“The men who don’t have college degrees are doing so badly in the job market that they don’t seem like good prospects to the women in their lives,” said Isabel Sawhill, co-director of the Center on Children and Families at the Brookings Institution, whose book on the topic, “Generation Unbound: Drifting Into Sex and Parenthood Without Marriage,” comes out Thursday.

Yet she said it‘s not entirely an economic phenomenon; it’s also one about shifting social roles. “I do think it has something to do with the fact that in the professional class, because men are doing very well, they aren’t threatened in any way by a wife that works or is doing very well herself,” she said. “Amongst working-class males, it may be a little more threatening if your wife or girlfriend is earning as much as you are and making new demands as a result.”

And as modern marriages have become more about love than about survival, it has become an indulgence that is easier for well-off people to take advantage of, said Justin Wolfers, an economist who writes for the Upshot and has studied marriage and divorce. The benefits of sharing passions are more likely to accrue to people who have the time and money to invest in them, he said.

Though marriage was once a steppingstone to economic stability, young adults now see financial stability as a prerequisite for marriage. More than a quarter of those who say they want to marry someday say they haven’t yet because they are not financially prepared, according to Pew.

“If you go back a generation or two, couples would literally take the plunge together and build up their finances and nest eggs together,” said Kim Parker, director of social trends research at Pew. “Now it seems to be this attitude among young adults to build up households before they get married.”

In other words, marriage has gone from being a way that people pulled their lives together to something they agree to once they have already done that independently.