From Oscar the Grouch to political commentators, some people seem to thrive on getting angry. But there’s a serious downside to nursing a grudge. Being a grump may actually be bad for your health.

According to Yale University researchers, how a heart handles anger helps predict who is at risk for a life-threatening irregular heartbeat. In fact, just thinking about a maddening event may send a jolt of electricity through the hearts of vulnerable patients, which could be a precursor to serious cardiac problems.

While doctors have long believed that hostile emotions such as anger or depression may contribute to heart disease risk, the Yale study went further and showed a link between angry thoughts and changes in the E-K-Gs of heart patients.

To test the theory, researchers gave E-K-Gs to sixty-two patients who had defibrillators implanted in their chests because of pre-existing heart disease. When asked to describe a moment that angered them, some experienced alterations in their E-K-Gs similar to those found when doctors test for irregular heartbeats on a treadmill. To put it another way, the emotional stress was creating the same effect as physical stress.

Those who showed a big anger surge were ten times more likely to need a lifesaving shock from their defibrillators over the next three years compared with those who remained calm.

While there’s no proof anger has the same effect on healthy people, it would probably be good for your heart … as well as your social life… to stay positive.