College of Liberal Arts and Sciences at Iowa State University

High temperatures, hot tempers

Study by Craig Anderson indicates that as the temperature outside rises, so does violent crime.
When the phone calls started to come, Craig Anderson, professor and chair of the psychology department, thought they were about his video game research.

Last spring, Anderson was in the media spotlight when his research on violent video games landed him in front of a U.S. Senate committee. The subsequent publicity resulted in media outlets from around the world contacting him.

This time the calls weren't on the study that said individuals who play violent video games were more aggressive than those that didn't. Aggression however does play a role in the psychology professor's latest research.

Anderson has published an article in the American Psychological Society's Current Directions journal entitled "Heat and Violence." That article caught the attention of Reuters news service and soon Anderson's phone was ringing off the hook.

The article says that hot temperatures can increase aggressive motives and behaviors in individuals. This conclusion is based on numerous studies conducted in both field and laboratory settings.

"Laboratory studies show that hot temperatures increase aggression directly by increasing feelings of hostility and indirectly by increasing aggressive thoughts" Anderson writes. "Results (from several field studies) show that global warming trends may well increase violent-crime rates. Better climate controls in many institutional settings (e.g., prisons, schools, the workplace) may reduce aggression-related problems in those settings.

"Other studies show that hotter cities have higher violent crime rates than cool cities," Anderson says. "Even when we statistically control for population and social and economic factors like poverty and unemployment rates, hotter cities will still have higher violent crime rates than cool cities."

The article considered three different types of studies of heat and aggressive behavior:

* Studies comparing geographic regions. Data in these field studies consistently show that violent crime rates are higher in hotter regions than in other regions of the United States.

* Studies comparing time periods. Field studies comparing aggression rates in hotter versus colder time periods also support Anderson's hypothesis. There are more than 2.6% more murders and assaults in the United States during the summer than other seasons of the year. He also noted that hot summers produce a bigger increase in violence than cooler summers; and violence rates are higher in hotter years than in cooler years.

* Laboratory studies of aggression behavior. "In affectively neutral and positive circumstances, hot temperatures cause increases in aggression," Anderson writes. "Recent lab studies show that even in affectively negative circumstances, heat causes increases in initial retaliatory aggression." For his field studies, Anderson studied violent crime rates in the US over a 50-year period. He also sampled the 50 largest metropolitan areas in the nation according to the 1980 census.

He found that increases in average annual temperature or global warming, has an increasing effect on murders and assaults in this country, even after controlling for a variety of other factors.

"For every one degree increase in average temperature, we can expect an increase of 4.58 additional murders and assault cases per every 100,000 people," Anderson said.

"There are obviously other factors involved," he continued. "I would never claim that temperature alone would be the main factor that causes violent crime to be higher. However, there is now considerable evidence from a variety of sources that suggesting that high temperature is one cause that contributes to violent behavior, including violent criminal activity."