The lead–crime hypothesis is a research area that involves a study of the

correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...

between elevated

blood lead level Blood lead level (BLL), is a measure of the amount of lead in the blood. Lead is a toxic heavy metal and can cause neurological damage, especially among children, at any detectable level. High lead levels cause decreased vitamin D and haemoglob ...

s in children and increased rates of crime, delinquency, and recidivism later in life. Lead is widely understood to be highly

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

to multiple organs of the body, particularly the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

. Individuals exposed to

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

at young ages are more vulnerable to

learning disabilities Learning disability, learning disorder, or learning difficulty (British English) is a condition in the brain that causes difficulties comprehending or processing information and can be caused by several different factors. Given the "difficult ...

, decreased

I.Q. An intelligence quotient (IQ) is a total score derived from a set of standardized tests or subtests designed to assess human intelligence. The abbreviation "IQ" was coined by the psychologist William Stern for the German term ''Intelligenzq ...

attention deficit hyperactivity disorder Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterised by excessive amounts of inattention, hyperactivity, and impulsivity that are pervasive, impairing in multiple contexts, and otherwise age-inap ...

, and problems with

impulse control Inhibitory control, also known as response inhibition, is a cognitive process – and, more specifically, an executive function – that permits an individual to inhibit their impulses and natural, habitual, or dominant behavioral re ...

, all of which may be negatively impacting decision making and leading to the commission of more crimes as these children reach adulthood, especially

violent crime A violent crime, violent felony, crime of violence or crime of a violent nature is a crime in which an offender or perpetrator uses or threatens to use harmful force upon a victim. This entails both crimes in which the violence, violent act is t ...

s. 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 Crime in the United States has been recorded since its founding. Crime rates have varied over time, with a sharp rise after 1900 and reaching a broad bulging peak between the 1970s and early 1990s. After 1992, crime rates began to fall year by ye ...

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 effect of legalized abortion on crime (also the Donohue–Levitt hypothesis) is a controversial hypothesis about the reduction in crime in the decades following the legalization of abortion. Proponents argue that the availability of abortion r ...

. Lead exposure during the years in question correlated with exposure to urban

poverty Poverty is the state of having few material possessions or little income. Poverty can have diverse social, economic, and political causes and effects. When evaluating poverty in ...

, due to close residential or primary school proximity with high-density motor vehicle traffic burning leaded gasoline or from residing in older, poorly maintained housing stock, much of which contained high levels of lead in the form of

lead paint Lead paint or lead-based paint is paint containing lead. As pigment, lead(II) chromate (, "chrome yellow"), lead(II,IV) oxide, (, "red lead"), and lead(II) carbonate (, "white lead") are the most common forms.. Lead is added to paint to acceler ...

, lead solder, or other lead-based building materials; additionally, municipalities with a low taxation base often continued to receive drinking water via degraded lead pipes rather than upgrading to modern infrastructure. The difficulty in measuring the effect of lead exposure on crime rates lies in separating the effect from other indicators of low

socioeconomic status Socioeconomic status (SES) is an economic and sociological combined total measure of a person's work experience and of an individual's or family's economic access to resources and social position in relation to others. When analyzing a family's ...

such as poorer schools, nutrition, and medical care, exposure to other pollutants, and other variables that are predictive of criminal behavior.

Background

Lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

is a naturally occurring

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

of bluish-grey color that has been used for multiple purposes in the history of human

civilization A civilization (or civilisation) is any complex society characterized by the development of a state, social stratification, urbanization, and symbolic systems of communication beyond natural spoken language (namely, a writing system). Ci ...

. Being soft and pliable, as well as resistant to

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

compared to other metals, has resulted in lead being used for many different items across time. Some of the earliest items made from lead were beads and jewelry dating back to 7th millennium B.C. Its malleability made lead an ideal choice for the Romans to build pipes for transporting water. Furthermore,

lead acetate Lead acetate can refer to: * Lead subacetate (Basic lead acetate), Pb3(OH)4(CH3COO)2 * Lead(IV) acetate (plumbic acetate), Pb(CH3COO)4 * Lead(II) acetate Lead(II) acetate (Pb(CH3COO)2), also known as lead acetate, lead diacetate, plumbous acetat ...

(also referred to as "sugar of lead") has been reported to have been used medicinally in the past. However, it was also noted that exposure to lead may have health consequences. The botanist

Nicander Nicander of Colophon ( grc-gre, Νίκανδρος ὁ Κολοφώνιος, Níkandros ho Kolophṓnios; fl. 2nd century BC), Greek poet, physician and grammarian, was born at Claros (Ahmetbeyli in modern Turkey), near Colophon, where his famil ...

was one of the first to write about the uses of lead.

Dioscorides Pedanius Dioscorides ( grc-gre, Πεδάνιος Διοσκουρίδης, ; 40–90 AD), “the father of pharmacognosy”, was a Greek physician, pharmacologist, botanist, and author of ''De materia medica'' (, On Medical Material) —a 5-vol ...

would later report that "the mind gives way" in individuals exposed to lead. Nonetheless, despite the hazards posed by lead, its durability made it useful and it was added to items such as

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

paint Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...

