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The lead-crime hypothesis proposes that the United States crime rates have dropped, in part, because of lower levels of lead exposure in the environment. No, it's not fewer pencil munchers (pencils don't use lead paint any more and the part that people call the "pencil lead" has never contained lead). No, it's not necessarily less lead paint eating. The main driver of the lower lead levels is elimination of leaded fuels.
This hypothesis stands as an alternative or supplement to other explanations including gun proliferation, the United States war on drugs, the legalization and subsequent increase in the number of abortions, or increasing rates of imprisonment and capital punishment. Demographic change – specifically, a reduction in crime-prone young males – was once proposed as a cause of the drop and forecast to cause increased crime in 2010, but it did not.
The lead-crime hypothesis offers a possible explanation for both the increase in crime rates that began in the 60s, as well as for the drop that began in the mid-90s and continues to the present.
- 1 Background
- 2 Biology
- 3 Crime
- 4 Evidence
- 5 Evidence against
- 6 Alternate explanations
- 7 Predictions and public policy
- 8 See also
- 9 External links
- 10 References
Lead, a poisonous heavy metal, has been used in many everyday products including paint, radiation protection, makeup, bullets, and plumbing, but most significantly in gasoline. Leaded gasoline was banned in the US for road vehicle use in 1995, although it remains in use for piston-engined aircraft and, until 2008, in NASCAR. In many parts of the world, lead pipes are no longer allowed. Lead paint was banned in the United States in 1978 although older buildings still have it. Although illegal in the US, kohl, a lead-based makeup, remains popular in South Asia, the Middle East, and North Africa.
Lead in the human body has different effects at different developmental stages. Unlike many chemicals, research shows no threshold of exposure above which symptoms emerge. Every bit of lead has an effect. In children and adults, the effects tend to be more neurologically focused. For children, though, some effects last beyond the lead exposure. At even low exposure levels, children experience increased ADHD, hearing impairment, and decreased IQ. Many of these impacts persist. Searle et al conclude that many of the persistent effects of childhood lead exposure seem more prominent in those with lower socioeconomic status. Regardless of exposure, many childhood effects, most commonly ADHD, persist into adulthood. Adults with low to moderate lead exposure as adults experience symptoms including depression, nervousness, irritability, and reduced IQ. Regardless of the effect, adults require greater exposure to induce symptoms than do children. Childhood exposure is key.
US Crime rates, expressed as crimes per 100,000 people, have dropped since the 1990's:
- Violent crime dropped from 758.2 in 1991 to 386.7 in 2012.
- Murder and non-negligent manslaughter, 1-2% of violent crimes, follows a slightly different pattern with relatively little change.
- Aggravated assault dropped from 441.9 in 1992 to 242.3 in 2012.
- Property crimes have shown a longer decline, peaking at 5,264 in 1979 to 2,859.2 in 2012.
Although other developed nations have different patterns of crime, usually less murder and violent crime, their crime rates have shown a similar pattern, albeit with slight differences in peaks and rates of decline.
Blood lead levels
The introduction of unleaded fuels reduced airborne lead pollution, reducing, in turn, human blood lead levels. In a Swiss study, average blood lead levels dropped almost in half over the ten years after the switch from leaded to unleaded fuels. In the United States, blood lead levels in children dropped by 79% between 1976 and 1991, the period over which leaded fuel was phased out. The same data showed that blood lead levels, although lower, remained higher among minorities and those with lower socioeconomic status.
International and local time lags
If the lead-crime hypothesis is true then one of the results would be that nations should not only have blood-lead level and violent crime curves of similar slopes and maxima but that the time gap between the phaseout of lead and the reduction in crime levels should be the same. What's more, since nations introduced and phased out leaded gasoline at different periods we should expect that the width of the crime curves vary only by these factors.
This would be particularly strong evidence for the lead-crime hypothesis because different nations have different social policy. OECD nations had different rates of abortion and abortion access, gun proliferation, racism, income inequality, incarceration, and drug proliferation during their crime waves, yet the curves remain inviolate. Moreover, Nevin claims that no nation as of yet breaks the international trend.
The lead-crime hypothesis also stands up to scrutiny within nations. For example, São Paulo, as the site of ethanol refineries, weaned itself from leaded gasoline years ahead of the rest of Brazil, and homicide rates have plummeted in São Paulo since 2000, despite holding steady in the rest of the country.
Considering nothing but airborne lead levels and violent crime rates, one study found that nearly a third of the variation in crime rates originated from concurrently measured lead levels. That correlation alone is compelling but hardly proves causation. Including a variety of socioeconomic factors halved the lead-crime association down to a still-notable level. Sure, social disruption and crime are associated, as is social disruption and poverty which, in turn, is associated with crime, but areas with higher airborne lead levels had higher violent crime rates than would be expected solely from socioeconomic factors. Property crime rates showed a similar pattern, but with less of a drop after incorporating socioeconomic factors.
