A while ago I speculated on why Americans and Europeans perceive temperature so differently. I pointed out that the United States is a much hotter country than many Europeans realize. A recent study has suggested that there is a strong link between heat and violence:
More specifically, for a degree Celsius of temperature increase (1.8 degrees Fahrenheit), Burke says there could be a 20 percent increase in civil conflict in Africa. The impact of warming varies by region, however; some places are more sensitive to small heat increases than others. In the United States, the estimate would be lower: For 1 degree Celsius of warming, he'd expect about a 1 percent increase in interpersonal conflicts, a category that includes crimes like assault and robbery but also road rage and fights at baseball games.
...Richard Larrick, a professor at Duke's Fuqua School of Business, explained in an e-mail the psychological research linking heat with shows of aggression.
"Researchers in social psychology have studied the relationship between temperature and aggression for many decades," Larrick said. That includes studies looking at links between a day's temperature and people engaging in real world behaviors ranging from honking horns to committing violent crimes. "Research in the laboratory," Larrick continues, "allowed for tightly controlled tests to show that changes in temperature directly lead to more aggression." Such research has shown, he notes, that "heat changes the way people feel and think, increasing anger and making thoughts of aggression increase."
It is important to underscore that the temperature-violence relationship is not deterministic. In their meta-analysis, Burke and his colleagues liken the situation to "the rise in car accident rates during rainy days" -- the rain ups the risk of accidents overall, but each accident is still contingent on the individual situation and choices (and mistakes) of the drivers involved.
Similarly, warmer temperatures seem to shift the overall background risk for violent conflict -- but whether someone commits a violent act remains dependent upon the specific circumstances and the individual.