, and eventually

gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...

. The widespread substance is also able to function as a shield against various forms of

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

. The use of leaded products such as lead paint and leaded gasoline have resulted in higher environmental levels of lead in the air and soil. Lead is also a stable element and does not break down in the environment, so it must be physically removed. Most cases of lead exposure occur via inhalation or ingestion, though transdermal exposure is also possible. Once in the body lead has a half-life of approximately 30 days if in the blood, but can remain in the body for 20 to 30 years if it has accumulated in bones and organs. Expanded scientific investigation into

organolead chemistry Organolead compounds are chemical compounds containing a chemical bond between carbon and lead. Organolead chemistry is the corresponding science. The first organolead compound was hexaethyldilead (Pb2(C2H5)6), first synthesized in 1858.''Main Grou ...

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 Blood lead level (BLL), is a measure of the amount of lead in the blood. Lead is a toxic heavy metal and can cause neurological damage, especially among children, at any detectable level. High lead levels cause decreased vitamin D and haemoglobi ...

(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 Neurology (from el, νεῦρον (neûron), "string, nerve" and the suffix -logia, "study of") is the branch of medicine dealing with the diagnosis and treatment of all categories of conditions and disease involving the brain, the spinal ...

problems and other health issues. Analyses of the role of lead exposure in the

have been ongoing for the past few decades. Lead can interfere with numerous neurotransmitter systems in the brain, most likely because of its ability to mimic calcium. Elevation of aminolevulinic acid from lead-induced disruption of heme synthesis results in lead poisoning having symptoms similar to

acute porphyria Porphyria is a group of liver disorders in which substances called porphyrins build up in the body, negatively affecting the skin or nervous system. The types that affect the nervous system are also known as acute porphyria, as symptoms are ...

. Exposure to lead can also alter brain structure and function. At the behavioral level, exposure to lead has been observed to cause increases in impulsive actions and social aggression, as well as the possibility of developing

(ADHD). Those conditions likely influence

personality Personality is the characteristic sets of behaviors, cognitions, and emotional patterns that are formed from biological and environmental factors, and which change over time. While there is no generally agreed-upon definition of personality, mos ...

traits and behavioral choices, with examples including having poor

job performance Job performance assesses whether a person performs a job well. Job performance, studied academically as part of industrial and organizational psychology, also forms a part of human resources management. Performance is an important criterion for org ...

, beginning a pattern of

substance abuse Substance abuse, also known as drug abuse, is the use of a drug in amounts or by methods which are harmful to the individual or others. It is a form of substance-related disorder. Differing definitions of drug abuse are used in public health, ...

, and undergoing

teenage pregnancy Teenage pregnancy, also known as adolescent pregnancy, is pregnancy in a female adolescent or young adult under the age of 20. This includes those who are legally considered adults in their country. The WHO defines adolescence as the period bet ...

. Evidence that lead exposure contributes to lower

intelligence quotient An intelligence quotient (IQ) is a total score derived from a set of standardized tests or subtests designed to assess human intelligence. The abbreviation "IQ" was coined by the psychologist William Stern for the German term ''Intelligenzqu ...

(IQ) scores goes back to a seminal 1979 study in ''

Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...

'', with later analysis finding the link particularly robust. The heavy metal lead can be found readily in the environment, especially in urban and industrialized areas. The majority of modern day environmental lead contamination can be traced back to leaded paint and the addition of

tetraethyllead Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb( C2H5)4. It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that al ...

and

tetramethyllead Tetramethyllead, also called tetra methyllead and lead tetramethyl, is a chemical compound used as an antiknock additive for gasoline. Its use is being phased out for environmental considerations. The National Institute for Occupational Safety a ...

to gasoline, though other sources have contributed as well. Though some of the hazards of lead exposure have been documented for centuries, recognition of the hazards posed did not appear to gain much traction until the 1960s with the Senate hearings of

Edmund Muskie Edmund Sixtus Muskie (March 28, 1914March 26, 1996) was an American statesman and political leader who served as the 58th United States Secretary of State under President Jimmy Carter, a United States Senator from Maine from 1959 to 1980, the 6 ...

that would help lead to the phaseout of leaded gasoline and lead-based paint in the 1970s. Blood lead levels would drop in a statistically significant way soon after the phaseout. In the decades since, scientists have concluded that no safe threshold for lead exposure exists. Though efforts to reduce environmental levels of lead were initially slowed down by the lead industry, the emergence of

Clair Patterson Clair Cameron Patterson (June 2, 1922 – December 5, 1995) was an American geochemist. Born in Mitchellville, Iowa, Patterson graduated from Grinnell College. He later received his Ph.D. from the University of Chicago and spent his entire profe ...

in the 1960s would lead to more meaningful changes. The establishment of the

United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...

