If you look at any of the data, you will see that women are waiting longer to have children, and more women are also not having children at all. From the linked article:
“In 1976, just 10 percent of all women ages 40 to 44 had no children. That percentage had jumped to 19 percent by 2010.”
I view this as a disturbing trend from an evolutionary perspective.
First, if you look back at hunter-gatherer birth patterns, women had their first child around age 18. So a woman today having her first child at age 30 is quite “late” from an evolutionary perspective. I’m not encouraging teen pregnancy or anything, but I am just stating that women in the ancestral environment had children earlier than they do today.
The more disturbing issue in my view is that many women seem to be growing indifferent to having kids. From anecdotal evidence, it seems that some women are not necessarily against kids, but they just never get around to having them. If a woman makes a choice to have or not have kids, then that’s one thing. But if a woman doesn’t make a choice and then just waits for the clock to run out, so to speak, then that’s another.
The second scenario is what I see happening more. Women get involved in their careers and put off getting married and having kids. Then suddenly they are in their late 30s and the window begins to close.
A point I want to make is that having children is not really supposed to be a rational cost-benefit decision. From an evolutionary perspective, people are just supposed to enjoy sex and then children come about shortly thereafter. It is only with the invention of contraception that having children becomes a more of a cerebral choice.
Given that having children was not really a decision in the Paleolithic environment, we may not have evolved with strong mental circuitry in regards to consciously choosing to have children. Given the numbers above that 20% of older women now have not had children, I think this issue deserves further research.