The truth of that depends on the accuracy of the title of:

Violence is a curvilinear function of temperature in Dallas: A replication. Rotton, James; Cohn, Ellen G. Journal of Personality and Social Psychology, Vol 78(6), Jun 2000, 1074-1081. doi: 10.1037/0022-3514.78.6.1074

http://psycnet.apa.org/psycinfo/2000-07798-005