The lead–crime hypothesis is the proposed link between elevated blood lead levels in children and later increases in crime. Children exposed to forms of lead at young ages are hypothesized to be more likely to develop learning disabilities, attention deficit hyperactivity disorder, and problems with impulse control. These problems are suggested to lead to the commission of more crimes as these children reach adulthood, especially violent crimes.
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 earlier rise 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. Although it has continued to be in wide use even into the 21st century, greater understanding of blood lead levels (BLLs) and other factors have meant that a new scientific consensus has emerged. No 'safe' level of lead in the human bloodstream exists as such; any amount can contribute to neurological problems and other health issues.
Medical analysis of the role of lead exposure in the brain note increases in impulsive actions and social aggression as well as the possibility of developing attention deficit hyperactivity disorder (ADHD). Those conditions likely influence personality traits and behavioral choices, with examples including having poor job performance, beginning a pattern of substance abuse, and undergoing teenage pregnancy. Evidence that lead exposure contributes to lower intelligence quotient (IQ) scores goes back to a seminal 1979 study in Nature, with later analysis finding the link particularly robust.
The international process of trying to lower the prevalence of lead has been largely spearheaded by the Partnership for Clean Fuels and Vehicles (PCFV). The non-governmental organization partners with major oil companies, various governmental departments, multiple civil society groups, and other such institutions worldwide. Efforts to phase-out lead in transport fuel achieved major gains in over seventy-five nations. In discussions at the 2002 'Earth Summit', institutions under the umbrella of the United Nations vowed to emphasize public–private partnerships (PPPs) in order to help developing and transitional countries go unleaded.
Research on lead–crime correlation
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After decades of relatively steady increases, crime rates in the United States started to sharply decline in the 1990s. The trend continued into the new millennium. Multiple possible explanations have come about, with academic studies pointing to complex, multifactorial causation as different social trends occurred at the same time. The fact that in the United States anti-lead efforts took place simultaneously alongside falls in violent crime rates attracted attention from researchers. Changes were not uniform across the country, even while increasingly stringent Environmental Protection Agency rules went into force from 1970s onward. Several areas had far greater lead exposure compared to others for years.
According to Jessica Wolpaw Reyes of Amherst College, between 1992 and 2002 the phase-out of lead from gasoline in the U.S. "was responsible for approximately a 56% decline in violent crime". While cautioning that the findings relating to "murder are not robust if New York and the District of Columbia are included," Wolpaw Reyes concluded: "Overall, the phase-out of lead and the legalization of abortion appear to have been responsible for significant reductions in violent crime rates." She additionally speculated that by "2020, all adults in their 20s and 30s will have grown up without any direct exposure to gasoline lead during childhood, and their crime rates could be correspondingly lower." According to Reyes, "Childhood lead exposure increases the likelihood of behavioral and cognitive traits such as impulsivity, aggressivity, and low IQ that are strongly associated with criminal behavior".
A 2011 study by the California State University found that "Ridding the world of leaded petrol, with the United Nations leading the effort in developing countries, has resulted in $2.4 trillion in annual benefits, 1.2 million fewer premature deaths, higher overall intelligence and 58 million fewer crimes", according to the United Nations News Centre. The executive director of the United Nations Environment Programme (UNEP) Achim Steiner argued that "Although this global effort has often flown below the radar of media and global leaders, it is clear that the elimination of leaded petrol is an immense achievement on par with the global elimination of major deadly diseases."
According to a May 2017 study, lead exposure in childhood substantially increased school suspensions and juvenile detention among boys in Rhode Island, suggesting that the phasing out leaded gasoline may explain a significant part of the decline in crime in the United States beginning in the 1990s.
- Biosocial criminology
- Environmental toxicology
- Lead abatement
- Lead poisoning
- Organolead chemistry
- Pollution control
- Statistical correlations of criminal behavior
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