If you’ve ever been crammed into a poorly ventilated bus in the summer or run errands when it’s 95 degrees with 90 percent humidity, you are probably familiar with how easily tempers can flare in hot weather. And there’s no shortage of experts trying to explain this phenomenon.
A popular 1986 study published in the journal Environment and Behavior attempted to explain the relationship between high temperatures and human interactions by testing motorist reactions to a car stopped at a green light in Phoenix, AZ. The researchers, Douglas T. Kenrick and Steven W. MacFarlane from Arizona State University, measured “interpersonal hostility” by the amount of horn-honking and found that it increased in line with temperatures.
In another 2016 study out of the Johns Hopkins University School of Medicine, maximum daily temperature was the weather factor most linked to a rise in trauma admissions to John Hopkins Hospital and police reports filed with the Baltimore Police Department.
“Of course, heat is not the only factor linked to aggression and violence, but it is an important factor,” writes Brad J. Bushman, Ph.D., professor of communications and psychology at Ohio State University, in an email. “It appears to be a linear function. The greater the number of hot days, the greater the number of violent crimes.” Interestingly, hot weather does not increase nonviolent crimes, according to Bushman.
In the past, scholars have attributed this phenomenon to either the General Aggression Model (which Bushman helped develop) or the Routine Activity Theory. The General Aggression Model suggests higher average temperature make people grouchy and therefore more aggressive. Routine Activity Theory proposes that warm temperatures allow people to be outside their homes more, therefore having more opportunities for social interaction—some of which can go very wrong.
But those studies are incomplete and it’s not just heat waves that ignite tempers, according to a controversial 2016 study Bushman co-authored called CLASH (CLimate Aggression, and Self-Control in Humans), published in the journal Behavioral and Brain Sciences. Bushman and lead author Paul A. M. Van Lange of Vrije Universiteit, Amsterdam, theorized that hot climates (and little variation in seasonal temperature) make people quicker to react aggressively or violently. The reason: It fosters a life strategy that is less concerned about the future (for example, when it comes to planning and planting crops or preparing for winter) and requires less self-control.
And because we can’t talk about hot weather without talking about climate change, Bushman provides a seemingly bleak view for the future: “When people think of the consequences of global warming, they typically think of crops, extreme weather, rising sea levels, etc.,” he says. “They do not even consider the fact that aggression and violence levels also increase.”