Hypothesis:
Up until recently, you only needed a so-so amount of interest in settling down and having children, in order to pass on your genes. Some people did, but others didn't, and while this was not entirely genes it wasn't entirely NOT genes either. But, since everyone got more or less pushed into marrying and starting a family anyway, by strong social pressures, there was no strong evolutionary advantage to a strong desire for family.
For about one generation now, the people who WANT kids, and are willing to be half of a committed couple in order to have more of them, are the ones having more kids. Others, either don't have any children, or only have one.
In a couple generations, they will look back at this time and wonder why people back then were so (relatively) uninterested in family.
Just a hypothesis. Could happen relatively quickly, though. Which would mean, in only a few generations.
For about one generation now, the people who WANT kids, and are willing to be half of a committed couple in order to have more of them, are the ones having more kids. Others, either don't have any children, or only have one.
In a couple generations, they will look back at this time and wonder why people back then were so (relatively) uninterested in family.
Just a hypothesis. Could happen relatively quickly, though. Which would mean, in only a few generations.