Instead of measuring actual airborne lead exposure, research that estimates the estimate nationwide lead exposure from gasoline and lead paint, showed that lead exposure from 23 years prior substantially predicted violent crime and, along with teen unemployment, accounted for 90% of the variation in historical violent crime rates. Using better estimates of preschool lead exposure, similar methodology replicated internationally showed the same trend across eight other developed nations in Europe, North American and Oceania.  Not surprisingly, the critical time for lead exposure is in childhood. Early exposure predicts subsequent crime rates.
Long History of Exposure
American blood lead levels are not as clearly established prior to the 1970s; however, they were likely very high by modern standards, likely comparable to or even higher than those seen in the 1970s. Use of lead paint actually peaked in the 1920s, with smaller peaks in the 1900s and 1940s. If lead was actually the cause of criminality, why did the rate of criminal activity only begin to rise in the 1960s, and then peak in the early 1990s, when lead exposure had begun more than half a century earlier? (Response: The poor quality of pre-1940s crime statistics makes the history of the crime levels hard to figure out, but there is some evidence that the rate of crime actually began to rise in the 1910s.) (Further response: the attached graph shows total vehicle numbers in the US from census data, showing that there were about 35m vehicles on the road in 1945, but 170M by 1985 - lead in petrol results in airborne lead pollution in high population areas, a much more likely vector for large-scale lead intake than paint-eating.)
The cause of the international decline in crime in the developed world since the 1990s is a cause of frequent speculation including numerous competing hypotheses.
One hypothesis is that an increase in gun ownership has discouraged people from committing crimes.
This is very unlikely; while per-capita gun ownership has risen in the US, the percentage of the population who owns guns has actually declined over time - while there are more guns in the United States, they are in the hands of a smaller percentage of the population than they were in the 1970s. Another flaw with this hypothesis is that other countries have seen a decline in gun ownership and have experienced falling crime rates in many cases.
Another hypothesis is that more stringent gun control laws have discouraged people from owning guns, in the belief that people with guns are more likely to commit crimes or that more severe crimes can be committed with guns.
This is also very unlikely; all locations and countries, with or without strict gun control laws, have seen similar declines in criminal behavior. Switzerland, which has about 70% as many guns as the US on a per-capita basis, has one of the lowest homicide rates in Europe.
Another hypothesis is that the crime rates have fallen because the criminals have been jailed en-masse. There is some evidence for this; people who commit crimes are vastly more likely to commit more crimes, and incarceration rates have risen dramatically since the late 1980s. By imprisoning all criminals until they are old enough that they are much less demographically likely to re-offend, you can, in principle, lower the crime rate.
While reasonable on the face of it, there are a few issues with it. The largest issue is that while incarceration rates in the US have risen dramatically while crime rates have fallen, crime rates in other countries have also gone down, without any similarly huge absolute increases in incarceration rates. A secondary issue is that incarceration rates have begun to fall since 2010 while crime rates have continued to fall, though supporters of incarceration have noted that criminals imprisoned in the late 1980s and early 1990s were unable to reproduce and thus may not have passed on the cultural memes/genes/broken family dynamics to produce more criminals.
Abortion and birth control access
The famous abortion-crime hypothesis forwarded by Freakonomics is suspect because of two reasons. The first is that it's not strictly causal i.e. cause precedes effect (crime rates were going up before Roe v. Wade) and the second is that it's not internationally expandable. For example, the Abortion Act of 1967 in the United Kingdom, all but legalizing abortion, occurred well before before Roe but the UK had a surge in crime after the United States' peak (though only by a couple years). Contrariwise, Canada experienced tighter restrictions in legalized abortions from 1969-1988 but had a crime wave similar in duration, peak, and decline of that of the United States.
A more plausible hypothesis is that greater availability of birth control in general may have caused the decline in the crime rates, and as more people used birth control in the late 1960s and 1970s, fewer unwanted children were born into unfortunate circumstances. This has the advantage of cutting across all countries and being an international trend. However, there remains a very large flaw in that crime rates were considerably lower in the 1950s than they are in modern times; the surge between the 1960s and 1990s and subsequent decline remains unexplained, and if lack of family planning was truly the cause, then crime rates should have been even higher before the mass availability of birth control in the United States. This is not the case.
