The General Social Survey asked respondents, "Now thinking about your mental health, which includes stress, depression, and problems with emotions, for how many days during the past 30 days was your mental health not good?"
Here is the mean number of bad mental health days listed by number of offspring for women ages 40-55:
Childless women average 3.5 bad days, while women with eight or more kids have a mean of 6.0 bad days.
Now it's possible that having more children to raise might make one crazy -- I have six kids myself and can testify that it ain't easy -- but it might be the case that people with mental health issues might have less control over their fertility. The large family size means the next generation has a greater number of genes linked to mental illness, and over many generations, you get a pretty crazy population.
In pre-modern times, mental illness would work against survival and mating success, but under modern conditions, we see the mentally healthy making every excuse against having kids, while people who simply don't have it together are reproducing like bunnies.
A perfect example of the current mess is a colleague of mine. She is tall, beautiful, athletic, healthy as a horse, and very smart (as least as smart as a progressive can be). She is also nurturing and is exactly the kind of person who should be having kids. But since the planet is going to melt at any moment, she doesn't want children. So instead of having a family, she and her husband help the environment by jetting to exotic places every other weekend. Not good.