4 key pieces of relationship advice from married psychologists who have worked with 40,000 couples

Renowned psychologists John and Julie Gottman have interviewed 40,000 couples in quest to find out what makes love last. Through their research at The Gottman Institute, they've learned how successful relationships survive conflict and express affection.

Here are four things that people in long-lasting partnerships do:

1. Fight to understand.

More than two-thirds, 69%, of conflicts in relationships are perpetual, meaning they never go away, according to data from The Gottman Institute.

Through their research they learned that just because a fight is reoccurring doesn't mean it has to be detrimental. In fact, you can handle any relationship conflict if you know how to fight right.

"So when we think about fighting right, whether talking about a perpetual problem or a solvable problem, what is the biggest mistake that the disasters of relationships make?" John Gottman said at a TED Talk in April. "The answer is that they fight to win, which means somebody has to lose. What do the masters do, instead? They fight to understand."

If you're fighting to prove a point and not to understand the other person's perspective, it's unlikely you'll be able to come to a resolution. Plus, the other person will be painted as the enemy.

Instead, ask questions that probe into why your partner came to their conclusion and what in their past might have led them there. This injects some empathy into the fight and allows both sides to express themselves without fear of judgment.

2. Make repairs.

In an interview with psychotherapist and bestselling author Esther Perel, The Gottmans said one commonality most successful couples had was their ability to do "repairs."

"They made repairs when their partner didn't receive a bid for connection," Julie Gottman told Perel. "They made repairs if they said the wrong thing, [if] they blurted out the wrong thing."

Making a repair doesn't mean buying flowers or treating your partner to an expensive dinner. It can be a question or a comment which acknowledges that you still see your partner as a person, not a villain.

John Gottman gives an example from one of his clients who just fought with his partner: "One guy said, 'Well now that we've destroyed each other's personalities, how about a piece of cheesecake.'"

3. Express positivity.

One of the Gottman's most concrete finding is that in successful couples, the ratio of positive to negative interactions during a conflict is five to one.

They discovered this during a longitudinal study during which they asked couples to try to resolve a disagreement in 15 minutes. They recorded the conflict, watched the tapes and categorized each interaction as positive or negative.

Positive interactions included a smile, touching the other person's hand, or simply saying "I understand." Negative interactions were insulting or blaming the other person.

"We watched couples, logged the data, then released them back into the wild," they wrote in their book "The Love Prescription: 7 Days to More Intimacy, Connection, and Joy."

"Six years later we followed up. And lo and behold: it was the couples who had maintained at least a five-to-one ratio (or more!) during conflict who were still happily together, still feeling the love."

4. Say 'thank you.'

There is a roster of characters in your life that you probably say "thank you" to regularly — your barista, your colleagues, a stranger who held the door open for you. But you might not be saying it to your partner all that much.

"A thriving relationship requires an enthusiastic culture of appreciation, where we're as good at noticing the things our partners are doing right as we are at noticing what they're doing wrong," the Gottmans wrote for CNBC Make It.

It might be the agreement that you do the dishes and your partner takes out the trash. Still, they would probably like to hear some appreciation for doing the chore. And don't just leave it at the two words, the Gottmans say. Add why you're grateful.

For example, you can say "Thanks so much for taking out the trash every night. I love waking up to a clean kitchen and that is a huge part of it and I really appreciate it."

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