Live Science

The married and cohabiting couples were matched on the length of their relationship and relationship satisfaction, Coan said. The key difference seems to be how people think of their relationship.

“When you’re asserting, in the same-sex couples, ‘We’re just living together, we’re only living together’ it means you haven’t really committed,” Coan said. “It means that you’re explicitly maintaining a little bit of emotional distance. You’re not locked in. I think that’s just enough to signal to your brain that you can’t outsource your stress response to your partner.”

In other words, people who hold back on commitment don’t fully trust their partner has their back, the study suggests.

There may be benefits to cohabiting, Coan said — at the very least, it’s cheaper for two adults to live together versus separately, which has its upsides. But the bar for emotional benefits seems to be higher, he said.

However, as the same-sex couples show, the marriage benefit may not be about the legal or religious recognition; rather, thinking of oneself as fully committed is key.

“Marriage is a proxy for trust and reliability,” Coan said.