...“There are really good studies showing that single people are more likely than married couples to be in touch with friends, neighbors, siblings and parents,” said Bella DePaulo, author of “Singled Out” and a visiting professor of psychology at the University of California, Santa Barbara.

But that doesn’t mean they’ll be single forever. “There are not just more types of families and living arrangements than there used to be,” said Stephanie Coontz, author of the coming book “Intimate Revolutions,” and a social historian at Evergreen State College in Olympia, Wash. “Most people will move through several different types over the course of their lives.”

At the same time, the old-fashioned family plan of stably married parents residing with their children remains a source of considerable power in America — but one that is increasingly seen as out of reach to all but the educated elite.

“We’re seeing a class divide not only between the haves and the have-nots, but between the I do’s and the I do nots,” Dr. Coontz said. Those who are enjoying the perks of a good marriage “wouldn’t stand for any other kind,” she said, while those who would benefit most from marital stability “are the ones least likely to have the resources to sustain it.”...