Conventional wisdom holds that violent crime increases during hot weather. Even our language is peppered with references to "hotheads" whose anger "simmers" until they either "lose their cool" and "blow up" or finally "cool down." Researchers, however, have started to question just how meaningful the link is between hot weather and aggressive behavior. Does violent crime really increase in hot weather?

Yes. Research supports the link between violence and heat: Many studies have concluded that violence does rise in hot weather, says Brandon Keim in Wired. Recently, Florida State University researchers found that, over two years, violent assaults consistently increased in Minneapolis, Minn., as temperatures rose toward the 80s. They attributed the change partly to "social opportunity" — when the temperature goes up, more people spend more time outside.
"The Hazy Science of Hot Weather and Violence"

At very high temperatures, crime drops: Even in research that shows violent crimes increasing in warmer weather, there's an upper limit, says James Alan Fox in The Boston Globe. "The rate of violence tends to decline when temperatures reach the 90s." When it gets that hot, people tend to withdraw and seek shelter from extreme heat by staying at home, and out of trouble.
"Heat wave has chilling effect on violent crime"

But global climate change could make things worse: The "heat hypothesis" is backed up by U.S. crime and weather statistics since 1950, says Iowa State University researcher Craig Anderson. And looking forward, we can expect murder and assault rates to rise by 34 crimes per 100,000 people for every eight degrees Fahrenheit the Earth's temperature rises. "When people get hot, they behave more aggressively. There's nothing new there and we're all finding the same thing."
"Iowa State researchers present study on how global climate change affects violence"