Hot years and serious and deadly assault: empirical tests of the heat hypothesis.

Two archival studies examined the relation between year-to-year shifts in temperature and violent and property crime rates in the United States. Study 1 examined the relation between annual average temperature and crime rate in the years 1950-1995. As expected, a positive relation between temperature and serious and deadly assault was observed, even after time series, linear year, poverty, and population age effects were statistically controlled. Property crime was unrelated to annual average temperature. Study 2 examined the relation between the average number of hot days (> or = 90 degrees F) and the size of the usual summer increase in violence for the years 1950-1995. As expected, a positive relation was observed between number of hot days and magnitude of the summer effect, even after time series and linear year effects were statistically controlled. For property crime, the summer effect was unrelated to number of hot days.