Keith Harries was misidentified in a story on climate and crime on yesterday's science page. He is chairman of the department of geography at the University of Maryland, Baltimore County. (Published 7/ 3/90)
I pray thee, good Mercutio, let's retire.
The day is hot, the Capulets abroad
And, if we meet, we shall not 'scape a brawl
For now, these hot days, is the mad blood stirring.
-- From "Romeo and Juliet,"
For Washington and cities like it across the United States, the killing season has begun. July and August, on average the year's hottest months, are also the nation's most violent months, when people rape, assault and kill each other more than at any other time of year.
This association between violence and weather has long been noted, of course. In one way or another, in culture and language, high temperatures have always carried ominous overtones. Blood does not freeze in the first flush of anger, after all. It boils. Tempers flare. Arguments get heated.
What no one really has proven, however, is why heat seems to have this effect and how large this effect is.
Do summer temperatures trigger biological changes that shorten tempers and heighten irritability?
Or does violence increase in the summer simply because people engage in more activities in warmer weather that lead to violence, such as drinking, gathering on the street or in public places and spending more time with their families?
And if all of this is true, how much of a difference does hot weather make? Would air conditioning every building in every hotbed of violent attacks bring down the crime rate? Studying the Effects of Heat
In recent years, using sophisticated field studies and laboratory experiments, psychologists have begun to explore these questions. But while some insights have been gained, unraveling the mysteries of the weather's effect on behavior remains difficult.
"We all seem to accept that irritability and the summer heat and behavioral changes are real," said Keith Harries, a geographer at Oklahoma State University. "But when it comes to explaining it in a scientifically acceptable way, things get a little trickier. The problem of finding an adequate way of measuring these effects is almost insurmountable."
Consider the most obvious effect of summer: People are outside their houses and workplaces more often.
"You've got more people at the beach, more people at recreational parks," said James Rotton, a professor of psychology at Florida International University in Miami. "What seems to be occurring is a shift from interactions with friends to those you don't know as well. People find themselves in situations where they are more likely to come into contact with a predator, someone who takes advantage of your activity."
Add to the equation alcohol -- which people drink more of in the summer and which has a known correlation with violence -- and some researchers say much of the hot weather effect can be explained away.
But how much? The effect of weather becomes most pronounced, according to some studies, during prolonged periods of very high temperatures, like heat waves. Do people really go outdoors more on those days than when it's 75 degrees and breezy? Wife-Beating Seems to Defy Theory
Some of the crimes that increase in hot weather also seem to contradict the social interaction theory. Take wife-beating, for instance. If people get out more in hot weather, then men should batter their wives less in the summer because they would have less time and opportunity to do it. But in a 1986 study, two Atlanta researchers showed the opposite was true. After analyzing the frequency of crisis calls to 23 different women's shelters over a four-year period, they found that the number of distress calls rose and fell with temperatures, peaking in the late summer months.
"The role of increased social interaction is important, but I don't think it explains the temperature correlations completely," said University of Missouri psychologist Craig A. Anderson. "The reason is that you get the same temperature-aggression correlation for all sorts of very different kinds of aggression. You get it with murders, assaults. You get it with rape. You get it with family assaults. Some of these things are those that are less sensitive to this explanation of increases in social contact."
Anderson and others are convinced that there is something peculiar to the heat itself that promotes aggression. In one study in Phoenix, for example, a researcher deliberately stalled his car in the middle of an intersection and then measured how quickly, how often and for how long people honked their horns in different types of weather. In what would seem to be a test of the heat effect and nothing else, the study found that those with air conditioning honked less than those without it, who in turn honked more frequently and with more emphasis the hotter it got.
This relationship between heat and behavior doesn't seem to exist for all temperatures. There appears to be a threshold level above which it becomes most apparent. Some research also suggests regional differences.
"We have found that people do react in a fashion that is aberrant under extreme weather conditions, but the conditions are relative," said University of Delaware climatologist Laurence Kalkstein. "What is extreme in northern cities is different from what is extreme in the south."
In the Northeast, Kalkstein says, temperature does appear to be the key. In a study of Baltimore, for example, he found that behavior was affected by climate -- as measured by the incidence of violent crime and increased mortality -- when temperatures rose into the mid-90s. But in Dallas, where those temperatures are the norm in the summer, behavioral changes were most marked during a combination of heat and extreme weather conditions such as storminess.
For Kalkstein, the key is what he calls stressful weather -- climatic conditions of any sort that come as a shock to people, making them uncomfortable and disrupting them psychologically. But this obviously differs from place to place.
"We don't find any good relationships in cities like San Francisco or Seattle because those places very rarely have extreme days," he said. "For certain West Coast cities, the weather is monotonously cool."
Other cities, by contrast, have lots of extreme days. New York has the most.
But determining exactly how much difference weather makes is difficult. One recent study suggested that the violent crime rate in a hypothetical city of 600,000 would increase by 7 percent if it experienced 42 more days of temperatures over 90 degrees.
But that was a hypothetical example. Accurately measuring weather influence on residents of a real city, researchers say, would depend on a thousand different variables. Do they have air conditioning? Do they have a ceiling fan? Is there a shade tree outside the window? And on and on.
"I don't want to say that 10 percent of the murders in such and such a place are caused by heat waves," Anderson said. "Murders aren't caused by heat waves. People get angry. They have fights. People get stabbed or shot. But my research demonstrates that high temperatures increase the likelihood of something happening. It is one of the many causes of violent crime."