The No. 1 thing we 'never ever' do to have a successful relationship: 2 married psychologists who studied 40,000 couples

As psychologists, we've studied more than 40,000 couples about to begin couples therapy.

We've also been happily married to each other for 35 years, so we know a thing or two about how to build a successful, long-lasting relationship. But that doesn't mean we don't make mistakes. We argue, we get frustrated, we snap at each other. We're human.

Still, there is one thing we've learned to never ever do: fight when we are emotionally flooded.

Emotional flooding is when you feel psychologically and physically overwhelmed. It often happens when our body senses danger during a conflict, and it prevents us from having productive conversations.

We've found that it's a common pattern in unhappy relationships.

Everyone has their own built-in meter that measures how much negativity and fear they can take in at a single moment. When it becomes too much, the nervous system goes into overdrive and we essentially enter "fight or flight" mode.

Here are some signs of emotional flooding:

These behaviors can harm both your partner's trust in you and the foundations of your relationship. You may stop communicating altogether and start to resent each other.

It's hard to stop yourself from acting out when you're emotionally flooded. You might say things you don't mean. But being mindful of your emotions and mental energy can prevent you from going too far.

When we realize we are flooded during an argument, we let each other know: "I'm feeling overwhelmed right now and need some time to myself."

Then we walk into separate rooms and do an activity that distracts or calms us down. This is important: We don't let ourselves stew in how upset we are. Instead, we might do a quick meditation or yoga session, read an article, or play a game on our phones. 

Then we continue the conversation at an agreed upon time — when we're feeling better. This exercise helps us remember that the end goal isn't for one of us to "win" or have the last word. The point is to work through challenges together as a team.

Disagreements are normal and, in some cases, healthy. Even when conflict goes south, a couple's capacity to course correct and repair their interaction requires knowing when to take a step back.

It's about being able to pause in the middle of a fight and make a peace offering. Remember, successful couples don't fight less than other couples — they fight better.

Dr. John Gottman and Dr. Julie Schwartz Gottman are the co-founders of The Gottman Institute and Love Lab. Married for over 35 years, the two psychologists are world-renowned for their work on relationship stability and divorce prediction. They are also the co-authors of "The Love Prescription: Seven Days to More Intimacy, Connection, and Joy" and "10 Principles for Doing Effective Couples Therapy." Follow them on Instagram and Twitter.

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