinct kicks in. You somehow know how to hold the newborn, despite the fact you have never done this. You try everything to keep it warm and comfortable. You realize that your body is perfectly shaped to hold a baby with one arm, and that the baby instinctively contracts its legs for a perfect and secure fit. Despite the fact that crying babies sounded the same to you all your life, you can suddenly distinguish the crying of your own child from that of others from the beginning and from a large distance. If your baby cries, you instinctively know what the problem is. Without making a conscious decision regarding this, your absolute number one priority is suddenly to keep this child alive, whatever it takes. The thought that you would fight anything or anyone attacking your child, until the bitter end, suddenly seems completely natural. All of this can not be made clear to people without children, which is basically why parents like to talk to other parents. Raising a child certainly is one of the most extreme things a human being is capable of, and yet most parents manage it. To me, it would not at all be surprising that not using this potential may lead to serious psychological problems.
I keep wondering how many of the psychological problems arise from people having children much much later in their lives than used to be normal.