New research shows - for the first time - that younger adults are more likely to have shared a home with a partner than a spouse, but that cohabitation doesn't deliver the same levels of happiness, trust and well-being that marriage can bring.

Some 59% of those 18 to 44 have had live-in partner without being married, according to a study by the Pew Research Center, compared with 50% who've walked down the aisle. That's a reversal from as recently as 2002, when more Americans in that age bracket had experienced marriage.

But while cohabitation is on the rise, data from Pew and other sources continue to show that married Americans enjoy greater overall happiness, as well as greater satisfaction with their relationships. The marriage happiness premium extends to nearly every aspect of a couple's relationship, with one notable exception: their sex lives.

Social scientists have known for some time that married people tend to be happier than their single counterparts. But the recent rise of cohabitation without marriage has complicated that picture somewhat, with modern relationships often involving a lot more ambiguity and nuance.

In recent years, the General Social Survey, a long-running nationally representative survey of American adults, began explicitly asking respondents about their current relationship status: are you married, cohabiting, or neither? The responses show a sort of relationship-happiness gradient, with married people reporting the most overall happiness, cohabitating partners reporting somewhat less happiness, and singletons were the least satisfied of all.

The Pew data offer additional clues as to what's driving these numbers. For starters, married and cohabiting couples give different reasons for why they chose their current relationship setup: married people are more likely to cite love and an eventual desire to have children, while live-in partners are more likely to note practical considerations, like convenience and finances.

These numbers dovetail somewhat with a study published earlier this year in the journal Demography. It found that happiness differences between married and cohabiting couples aren't necessarily because marriage causes people to become more happy. Rather, the authors write that "cohabitation is a symptom of economic and emotional strain."

This may particularly be the case in the United States, where health care costs are high and the social safety net is porous. Couples deciding to move in together in order to reduce their health insurance costs or save money on monthly expenses may be facing greater economic challenges in life than those who are able to prioritize things like commitment and children in their search for a spouse.

The Pew data show, for instance, that the chief reason cohabiting partners offer for not being married is a lack of financial readiness.

Across a battery of other questions, the Pew data show that married couples enjoy greater levels of satisfaction with their relationships. Married people are more likely to say they're happy with their partner's parenting techniques, the division of household chores, their partner's work/life balance, and the level of communication within the relationship.

But the married and cohabiting are on par in one realm: sex. Similar percentages of each report being very satisfied with their sex life. Among married people in particular, women report greater levels of satisfaction with their sex lives than men. This gap does not exist among cohabiting couples.

Similar patterns exist among measures of relationship trust, with married adults more likely to report trust in their partner to be faithful, act in their best interests, tell the truth and handle money responsibly.

Overall, the data from Pew and elsewhere show that from a life satisfaction perspective, marriage is typically the best choice for the couples who are able to make it. But it suggests that financial difficulties are a key reason why many couples are choosing to cohabitate rather than get married.

This article was written by Christopher Ingraham, a reporter for The Washington Post.