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Why higher temperatures lead to more crime

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Every summer, the same alarming trend occurs in American cities: there are more shootings and violence compared to the months before.

The trend is played up by local media outlets every year, with reports of shootings and homicides occurring at what seems like an alarming rate. This kind of reporting can then frighten locals, who may fear their city is taking a turn for the worse.

But the cause isn't necessarily that the social fabric of America is breaking down. There's a much simpler explanation: warmer weather leads to more violence, and some months and years, including 2015, appear to be more violent even as crime trends down in the long term.

Warmer weather and seasons lead to more crime

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It's a long-established fact of crime: it tends to happen more when it's warmer out.

There are two big explanations for why this happens. One potential reason is that the heat itself can make people more aggressive. The other is that people are more likely to be out when it's warm — and therefore more likely to be perpetrators or victims of crime.

The empirical evidence, summarized by researchers at University of North Carolina Chapel Hill and North Carolina State University, has mostly backed the latter explanation: people are more likely to commit or be struck by crime just because the warmer seasons put them outside more — although the North Carolina researchers did find some evidence that the heat by itself could make people more aggressive and lead to more violent crime.

"When it's warmer and when people want to go out more, they're putting themselves in places in which they're more likely to be victimized," John Roman, a senior fellow at the Urban Institute's Justice Policy Center, said.

It's not just the weather. Other events during the summer, like school breaks, can put more youth in the streets. And since young people are the main perpetrators of crime, summer break means there are more potential criminals in the streets with free time during the summer.

"There are seasonal effects on homicide and other crimes," Richard Rosenfeld, a University of Missouri St. Louis professor, wrote in an email, "but they are as likely to reflect seasonal changes such as school closings as changes in temperature."

The weather-driven increase in crime can be seen in, for example, Baltimore's shooting statistics: in 2014, shootings dropped during the colder months and rose during the warmer ones, with July taking the lead as the most violent month.

So when looking at any recent increase in violence, it's important to keep the weather and seasons in mind.

Crime fluctuates from month to month and year to year

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Beyond the weather, crime can also fluctuate from month to month and year to year for other reasons, some of which researchers may not figure out until years later. In 2005 and 2006, violent crime mysteriously ticked up before continuing its long-term decline. And in 2015, it seems like violent crime is rising in cities all over the country — with reports of more violence coming out of New York City, Baltimore, Chicago, St. Louis, New Orleans, Dallas, and Cincinnati.

This defies the long-term trends that show violent crime trending down in the US. But Roman argued that there are going to be some years with upticks while a majority of years show a fall — it's simply unlikely that every single year will see a drop.

What's behind this year's uptick? Roman suggested that an improving economy could be pushing people to go out more, exposing them to more crime. Or, he said, it could just be that with more people moving to cities, there are more potential victims of crime. "We're talking a lot about the levels of violence, but not the rates of violence," Roman said. (This wouldn't apply to cities like Baltimore and St. Loluis that have seen their populations steadily decline over the past several decades.)

Another possibility is that we're simply measuring crime better, with the advent of tools like CompStat that let police more accurately track violence. "The chance that a shooting gets recorded as a shooting or the chance that a violent event gets recorded as a violent event is much higher today than it was even 15 years ago," Roman said.

Roman cautioned that people shouldn't make too much of the fluctuations, especially when looking at a window as narrow as one month. So although 42 homicides in May 2015 is the worst month for Baltimore in decades, it's such a small number in terms of statistics that it could prove to be an empirical anomaly in the long term.

"You can always find one weekend or month that is really, really bad," Roman said, "but that doesn't mean the world is getting worse."

But even if the world were actually getting worse, it's very likely not just one thing — such as a slowdown in arrests, as is occurring in Baltimore — contributing to it. When the Brennan Center for Justice looked at why crime has been dropping for decades in the US, it had to look at more than a dozen different possible factors, ranging from mass incarceration to a reduction in the amount in lead found in gasoline. Similarly, any increase in crime should be looked at through multiple possible variables, from the weather to social unrest.

"There's so many different factors in crime," Inimai Chettiar, director of the Brennan Center's Justice Program, said, "that it's very, very difficult to say one thing caused it."