Surprisingly, though, the same effect held true for same-sex couples who considered themselves married — even though they were not legally different than the same-sex couples who considered themselves to be cohabiting.
"There's a pretty strong and predictable regulation effect in the married couples and no regulation at all in the cohabiting couples," Coan said.
"When you're asserting, in the same-sex couples, ' We're living together' it means you haven't really committed," Coan said.
"It means that you're explicitly maintaining a little bit of emotional distance. 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.
But there were only 16 participants, all married women.
"We really needed to replicate the findings," he said.
The effect occurred in the hypothalamus, an almond-sized structure buried deep in the brain that helps regulate some of the body's responses to stress, including increased blood pressure.