Thank the Clean Air Act for significantly reducing violent crime rates in the U.S., says researcher Jessica Wolpaw Reyes. The legislation was behind the phaseout of leaded gasoline in the 1970s and ’80s, which significantly reduced blood levels of the heavy metal in Americans. The arc of lead-exposure rates seems to match the arc of violent crime in the U.S., says Reyes, but with a 20-year lag — enough time for children to reach their most violence-prone years. Brain damage from lead poisoning has been shown to make children less intelligent and, in some cases, more aggressive and impulsive.