Are heatwaves a hotbed for crime?

NY CRIME

It was likely no coincidence New York’s most violent week of the year occurred as mid-Atlantic US faced scorching temperatures

You can’t ignore the headlines. Violent crime has been up in New York in recent weeks. What’s causing the increase?

One interesting theory that has popped up is that the crime wave is a consequence of the hot weather. But does hot weather actually cause increases in crime?

A Wall Street Journal study of New York City weather patterns from 2002 to 2009 apparently found there was no increase in crime in the summer as opposed to other times of the year. That study, however, is in a distinct minority.

Almost all the established academic literature that examines not just months, but the temperatures within those months has determined that there is a high correlation between increased temperature and elevated crime levels.

Why does hot weather contribute to rising crime rates?

One theory holds that people are more easily agitated in the heat because adrenaline and testosterone levels rise in the warmer temperatures. If higher temperatures were causing greater crime rates, then we should see crime incidences peak when temperatures are at their highest.

A second theory is that more crime is committed when more people gather in public. During the summer, people – including, say, troubled teenagers who might otherwise be in school – spend more time outside, creating more opportunity for interactions of all sorts, including criminal behavior.

If the community gathering hypothesis were right, then we should witness a decline as temperatures become very high because fewer people gather outside at extreme temperatures.

Ellen Cohn and James Rotton studied two years of Minnesota weather and found that once you got above 75-80F, there was a rapid decrease in crime. John Hipp et al also provide considerable support for the community hypothesis. They looked at cities across the United States in the early 1990s and discovered that crime rates go up during the summer regardless of the relative temperature increase to winter temperatures (that is, comfort levels are not the likely cause). They also determined that places with more public gathering places were more susceptible to crime in the summer months.

The Cohn and Rotton study, however, has one fatal flaw: it doesn’t control for time of day. Higher temperatures tend to occur during the day, but crime tends to happen at night. If you control for time of day and utilized the Cohn and Rotton data as Brad Bushman, Morgan Wange, and Chris Anderson did (pdf), you see that crime goes up significantly during warmer nights. During warm days, there is also an increase in the crime rate, but it plateaus once the mercury spikes past 75F.

John Simister has consistently shown that temperature rises are associated with crime increases in every city across the world, regardless of how hot they are. Summer months when the temperature had been warmer than average have tended to show greater crime levels than summer months that had below-average temperatures.

One interesting study of Major League Baseball batsmen who’d been hit by pitches also offers support for the anger theory. It turns out that more batters are “beaned” by the pitcher when the temperature goes above 90F (well above the Cohn/Rotton breaking point).

Still, one cannot dismiss the social (or community) hypothesis. Beyond the studies listed, a separate Cohn and Rotton investigation looked at environments that had access to climate controls such as air conditioning and those that did not. They found that crime rates in Dallas rose with temperature before dropping off in non-climate controlled areas, while no such drop occurred in climate-controlled places.

However, even if the 75-80F cut-off is incorrect, that doesn’t mean there might not be a higher threshold point at which crime rates will drop off. A study of Columbus, Ohio weather in 2007, for example, found crime rates rose with high temperatures for a given date, but fell off once the thermometer registered 85F or above.

So we have a lot of suggestive evidence, but much of it somewhat contradictory. We don’t have enough consistent information to be certain that either the aggression hypothesis or the community theory is correct. My hunch is that they probably both are: there is going to be a point where fewer people are outside and so crime drops; and it seems plausible that people are more irascible when subjected to extremely hot outside temperatures.

Both theories agree, though, that crime waves do generally track heat waves. That could be quite troublesome for the simple reason that if climate change is causing higher average temperatures for most of us, then we should expect the crime rate to rise in coming decades also.

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