From an evolutionary perspective, childlessness is as bad as death: either way, your genes die with you. (Unless you’re a bee or ant, which you’re not.) Quite obviously, you are descended from people who successfully reproduced, not from the millions of creatures throughout Earth’s history who didn’t. Adaptations that led to your ancestors reproducing got passed down to you, while adaptations that failed to make your ancestors reproduce didn’t. As a result, the vast majority of us have a rather strong inclination to do whatever it takes to reproduce.
I know this is incredibly basic stuff, but you wouldn’t believe the number of people I have encountered who swear that humans do not possess instincts related to reproduction.
Males and females have followed a different historical path toward reproduction, for both the obvious reasons (women bear the brunt of childbearing,) and some less expected ones. Like that, while about 80% of women historically reproduced, it looks like only 40% of men did.
What happens when people don’t have children? On a biological level, something needs to kick in and get them out there, where they can meet people and fuck. Or overthrow society, kill all of the other males, and then fuck. After all, the continuing existence of society means nothing when you don’t have kids.
When men have children, their testosterone drops. The more time men spend around their children, the more their testosterone drops. This is very sensible: it prevents men from murdering their children.
Since American society has, since its founding, afforded enough resources for most men to marry and have children, the normal state for American males by their thirties has been relatively low testosterone. Today, however, millions of people are choosing not to have children, do not live with their children, or are delaying childbearing for decades.
Men who do not have children/live with them do not have this drop in testosterone. They have the testosterone of evolutionary failure, of increasing aggression until, well, they reproduce or die. (Or just get old.)
If I really wanted to embarrass people, I could tell of some of the absolutely nutty things men I know have done to try to get laid. Rational thought and risk assessment go completely out the window. People act like they have gone mad.
The evolutionary pressures on women have been different, but I can’t imagine that they were non-existent. Like men, they probably find pregnancy and babies calming–meeting people requires aggressive, social behavior that (often) leads to violence, but raising babies requires being a quiet, responsible homebody. It’s probably not a coincidence that childbirth triggers, in some women, actual depression.
Women with no children seem, at least anecdotally, highly aggressive. Their willingness to overthrow society is well-known.
On the plus side, childlessness probably drives a certain amount of creativity. The childless can take more risks, and are driven to succeed. But this is not to say that high levels of aggressive hormones are, long term, great for your brain or overall health.