Psychologist and eHarmony CEO Neil Clark Warren says these are the 5 most important factors for a lasting relationship

With Valentine's Day right around the corner, many couples may be asking themselves the question: How will I know if this one's going to last?

While there's no exact formula, we sat down with the CEO of the dating website eHarmony, Neil Clark Warren, and asked him what he thinks are the most important factors in successful relationships.

Before founding eHarmony, Warren spent nearly 40 years as a psychologist specializing in marriage counseling. In his experience, whether couples stay together or split apart depends on whether they have what he calls "broad-based compatibility."

eHarmony determines whether couples have this based on 29 aspects of personality, such as character, sociability, and intellect.

According to Warren, these are the top five things successful couples share:

1. Both partners are emotionally healthy

One of the most important things for a strong relationship, Warren said, is that both partners are mentally healthy, and do not have disorders such as bipolar disorder or severe depression. In a 2014 study of the spouses of 60 patients with depression and 65 patients with bipolar disorder, members of both groups reported high levels of burden. Spouses of bipolar patients reported much lower satisfaction with their sex lives, too.

2. And have what he calls "healthy characters"

In addition to emotional health, it's also vital that both partners have healthy characters - and honesty is especially important, Warren said.

A 2015 study of 3,459 Korean adults that included 542 pairs of identical twins and 122 pairs of fraternal twins surveyed people about their temperament and character. They found that spouses reported character traits that were more similar to each other than to those of their children, parents, or siblings - and the similarity increased the longer they had been married.

3. They have similar levels of intelligence

According to Warren, the most compatible couples tend to be within one standard deviation of each on the Wechsler Adult Intelligence Scale, one of the most commonly used tests of human intellect. If one partner is significantly smarter than the other, it tends not to work, he said.

It also turns out that in general, women are attracted to intelligent men. Psychologist Geoffrey Miller argues in "The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature" that the human brain, much like a peacock's tail, has evolved through generations of sexual selection in which women chose to have children with brainier men. But when it comes to men, research suggests they like the idea of dating a smarter woman, but don't necessarily want to in practice.

4. They share a sense of humor

It may seem intuitive, but having a sense of humor is incredibly important in a relationship. Humor acts as a
"social lubricant" that keeps a couple's interactions running smoothly, Warren told Business Insider.

Studies support this, but they make an important distinction between "wit" (spontaneous jokes that are genuinely funny and require intelligence) and preplanned jokes and one-liners (which aren't very funny or intelligent).

5. They are kind to each other

This was the number-one thing that couples report as being important for a relationship, Warren said. And there's research to back this up: A 2015 study of more than 2,000 couples in five cultures found that kindness, in addition to sex, finances, division of labor, and raising children, was a key factor in whether a marriage was successful.

Of course, many other factors contribute to a healthy, lasting relationship. But it's probably safe to say that compatibility on these five are a good starting point.