Bouts of anger, frustration, and even resentment are inevitable for long-term couples, but certain researchers believe one emotion is more damaging to relationships than others.

In the 1970s, a psychology researcher named Paul Ekman identified six universal facial expressions that all humans share; fear, anger, surprise, happiness, sadness, and disgust. In a study published in 1972, Ekman noted that all humans — from Americans to the most isolated tribes in Papua New Guinea — shared these basic emotions. Later, he added one more: Contempt, which can be defined as something of a mixture of anger and disgust.

In the 1980’s and 90’s, another psychologist was studying what makes marriages work or end in divorce. John Gottman started the “Love Lab,” in which he listened to thousands of hours of couples interacting and talking. Contempt was listed as one of the four destructive behaviors that can destroy a relationship, along with defensiveness, silent treatment, and criticism. But Gottman believed that contempt was the worst of all. It was, in fact, the number one predictor that a relationship was heading for divorce.

Contempt is an emotion, but it’s also a facial expression — identified in particular by an asymmetrical smile, perhaps what’s better called a sneer or smirk (Martin Shkreli, anyone?) One corner of the lip is raised higher than the other, and the emotion is characterized by a sense of superiority or looking down on someone. A look of contempt can communicate many negative things to a person; it belittles and degrades them, and makes them feel unworthy.

“You would think that criticism would be the worst… but if I speak from a superior plane, that’s far more damaging, and contempt is any statement made from a higher level,” Gottman said in an interview for Malcom Gladwell’s book, Blink. “A lot of the time it’s an insult: ‘You are a bitch. You’re scum.’ It’s trying to put that person on a lower plane than you. It’s hierarchical.”

According to the Relate Institute, contempt doesn’t happen immediately; it’s often the result of a buildup of problems that were never resolved. “Often, a contemptuous interaction is like a volcano erupting — there is a lot of build up over a long period of time that finally explodes,” the website states. “In relationships, this means that maybe there are issues that have been bothering you for a long time, but you haven’t said anything to your partner about it.”

Interestingly, Gottman also found that couples who were contemptuous of one another were also more vulnerable to catching infectious illnesses like colds or the flu, according to The Gottman Institute. The stress of chronic negative thoughts about a partner may actually weaken your immune system, adding to the spiral of toxicity of an unhealthy relationship. Plus, the more contempt you harbor, the more likely you will act aggressively not only towards your partner — but towards others as well, according to recent research.

Frequent, consistent contempt may spell the end of a relationship; occasional contempt during the heat of the moment won’t always mean you’re going to break up. It all depends on how you make up after an argument that makes the difference, Gottman found in his research. The way to deal with contempt is to talk about it — talk about the problems instead of allowing them to fester, and avoiding responding to contempt with contempt.