Scientists have long known that even low levels of lead can cause brain damage increasing the likelihood of behavioral and cognitive traits such as impulsivity, aggressivity, and low IQ that are strongly associated with criminal behavior. The New York Times has a story on how the phase out of leaded gasoline
starting with the Clean Air Act in 1973 may have led to a 56% drop in violent crime in the United States in the 1990s. Amherst Economics Professor Jessica Wolpaw Reyes discovered the connection and wrote a paper comparing the reduction of lead from gasoline between states
(pdf file) and the reduction of violent crime by constructing a panel of state-year observations linking crime rates in every state to childhood lead exposure in that state 20 or 30 years earlier. The theory will be put to the test as children grow up in Indonesia, Venezuela and sub-Saharan Africa, where leaded gasoline has just recently been phased out. Meanwhile, the list of countries that still use lead in gas -- Afghanistan, Serbia and Iraq, as well as much of North Africa and Central Asia -- does not rule out a connection with violence.