John MacDonald, a professor of criminology and sociology, and emergency medicine physician Eugenia South, collaborated on a prior study on environmental stressors and their impact on crime. MacDonald says the lessons learned can be applied to combating heat’s relationship to violence. (Photo: Eric Sucar/University Communications)

By Blake Cole

There are many ways to beat the summer heat: retreating to an air-conditioned space or taking a dip in a pool, to name a few. But when relief is hard to come by—especially common in communities in which resources are stretched thin—data show that high temperatures, combined with high-stress situations, can lead to violent confrontations.

“There’s long been a theory connecting temperatures and crime,” says John MacDonald, a professor of criminology and sociology who researches environmental stressors and their relationship to crime. “Southern states and southern Europe have higher rates of reported homicide and violent crime than northern states or northern parts of Europe. The classic challenge has been singling out the connection from other variables like higher rates of poverty.”

The research has become more sophisticated in recent times, charting specific instances like heat waves and temperature fluctuations in a particular location, and has consistently shown increases in violent crime in comparison to cold weather.

In a temperature-crime analysis published in the Journal of Urban Health, Drexel University researchers polled Philadelphia crime data from 2006 to 2015 and found that during warmer months, violent crime rose by almost double digits when the heat index was 98 degrees compared to days when the temperature was above 57 degrees. During colder months—October through April—crime was up 16 percent on 70-degree days as opposed to 43-degree days.

And it’s not just limited to the U.S., as MacDonald suggests. The BBC reported this past July that data from the London Metropolitan Police show that between April 2010 and June 2018, violent crime was, on average, 14 percent higher when the temperature was above 68 degrees than when it was below 50 degrees, and that harassment and weapons possession offenses were each 16 percent higher.

One factor is community stability, MacDonald says. “Communities that are resource-deprived and don’t have access to air conditioning, or a shopping mall to go to in order to avoid the heat for the day, are under more intense community pressure, and crime is a natural consequence of that.”

And the increase in crime isn’t only related to physiological stressors, he adds. “There is general consensus from a number of studies that finds when you have more people out and about, that there is less street crime. There are more stores open and more eyes on the street. Even if there are more opportunities, there’s a protective effect. During a heat wave there are going to be fewer people out policing each other. There also tends to be increased alcohol consumption due to people trying to rehydrate, which leads to a perfect storm of bad judgment.”

So how does a city combat a phenomenon dictated by nature? MacDonald has studied a similar dynamic when it comes to habitat and crime. In a recent study, he and Penn Medicine physician Eugenia South, found that mental health improved when vacant lots were remediated.

“There’s a physiological response when people are exposed to green lots. Their heart rate tends to slow down. Where do people often go when they take a vacation? To the ocean to look out on the water and cool off, or they go to the mountains. Is that just tradition or is there something about a comfortable environment that decreases irritability and other gateway emotions to violence? This same dynamic applies to temperature.”

When it comes to heat, MacDonald says there are practical policy implications to be found that would improve both the community and personal well-being.

“If we have this spike in serious assaults and homicides during heat waves, you could imagine interventions like providing more access to cooling centers and access to drinking water,” MacDonald says. “Hospitals could staff up and you could have better police presence in high-risk areas. Better preparation is really the key.”