“We know from studies we have done and that others have done that many people are forgoing marriage for economic reasons, and we do see that here, with many cohabitants saying they are not far enough along in their career to get married yet,” she said.

It used to be considered somewhat taboo for a couple to live together if they were not married — hence the term “living in sin” — but those attitudes have changed, researchers said.

A slim majority of Americans, 53 percent, said society would be better off if long-term couples got married. But 69 percent of Americans said it was acceptable to live with a romantic partner even if you have no plans to get married, while 16 percent said it was O.K. only if a couple sees a wedding in their future. A majority also said unmarried couples could raise children just as well as married couples could.

But all this acceptance does not mean there are no troubles in paradise for unmarried couples.

According to the survey, unmarried couples report significantly less satisfaction in their relationships than do married couples, who report higher levels of trust in their partners’ honesty, fidelity and spending habits. It said that 58 percent of married adults said their relationship was “going very well,” compared with 41 percent of unmarried people who live with a partner.

That pattern is true across a broad range of areas: Married people are more likely than unmarried cohabitants to say they are “very satisfied” with the division of household chores (46 percent to 37 percent); with their partner’s communication skills (43 percent to 35 percent); and how well their partner balances work and personal life (43 percent to 35 percent).