Lead–crime hypothesis

The lead–crime hypothesis proposes that the sharp decline in crime rates in the industrialised world in the 1990s and beyond, after decades of increased crime, can be attributed to the reduction of lead poisoning. Lead is widely understood to be toxic to multiple organs of the human body, particularly the human brain. Concerns about even low levels of exposure began in the 1970s; in the decades since, scientists have concluded that no safe threshold for lead exposure exists. The major source of lead exposure during the 20th century was leaded gasoline. The hypothesis argues that the removal of lead additives from motor fuel, and the consequent decline in children's lead exposure, explains the fall in crime rates in the United States beginning in the 1990s. This hypothesis also offers an explanation of the rise in crime in the preceding decades as the result of increased lead exposure throughout the mid-20th century. Other explanations have been proposed, including situational crime prevention and interactions between many other factors with complex, multifactorial causation.

The lead–crime hypothesis is not mutually exclusive with other explanations of the drop in U.S. crime rates, which includes analysis of the hypothesized legalized abortion and crime effect. The difficulty in measuring the effect of lead exposure on crime rates is in separating the effect from other indicators of poverty such as poorer schools, nutrition, and medical care, exposure to other pollutants, and other variables that may lead to crime.