Get on the same page ahead of time about whether you're looking to get married someday or are happy to keep things how they are indefinitely.

According to Tatkin, if one of your goals is to treat cohabitation like a "trial session" to see how you'd fit as a married couple, you should think about how you're going to define success.

A 2014 University of North Carolina at Greensboro study found that American women who cohabitate or get married at age 18 have a 60% divorce rate, but women who wait until 23 to make either of those commitments have a divorce rate around 30%.

"The longer couples waited to make that first serious commitment [cohabitation or marriage], the better their chances for marital success," The Atlantic reported. According to a 2005 study by the University of Pavia in Italy, it lasts about a year. • The adult: Does each person think the other is bright? While having symmetry across all three is ideal, people often get together to "balance each other." For instance, one may be nurturing and the other playful.

This is especially true of couples who are already at odds over whether or not to marry.

Living with somebody opposed to marriage won't make them change their mind, and moving in with somebody who expects marriage won't pacify their need for commitment.

Think back to that time your dog was rushed to the emergency vet at 3 A. or your car got stranded in a snowy ditch for four hours.

Were you able to figure things out together without turning on each other too quickly, or at least able to work around each other's sore spots?

Helen Fisher, a psychologist and relationship expert, told Business Insider that it's unclear when exactly the "in love" feeling starts to fade, but it does so "for good evolutionary reasons," she said, because "it's very metabolically expensive to spend an awful lot of time just focusing on just one person in that high-anxiety state." Back in the 1950s and '60s, Canadian psychologist Eric Berne introduced a three-tiered model for understanding a person's identity. Controlling for premarital happiness, the study concluded that marriage leads to increased well-being — and it does so much more for those who have a close friendship with their spouses. I mean, you adore them — but they constantly leave hair in the shower.They tell the worst jokes — but they're always there to comfort you after a hard day.How are you supposed to lasso that magical unicorn before it flies away?With Valentines Day coming upon us, I’ve updated this earlier post to make it fifteen needed questions you need to ask you and your dating relationship.