in 1970 and the influence of the

Consumer Product Safety Commission The United States Consumer Product Safety Commission (USCPSC, CPSC, or commission) is an independent agency of the United States government. The CPSC seeks to promote the safety of consumer products by addressing “unreasonable risks” of inj ...

would help ensure that gasoline and paint could only contain trace amounts of lead. Furthermore, several major legislative acts were passed to help reduce the amount of lead being introduced into the environment, including the Clean Air Act of 1970 and the Lead Based Paint Poisoning Prevention Act. 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 A non-governmental organization (NGO) or non-governmental organisation (see spelling differences) is an organization that generally is formed independent from government. They are typically nonprofit entities, and many of them are active in h ...

partners with major

oil companies The following is a list of notable companies in the petroleum industry that are engaged in petroleum exploration and production. The list is in alphabetical order by continent and then by country. This list does not include companies only involved ...

, various governmental departments, multiple

civil society Civil society can be understood as the "third sector" of society, distinct from government and business, and including the family and the private sphere.2002 Earth Summit, institutions under the umbrella of the

United Nations The United Nations (UN) is an intergovernmental organization whose stated purposes are to maintain international peace and international security, security, develop friendly relations among nations, achieve international cooperation, and be ...

vowed to emphasize public–private partnerships (PPPs) in order to help developing and transitional countries go unleaded.

Research on lead–crime correlation

The lead–crime hypothesis arose out of the confluence of several events, primarily the decrease in crime rates in the 1990s and the reduction of environmental lead pollution in the 1970s. After decades of relatively steady increases,

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 The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

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 A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...

rules went into force from 1970s onward. Several areas had far greater lead exposure compared to others for years. While there is strong evidence indicating that genetics influence the development of violent and aggressive behavior, more recent attention has focused on environmental factors such as lead exposure. Though there is anecdotal evidence suggesting that knowledge of a relationship between lead exposure and behavior dates back centuries, direct observations would not be documented until the late 1800s. Research in the mid-1900s observed that children previously treated for lead poisoning displayed a series of aberrant behaviors, including violence and aggression. Further research has yielded similar results, finding that past lead exposure functions as a predictor for criminal activity. Nation-wide analyses have also demonstrated positive associations between air-lead concentrations and measures of criminality and homicide. A meta-analysis of studies examining the relationship between lead and conduct problems arrived at a similar conclusion, suggesting that the magnitude of the relationship between lead exposure and behavior is comparable to the relationship between lead exposure and I.Q. While the scientific literature suggests there is a relationship between lead exposure and behavioral issues such as delinquency and criminality, directly relating these observations to the decrease in overall criminality is more difficult. According to Jessica Wolpaw Reyes of

Amherst College Amherst College ( ) is a private liberal arts college in Amherst, Massachusetts. Founded in 1821 as an attempt to relocate Williams College by its then-president Zephaniah Swift Moore, Amherst is the third oldest institution of higher educatio ...

, 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." 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". Achim_Steiner-IMG 0837

A 2011 study by the

California State University The California State University (Cal State or CSU) is a public university system in California. With 23 campuses and eight off-campus centers enrolling 485,550 students with 55,909 faculty and staff, CSU is the largest four-year public univers ...

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 The United Nations Environment Programme (UNEP) is responsible for coordinating responses to environmental issues within the United Nations system. It was established by Maurice Strong, its first director, after the United Nations Conference on th ...

(UNEP)

Achim Steiner Achim Steiner (born 17 May 1961) is a Brazilian-born environmentalist who currently serves as the administrator of the United Nations Development Programme and chairman of United Nations Sustainable Development Group. Before joining UNDP, he w ...

argued that "Although this global effort has often flown below the radar of hemedia 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 Rhode Island (, like ''road'') is a U.S. state, state in the New England region of the Northeastern United States. It is the List of U.S. states by area, smallest U.S. state by area and the List of states and territories of the United States ...

, 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. A 2018 longitudinal study conducted in New Zealand found only a weak association between childhood lead levels and criminal conviction, which was no longer significant after controlling for sex. In New Zealand, there is no correlation between lead exposure and socioeconomic status, thus social class does not act as a confounder. The authors conclude that "past studies of the association between BLL and crime, in which high BLL and low socioeconomic status were associated, may not have completely overcome confounding". Data from the same cohort was published the next year as well and has been cited in review papers as evidence of lead poisoning having long-lasting consequences for mental health and personality. A 2022 meta-analysis, which pooled 542 estimates from 24 studies and corrected for

publication bias In published academic research, publication bias occurs when the outcome of an experiment or research study biases the decision to publish or otherwise distribute it. Publishing only results that show a significant finding disturbs the balance o ...

, found that the estimates indicated that "the abatement of lead pollution may be responsible for 7–28% of the fall in homicide in the US," leaving 93-72% unaccounted for. It concluded that "Lead increases crime, but does not explain the majority of the fall in crime observed in some countries in the 20th century. Additional explanations are needed".

Background

Research on lead–crime correlation

See also

References

Further reading