Unmarried couples who live together or have kids together are more likely to be in relationships where one partner is much less committed, compared to couples who don’t live together or are child-free, according to a recent study.

These couples are stuck in what academics poetically like to call “asymmetrically committed relationships (ACRs).” These are relationships in which the level of investment is lopsided; one partner is much more into being a couple than the other. Think: Kelly and Ryan on The Office. Carrie and Aidan on Sex and the City. Or just listen to almost any country or R&B songs. This type of couple is not new (hello, Jacob and Leah) and they have been studied by psychologists for decades, but some researchers have become newly interested in the dynamic as marriage rates decline.

The authors of the new study, which was published in the Journal of Social and Personal Relationships in November, say this type of relationship is on the rise, because the traditional roadblocks that would have prevented couples like this from lasting very long are becoming less obstructive.

Researchers at the University of Denver followed more than 300 unmarried heterosexual couples between ages 18 and 34 for about two years. About 35% of those couples were deemed by the researchers to be in a relationship in which one partner was substantially more committed than the other—the dreaded ACR.

In two-thirds of the cases, it will surprise nobody to read, it was the guy who was the less enthusiastic partner. However, in the third of relationships where the woman was the tepid participant, a breakup was much more likely within two years. More than half of those relationships ended, compared to fewer than 30% of the couples who had an apathetic guy or 34% of relationships where both had the same amount of commitment.

Interestingly, the more committed partners in the asymmetrical couples were even more devoted to making the relationship work than partners whose level of interest was about equal. They were making up for their lovers’ low levels of interest. This inequality often has dire results: Both committed and less committed partners report more fighting and assaults within the relationship. “This finding is noteworthy,” the study says, “given that high levels of commitment usually inhibit conflict and aggression.”

Why do living together and having kids correlate with unequal amounts of commitment? Scott Stanley, a research professor in the psychology department at the University of Denver, who is one of the authors of the study, says it’s because those relationships should actually have broken up already, but are being kept together because it’s much more difficult to split when you have to move out or leave a child.

“There are strong reasons to believe that [there are more ACRs than before], because it’s so much easier to transition into situations where you have made it harder to break up before you know as much as you may end up wishing you knew about your partner and your partner’s commitment to you,” Stanley says.

He’s also coined a term for the way couples often move in together as a phase of their dating life, rather than as a precursor to marriage: cohabidating. “Some of those couples will get stuck together,” he says. “Some will have a child. Some will build up a lot of inertia making it harder to part ways when the relationship was not very well vetted in the first place.”

Stanley also believes that the modern tendency of marrying much later and after many more dating/having a fling/living together experiences is affecting people’s long-term relationships. “People are spending more and more time with consequential relationship experiences prior to when they were ready to settle down,” he says. “A lot of that stuff in those years (like the 20s) isn’t nothing. It’s stuff that for some—though not all, by any means—will have effects that last.” Past relationships, particularly those in which couples live together or have a child, will have an impact on future relationships, so many couples are not starting their time together with a blank slate.

In other words, Stanley says, “Not everything stays in Vegas.”

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