Why Does Violent Crime Go Up When It's Hot?

Calm down there, sir, you're mad at weather. Image credit: Jan Mika / Shutterstock.com

Are you one of those people that gets cranky or more aggressive when you're hot? Well, the good news is, it's not just you. The bad news is, it's an actual phenomenon and when the temperature goes up, violent crime does too.

It's difficult to quantify crankiness, but there have been numerous attempts to take a look at the relationship between the temperature and levels of aggression in humans. In one experiment, 64 students were given personal evaluations from a stranger and then offered an opportunity to give that person an electric shock. Some were given positive feedback, and some negative, while the room they were in was either uncomfortably hot or uncomfortably cold. The team found that as the temperature went up, the students would become more aggressive and choose to electrocute their evaluator (don't worry, they were pretending), until it got hotter still, at which point they would become less aggressive again.

As interesting as this is, it hardly beats real-world data, but fortunately there's quite a lot of that too.

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One study in Los Angeles looked at temperatures in the city in comparison to the crime rate, finding that "overall crime increases by 2.2% and violent crime by 5.7% on days with maximum daily temperatures above 85 degrees Fahrenheit (29.4° C) compared to days below that threshold". The trend can be seen in London as well as the US, where the murder rate rises by about 3 percent when it's hot

In Mexico, where a much larger study looked at court data from 1997–2012, compared to data from the Mexican National Weather Service. "The research shows that an increase of 1°C linearly increases the accusation rate of all types of crimes by 1.3%," the team wrote in their study. "The crime accusation rate is expected to be one third higher during a hot day (>32°C) than during a cold day (<10°C)."

Where the correlation has been found, it has mainly been between violent crime and temperature, with non-violent crimes remaining relatively unaffected. Nobody, it appears, gets hot and fancies a spot of fraud.

So, what's causing it? Well, it probably can't be put down to one single factor, and results are contradictory. The study of Mexico courts put it down to a change in behavior of those who ended up committing offenses on hot days: largely that the offenders had been drinking, and were outside more when the weather was hot and the evenings mild. 

However, the hotter temperatures in the student study did not make them consume more alcohol, nor wait until the evening to shock their evaluator. Nor did it make baseball pitchers hit more batters with their pitches (possibly intentionally) or NFL players give away aggressive penalties, both of which appear to be affected by the temperature in fairly sizable studies. We can also assume that drivers without air conditioners aren't honking more because they have been pounding more beers before they drive.

Other explanations for this include hotter temperatures making us uncomfortable, and getting worse sleep at night makes us more stressed.

“Heat makes us feel physically uncomfortable," as Professor of Psychology at Knox College puts it in Psychology Today. "We’re more inclined to aggressive thoughts and to interpret things in a negative way”. 

[H/T: Psychology Today]

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