Scientists may have just discovered the true law of attraction. 

By Grace Elkus
Updated July 15, 2015

Whether we see it on the big screen or witness it in real life, the concept “opposites attract” is a fascinating one. But does the expression hold true?

According to a study conducted by Northwestern University, it might depend on the length of time partners know each other before becoming romantically involved. The results showed that couples who began dating soon after meeting were more similar in attractiveness than those who started off as friends—and this isn’t just a coincidence. Physical attractiveness is a more important factor to these couples, whereas it becomes less relevant for friends-first couples. The findings are published in Psychological Science, a journal of the Association for Psychological Science.

Data was collected from 167 couples—67 dating and 100 married. The length of relationship ranged from three months to 53 years, with the average length being eight years and eight months. The couples were asked to talk on camera about how they had changed over the course of their relationship, and independent coders used a -3 to 3 rating scale to indicate the physical attractiveness of each partner.

The results supported the researchers’ hypothesis—the longer a couple had known each other before dating, the larger the attractiveness gap. The results also show that length of acquaintance has the ability to shift the dynamics of sexual competition, which says that a person’s success in the dating “market” is based on desirability. “Having the time to interact with others in diverse settings affords more opportunities to form unique impressions that go beyond one’s initial snap judgments,” Lucy Hunt, lead researcher of the study, said in a statement.

The level of attractiveness didn’t have an effect on relationship satisfaction—so couples in both categories can breathe a sigh of relief.