SINCE the 1990s, we’ve seen two broad social changes that few observers would have expected to happen together.

First, youth culture has become less violent, less promiscuous and more responsible. American childhood is safer than ever before. Teenagers drink and smoke less than previous generations. The millennial generation has fewer sexual partners than its parents, and the teen birthrate has traced a two-decade decline. Violent crime — a young person’s temptation — fell for 25 years before the recent post-Ferguson homicide spike. Young people are half as likely to have been in a fight than a generation ago. Teen suicides, binge drinking, hard drug use — all are down.

But over the same period, adulthood has become less responsible, less obviously adult. For the first time in over a century, more 20-somethings live with their parents than in any other arrangement. The marriage rate is way down, and despite a high out-of-wedlock birthrate American fertility just hit an all-time low. More and more prime-age workers are dropping out of the work force — men especially, and younger men more so than older men, though female work force participation has dipped as well.

You can tell different stories that synthesize these trends: strictly economic ones about the impact of the Great Recession, critical ones about the infantilizing effects of helicopter parenting, upbeat ones about how young people are forging new life paths.