15 relationship facts everybody should know before getting married

• Relationships are complicated, and every relationship is different.
• That said, scientists have identified some predictors of success in relationships that are important to consider before getting married.
• For example: getting excited for each other's news is a good thing and moving in to "test" the relationship usually isn't.

Thinking about popping the question?

Before you do, consider the large and growing body of scientific research on relationships: what strengthens and weakens them and what predicts long-term success versus dissolution.

Below, we've put together a list of 15 nontrivial facts about relationships to consider before you hire a wedding planner.

This is an update of an article originally posted by Drake Baer.

SEE ALSO: 10 myths about dating too many people believe

If you wait until you're 23 to commit, you're less likely to get divorced.

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.

The 'in love' phase lasts about a year.

The honeymoon phase doesn't go on forever.

According to a 2005 study by the University of Pavia in Italy, it lasts about a year. After that, levels of a chemical called "nerve growth factor," which is associated with intense romantic feelings, start to fall.

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."

Two people can be compatible — or incompatible — on multiple levels.

Back in the 1950s and '60s, Canadian psychologist Eric Berne introduced a three-tiered model for understanding a person's identity. He found that each of us have three "ego states" operating at once:

• The parent: What you've been taught

• The child: What you have felt

• The adult: What you have learned

When you're in a relationship, you relate on each of those levels:

• The parent: Do you have similar values and beliefs about the world?

• The child: Do you have fun together? Can you be spontaneous? Do you think your partner's hot? Do you like to travel together?

• The adult: Does each person think the other is bright? Are you good at solving problems together?

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.