Research Links Childhood Lead Exposure to Changes in Violent Crime ( PDF file ) : ""...variations in childhood gasoline lead exposure from 1941 to 1986 explain about 90% of the variation in violent crime rates from 1960 to 1998.....variations in childhood paint lead exposure from 1879 to 1940 explain about 70% of the variation in murder rates from 1900 to 1960....extensive research demonstrating that childhood leadexposure reduces IQ levels later in life....a strong association between low IQ and criminal behavior....a statistical analysis of UnitedStates crime rate data and lead consumption data to determine whether changes in population lead exposure could explain subsequent trends in violent crime....relationship between population exposure to gasoline lead from 1941 to 1986 and the resulting trend in violent crime rates from 1964 to 1999. The23-year time lag is consistent with a typical age of about 23 for violent crime offenders, and research showing that a child'sbrain is most vulnerable to lead poisoning in the first few years after birth."