The link between violence and hot weather is so intuitive that it's embedded in our language: Hotheads lose tempers that flare, anger simmers and comes to a boil, and eventually we cool down.

So what does science have to say? Do tempers truly soar with temperature? The answer, appropriately enough for these triple-digit days, is hazy and hotly contested.

To be sure, extensive literature exists on hot weather and violence, stretching from poorly controlled regional studies in the late 19th and early 20th centuries – oh, those hot-blooded southerners! – to more sophisticated modern analyses. This doesn't just apply to the United States, but countries like England and Wales and New Zealand.

But whether weather is cause or coincidence is difficult to determine.

Perhaps the most detailed studies, led by psychologists Ellen Cohn and James Rotton of Florida State University, involved violent crime over a two-year period in Minneapolis, Minnesota. Cohn and Rotton classified assaults according to time of day, day of week, and month and temperature. They ultimately concluded that violence rose with temperature, but only to a point.

Around 80 degrees Fahrenheit, assault rates started to fall, a trend that dovetailed with a hypothetical explanation for heat-induced violence in which being uncomfortable provokes competing tendencies of both aggression and escape. At low to moderate levels of discomfort, people lash out, but at high levels they just want to flee.

But the results also fit with a sociological rather than psychological explanation. According to so-called Routine Activities Theory, many forms of violent crime are functions of social opportunity, and increase when more people spend more time outside. When it becomes so hot that people retreat inside, crime falls. Cohn and Rotton supported this explanation.

Cohn and Rotton's interpretations of the numbers, however, were contested by Iowa State University psychologist Craig Anderson, who felt they hadn't fully accounted for time-of-day effects. His own take on the data (.pdf) produced a linear relationship between heat and violence, with assault rates peaking at the highest temperatures.

Anderson's model of one psychological explanation for heat-induced aggression. Journal of Personality and Social Psychology

A straight-line relationship supports various psychological and physiological processes.

In hot weather, the body exhibits changes – increased heart rate, blood circulation and sweating, and metabolic changes – associated with sympathetic nervous system activity, which in turn is linked to fight-or-flight responses. Hot weather also increases testosterone production, tilting that equation towards fight.

Anderson warns that physiological explanations are still very speculative, but thinks there's evidence that heat fuels anger through a variety of psychological effects. Physical discomfort is usually associated with negative, painful memories, so that hot weather could prime people to react badly.

Studies show that in hot weather people are more likely to interpret neutral signals as signs of hostility and less likely to condemn violence. Overheated people simply have trouble thinking straight.

But even as the true nature of links between violence and hot weather remain uncertain, one 1976 study stands out in its simplicity and practicality: Angry people can be soothed with a cold drink.

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*Image: Jeffrey Smith/Flickr

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