There may also be a medical reason for the decline in crime. For decades, doctors have known that children with lots of lead in their blood are much more likely to be aggressive, violent and delinquent. In 1974, the Environmental Protection Agency required oil companies to stop putting lead in gasoline. At the same time, lead in paint was banned for any new home (though old buildings still have lead paint, which children can absorb).
Tests have shown that the amount of lead in Americans’ blood fell by four-fifths between 1975 and 1991. A 2007 study by the economist Jessica Wolpaw Reyes contended that the reduction in gasoline lead produced more than half of the decline in violent crime during the 1990s in the U.S. and might bring about greater declines in the future. Another economist, Rick Nevin, has made the same argument for other nations.
Maybe the EPA might actually be doing some good!