Tyler Cowen cites research from Rachel Sheffield and Scott Winship that shows this does not result from economic problems or from a lack of "marriageable men", for example because so many are in prison. The main reason people aren't marrying is that they don't feel the need to:
– Rather than economic problems causing the increase in family instability, we argue that rising affluence is a better explanation. Our story is about declining co-dependence, increasing individualism and self-fulfillment, technological advances, expanded opportunities, and the loosening of moral constraints. We discuss the paradox that associational and family life has been more resilient among the more affluent. It’s an argument we advance admittedly speculatively, but it has the virtue of being a consistent explanation for broader associational declines too. We hope it inspires research hypotheses that will garner the kind of attention that marriageability has received.
– The explanation section closes with a look at whether the expansion of the federal safety net has affected family instability. We acknowledge that the research on select safety net program generosity doesn’t really support a link. But we also show that focusing on this or that program (typically AFDC or TANF) misses the forest. We present new estimates showing that the increase in safety net generosity has been on the same order of magnitude as the increase in nonmarital birth rates.
Obviously this study isn't the last word on the issue but I think this is an important point. Marriage used to be very much about economic security. In peasant societies a family was an economic unit as much as a biological or emotional one, and much of this lingered into recent times. The modern model, developed by the Victorians, saw the family as a secure refuge from a tumultuous world. Now that single people can easily support themselves and draw on a raft of professional services for everything from laundry to pet sitting, there is simply less need to have partners. I think this economic reality has slowly fed back into the culture, leading to less of a felt need to marry and settle down, leading to later marriage and smaller families.
I don't see anything wrong with this; after all many people entered or stayed in bad marriages because they felt they had no choice, and getting away from that is great. But living alone has its own dark side.