Thelead–crime hypothesis proposes that thesharp decline in crime rates in the industrialised world in the 1990s and beyond, after decades of increased crime, can be attributed to the reduction oflead poisoning. Lead iswidely understood to be toxic to multiple organs of the human body, particularly thehuman 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.[1][2] The major source of lead exposure during the 20th century wasleaded 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 incrime rates in the United States beginning in the 1990s.[3] This hypothesis also offers an explanation of therise in crime in the preceding decades as the result of increased lead exposure throughout the mid-20th century.[4]Other explanations have been proposed, includingsituational crime prevention and interactions between many other factors withcomplex, multifactorial causation.[5]
The lead–crime hypothesis is not mutually exclusive with other explanations of the drop in U.S. crime rates, which includes analysis of the hypothesizedlegalized abortion and crime effect.[4] The difficulty in measuring the effect of lead exposure on crime rates is in separating the effect from other indicators ofpoverty such as poorer schools, nutrition, and medical care, exposure to other pollutants, and other variables that may lead to crime.[3]
Lead, a naturally occurringmetal of bluish-grey color, has been used for many purposes in the history of humancivilization. Advantages include being somewhat soft and pliable as well as resistant tocorrosion compared to other metals. The widespread substance is also able to function as a shield against various forms ofradiation.[1]
Expanded scientific investigation intoorganolead 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 ofblood lead levels (BLLs) and other factors have meant that a new scientific consensus has emerged. No 'safe' level of lead in thehuman bloodstream exists as such; any amount can contribute toneurological problems and other health issues.[1]
Medical analyses of the role of lead exposure in thebrain note increases in impulsive actions and social aggression as well as the possibility of developingattention deficit hyperactivity disorder (ADHD). Those conditions likely influencepersonality traits and behavioral choices, with examples including having poorjob performance, beginning a pattern ofsubstance abuse, and undergoingteenage pregnancy. Evidence that lead exposure contributes to lowerintelligence quotient (IQ) scores goes back to a seminal 1979 study inNature, with later analysis finding the link particularly robust.[1][5][citation needed]
The international process of trying to lower the prevalence of lead has been largely spearheaded by the Partnership for Clean Fuels and Vehicles (PCFV). Thenon-governmental organization partners with majoroil companies, various governmental departments, multiplecivil 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 the2002 'Earth Summit', institutions under the umbrella of theUnited Nations vowed to emphasizepublic–private partnerships (PPPs) in order to helpdeveloping and transitional countries go unleaded.[2]
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Commentators and researchers have noted that, after decades of relatively steady increases,crime rates in the United States started to sharply decline in the 1990s. The trend continued even into the new millennium. Multiple possible explanations have been suggested, with academic studies pointing tocomplex, multifactorial causation concurrent with various social trends.[5]
TheeconomistsSteven D. Levitt andJohn J. Donohue III, of theUniversity of Chicago andStanford University, respectively, have argued that the decline in U.S. crime rates was the combined result of an increase in the number of police, hikes in size of the prison population, waning of the spread ofcrack cocaine, and the widespread legalization ofabortion from the 1970s onward. Possible other factors include changes inalcohol consumption. Later studies have upheld many of these findings while disputing others.[5][6]
While noting thatcorrelation does not imply causation, the fact that in theUnited 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 stringentEnvironmental Protection Agency rules went into force from the 1970s onward. Several areas had far greater lead exposure compared to others for years.[5][7]
A 2007 report published byThe B.E. Journal of Economic Analysis & Policy, authored by Jessica Wolpaw Reyes ofAmherst College, found that between 1992 and 2002 thephase-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," the author concluded that "[o]verall, 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."[5]
In 2011, a report published by the official United Nations News Centre remarked, "Ridding the world of leaded petrol [...] has resulted in $2.4 trillion in annual benefits, 1.2 million fewer premature deaths, higher overall intelligence and 58 million fewer crimes". TheCalifornia State University did the specific study. ThenU.N. Environment Programme (UNEP) executive directorAchim Steiner argued, "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."[2]
In a 2013 article,Mother Jones ran a report byKevin Drum arguing:
Needless to say, not every child exposed to lead is destined for a life of crime. Everyone over the age of 40 was probably exposed to too much lead during childhood, and most of us suffered nothing more than a few points of IQ loss. But there were plenty of kids already on the margin, and millions of those kids were pushed over the edge from being merely slow or disruptive to becoming part of a nationwide epidemic of violent crime.[7]
Drum writes:
We now have studies at the international level, the national level, the state level, the city level, and even the individual level. Groups of children have been followed from the womb to adulthood, and higher childhood blood lead levels are consistently associated with higher adult arrest rates for violent crimes. All of these studies tell the same story: Gasoline lead is responsible for a good share of the rise and fall of violent crime over the past half century.[8]
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".[5]
A May 2017 study by Anna Aizer and Janet Currie found that lead exposure in childhood substantially increased school suspensions and juvenile detention among boys inRhode Island, suggesting that the phasing out of leaded gasoline may explain a significant part of the decline in crime in the United States beginning in the 1990s.[3][9]
The first meta-analysis of the lead-crime hypothesis was published in 2022.[10] "The Lead-Crime Hypothesis: A Meta-Analysis", authored by Anthony Higney, Nick Hanley, and Mirko Moro consolidates findings of 24 studies on the subject. It found that there is substantial evidence linking lead exposure to a heightened risk of criminal behavior, particularly violent crimes. This aligns with earlier research suggesting lead exposure may foster impulsive and aggressive tendencies, potential precursors to violent offenses. The study concluded that, while a correlation between declining lead pollution and declining criminality is supported by research, it is likely not a significant factor in reduced crime rates, and that the link is generally overstated in lead-crime literature.
The study's implications point towards the potential benefits of reducing lead exposure to decrease crime rates. Such reductions could be achieved through initiatives like removing lead from products like gasoline and paint, water pipes and enhancing lead abatement measures in schools and residences.
