I did not read the article, so this is just a hot-take. My hunch is that to have children, one must already be happier than others to even consider it. Here, lots of childless couples carry some heavy childhood trauma, making them, on average, less happy, not wanting to pass on their burden and genes to someone else, and thus, less likely to have children. Basically, it's a chicken-and-egg problem. Happy coz of no kids, or no kids coz of not happy, that is the question. Just a hunch though.