Lead–crime hypothesis

After decades of increasing crime across the industrialised world, crime rates started to decline sharply in the 1990s, a trend that continued into the new millennium. Many explanations have been proposed, including situational crime prevention and interactions between many other factors complex, multifactorial causation. 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. Proponents of the lead–crime hypothesis argue 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 in crime in the preceding decades as the result of increased lead exposure throughout the mid-20th century. The lead–crime hypothesis is not mutually exclusive with other explanations of the drop in US crime rates such as the 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. Lead, a naturally occurring metal of bluish-grey color, has been used for multiple purposes in the history of human civilization. Advantages include being somewhat soft and pliable as well as resistant to corrosion compared to other metals. The widespread substance is also able to function as a shield against various forms of radiation. Expanded scientific investigation into organolead chemistry and the varied ways in which human biology changes due to lead exposure took place throughout the 20th century.