Other research has found that hotter days, months and seasons produce higher-than-normal crime rates. The December 1997 issue of the Journal of Personality and Social Psychology, Vol. 73, No. 6, 1213?1223 reported on two studies that examined the relation between hotness of year and violent crime rate in the United States from 1950 to 1995. One study found that summer produced more violent crime?murders and assaults?than the other seasons. If high temperature was a direct cause of the summer effect, then years with more hot days (days in which the maximum temperature is at least 90F) would have somewhat larger increases in murder and assault than years with fewer hot days. This prediction was confirmed by the data.

A second study examined the relation between the average temperature for each year and the corresponding murder and assault rate for the same 46-year period. If hot temperatures have a direct effect on violent behavior, then hotter years should (on average) produce higher violent crime rates. This is exactly what happened. The combined murder-and-assault rate was consistently higher in hotter years than in cooler ones. These results occurred even when the data were statistically controlled for the poverty rate, age shifts in the U.S. population and the general upward drift of violent crime during the period.

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