The black-and-blue rule of baseball—if your pitcher beans our batter, our pitcher will bean yours—it turns out, is highly dependent on the weather. Richard P. Larrick, a professor at Duke University’s Fuqua School of Business, and his colleagues examined every at bat in every Major League Baseball game since 1952, keeping an eye peeled for retribution pitches. They then calculated that of the roughly 190,000 at bats that occur every season, about 1,550 result in the batter being struck by a pitch. When they overlaid game data with weather data, they discovered that batters have a 27 percent chance of being hit by a retribution pitch in 95 degree Fahrenheit weather, compared with an only 22 percent chance in 55 degree F weather. “We don’t think that heat increases aggression in general,” Larrick says, “but that it increases a special type of aggression: retribution.”

The findings jibe with results from previous studies on the psychological effects of heat, which have shown that people in hotter rooms have a lower threshold for revenge and are more likely to view others’ actions as hostile. That may explain why violent crime increases in the summer months, although researchers have yet to determine whether temperature plays a role or if attacks go up because more people are interacting on the streets. Larrick says that his research could help get to the bottom of the mystery. “Studying baseball is helpful because it removes the confounding variables of real life. It’s controlled,” he says—as long as fans stay in their seats.