“I pray thee, good Mercutio, let’s retire; The day is hot, the Capulets abroad, And, if we meet, we shall not ‘scape a brawl, For now, these hot days, is the mad blood stirring.”
— William Shakespeare, Romeo and Juliet, Act 3, Sc 1
One common belief shared by writers, philosophers, and lay people alike is that hot temperatures increase aggression and violence. Research evidence is consistent with this common belief. The evidence from laboratory experiments, field experiments, correlational studies, and archival studies of violent crimes indicates that hotter temperatures are associated with higher levels of aggression and violence. Studies that compare the violence rates of regions that differ in temperature have generally found that hotter regions have higher violent crime rates. Time period studies generally have found higher violence rates in hot years, hot seasons, hot months, and hot days. In one study, for example, we analyzed temperature and crime rate data in the United States for a 45-year period and found that murder and assault rates were higher during hotter years than during cooler years, and were higher during hotter summers than during cooler summers. Nonviolent crimes were not affected by temperature. Field and archival studies have found similar results. For example, in baseball games, the hotter the temperature, the more common it is for the pitcher to hit the batter with a pitched ball (which hurts a lot!).
The belief that hot temperatures are linked to aggressive and violent behavior has even crept into the English language, as indicated by common phrases such as “hot-headed,” “hot-tempered,” “hot under the collar,” and “my blood is boiling.” Indeed, research has shown that even words related to hot temperatures can increase aggressive thoughts and hostile biases. In both experiments, participants completed a scrambled sentences task in which they chose 4 of 5 words to make a grammatically correct sentence. By random assignment, participants completed one of three versions of the task: heat prime, cold prime, or neutral prime. For the heat prime, 6 of the 13 sets contained words related to hot temperature (i.e., boils, burning, hot, roasted, sunburn, sweats). For example, the 5 words “boils, eggs, she, the, bricks” would be constructed to form the 4-word sentence “She boils the eggs.” For the cold prime, 6 of the 13 sets contained words related to cold temperature (i.e., cold, defrosted, freezes, frostbite, frozen, shivers). For example, the 5 words “cold, Judy, felt, very, cleans” would be constructed to form the 4-word sentence “Judy felt very cold.” For the neutral prime, all 13 sets contained words unrelated to temperature. For example, the 5 words “green, the, grass, is, pusher” would be constructed to form the 4-word sentence “The grass is green.” None of the words used in any of the conditions were related to aggression or violence. In Experiment 1, participants completed a word stem completion task in which some word stems (e.g., “k i _ _”) could be completed as either aggressive (e.g., “kill,” “kick”) or nonaggressive (e.g., “kiss,” “kind”) words. In Experiment 2, participants read an essay about a man named Donald, which contained several ambiguous behaviors that could be interpreted as hostile (e.g., “Donald refuses to pay his rent unless his landlord repaints his apartment”). After reading the essay, participants rated Donald on a series of traits, including aggressive ones (e.g., hostile, angry). In both experiment, exposure to hot temperature words, relative to cold temperature and neutral words, increased aggressive thoughts and hostile perceptions. These findings show a strong link in memory between words related to hot temperatures and aggressive thoughts and hostile biases.
One interesting fact about heat is that it has opposite effects on perceived arousal and physiological arousal (e.g., heart rate). When people are hot, they think they are less aroused. They use terms such as “I feel drained,” “I feel sluggish,” and “I’m dragging.” But if you measure their heart rate, it actually increases with increase in temperature. And that can be a recipe for disaster. Researchers have found that arousal caused by one event can sometimes be transferred to something else, thereby increasing one’s reaction to it. Aggression can be increased by this “excitation transfer.” In a provocative situation, arousal deriving from heat can be mistaken for anger and can therefore increase aggression. For example, if a driver cuts you off in traffic, you’re much more likely to honk at them, flip them off, or tailgate them if it’s a hot day rather than a cool day. The arousal from the heat is mislabeled as “anger.”
When people think of the consequences of global warming (the observation that the weather all over the world is getting a little hotter year by year), they focus mainly on the impact of rising temperatures on agricultural crops, flooding, draughts, forest fires, hurricanes, and other natural disasters. However, there is also an impact of global warming on violent crimes. Most global warming experts predict that temperatures will rise between 2°F and 8°F (1.1°C to 4.4°C) by the middle of this century. If temperatures rise 2°F (1.1°C), the number of assaults and murders in the United States is predicted to rise by more than 25,000 each year. If temperatures rise 8°F (4.4°C), the annual predicted rise in assaults and murders is more than 80,000 each year.
In summary, research has consistently shown a link between heat and aggressive and violent behavior. Being aware of the link is important so you do not mislabel heat-induced arousal as anger. Adding air conditioners to environments where tempers might flare, such as prisons, psychiatric wards, schools, homes, and motor vehicles might be a good investment of money. One way to calm hot tempers is to try to stay cool.
 Anderson, C. A., Anderson, K. B., Dorr, N., DeNeve, K. M., & Flanagan, M. (2000). Temperature and aggression. In M. Zanna (Ed.), Advances in experimental social psychology (Vol. 32, pp. 63–133). New York: Academic Press.
 Anderson, C. A., Bushman, B. J., & Groom, R. W. (1997). Hot years and serious and deadly assault: Empirical tests of the heat hypothesis. Journal of Personality and Social Psychology, 73, 1213-1223.
 Reifman, A. S., Larrick, R. P., & Fein, S. (1991). Temper and temperature on the diamond: The heat-aggression relationship in major league baseball. Personality and Social Psychology Bulletin, 17, 580–585.
 DeWall, C, N., & Bushman, B. J. (2009). Hot under the collar in a lukewarm environment: Hot temperature primes increase aggressive thoughts and hostile perceptions. Journal of Experimental Social Psychology,45(4), 1045-1047.
 Anderson, C.A, Deuser, W.E., DeNeve, K. (1995). Hot temperatures, hostile affect, hostile cognition, and arousal: Tests of a general model of affective aggression. Personality and Social Psychology Bulletin, 21, 434-448.
 Zillmann, D. (1979). Hostility and aggression. Hillsdale, NJ: Erlbaum.