He said: “Lead in gasoline can explain up to 90% of the increase and decline in violent crime over the last half -century. Drum is referring to work done by Rick Nevin, a consultant working on a study for the U.S. Department of Housing and Urban Development. Mr. Nevin determined that as the number of cars on the road using leaded gasoline increased between the 1940s and 1970s, lead emissions were higher. During this period, the crime rate increased dramatically. Once unleaded gasoline replaced leaded gasoline and emissions fell, crime rates began to decline.