Economic inequality and poverty
The largest evidence against the prevailing notion that economic inequality was the primary cause behind the post-Atomic Age crime waves so many OECD nations experienced is that economic inequality has been going upwards almost continually since the 80s but the crime waves and their decline continued steadily. The Gini coefficient has been rising monotonically for the United States but crime has continued to plummet after peaking in the 90s.
What's more, economic inequality does not alone explain why the rise and fall of crime was concentrated in urban areas.
Economic inequality is a poor measure of poverty in any case, used primarily for political purposes; absolute poverty has declined over the same time span, and poor people have benefited from increasingly higher standards of living. However, this was occurring during the time of the surge in crime, and if destitution was the cause of the increase in crime, why weren't people born during the Great Depression especially likely to become criminals in the 1950s and early 1960s, versus people born in the relatively prosperous 1950s and 1960s who committed crimes in the 1970s and 1980s? One possible explanation might be that poverty needs to be paired with expectations of future prosperity. In the continuous post-WWII boom in the U.S., expanding legal economic opportunities might have been seen as a more attractive alternative to crime, while the economic problems in the 1970s and 1980s could have increased the attractiveness of the "crime economy" instead.
Predictions and public policy
The most salient prediction of the lead-crime hypothesis is that crime and incarceration rates will continue to go down in countries that phased out leaded gasoline. While the incarceration rates of age groups that were maximally exposed to aerosolized lead will still be record-settingly high as they age, the overall incarceration rates will continue to down -- which is what we have seen. Unfortunately, for the few countries that have only recently phased leaded gasoline out such as Venezuela and Indonesia, their crime rates are predicted to skyrocket for exposed age groups.
For those wanting a prospective study, Dr. Jianghong Liu is tracking 1341 children aged 3 to 5 years in China's Jiangsu Province that have elevated blood-lead levels. Already the children are uniformly showing behavioral problems directly correlated with increased blood-lead levels.
As far as the United States goes, since black people have an elevated exposure to lead due to their concentration in urban areas and likeliness of living in homes with lead paint, the IQ gap, contrary to what The Bell Curve predicts, should shrink over time. Indeed, the black-white IQ gap of children was already shrinking when the book was released in 1994 . A 2006 paper showed that it has decreased slightly further in the ten years since.
- The Bell Curve - Rick Nevin extensively referred to this book to formulate this hypothesis, only to show that a lot of the assertions, especially those having to deal with the supposed genetic inferiority of racial minorities were wholly without merit in light of this hypothesis.
- America's Real Criminal Element: Lead, Mother Jones
- Where have all the burglars gone?
- US crime rate is down: six key reasons
- My Black Crime Problem, and Ours
- Searle et al 2014.Tracing the long-term legacy of childhood lead exposure: A review of three decades of the Port Pirie Cohort study
- CDC Lead Toxicity What Are the Physiologic Effects of Lead Exposure?
- Wietlisbach et al 1995."Time Trend and Determinants of Blood Lead Levels in a Swiss Population over a Transition Period (1984-1993) from Leaded to Unleaded Gasoline Use"
- Pirkle et al 1994.The decline in blood lead levels in the United States. The National Health and Nutrition Examination Surveys (NHANES)
- Drum 2013. America's Real Criminal Element: Lead
- 2013 Nevin Lead Exposure and Murder in Latin America
- Stratesky and Lynch 2004. The Relationship between Lead and Crime
- Nevin 2000. How Lead Exposure Relates to Temporal Changes in IQ,Violent Crime, and Unwed Pregnancy
- The same study also predicts unwed pregnancy, where total lead emissions have nearly the same impact as the Roe vs. Wade decision that legalized abortion in the US. As if that weren't enough, the article also takes an extended look at IQ and almost seems to suggest that the infamous results from the Bell Curve can be explained largely by increased lead exposure in minorities leading to lower IQ results.
- Nevin 2007. Understanding international crime trends: The legacy of preschool lead exposure
- From the National Opinion Research Center, General Social Survey: Trends in Gun Ownership in the United States, 1972-2014
- Wonkblog: There are now more guns than people in the United States
- 2013 The Canadian Press Canada's Crime Rate Lowest Since 1972
- 2013 Political Calculations The Major Trends in U.S. Inequality Since 1947
- 2011 Florida The Atlantic Don't Fear the City: Urban America's Crime Drops to Lowest in 40 Years
- From 2002 to 2012, the incarceration rate fell 51% for men ages 18-19, 30% for men ages 20-24, and 21% for men ages 25-29....The prison population has fallen at a slower rate because steep incarceration declines for young men have been offset by rising incarceration for older adults. From 2007 to 2012, the incarceration rate increased by 21% for men ages 45-49 and surged 45% for men ages 50-64
- Hoffman 2007 New York Times Criminal Element
- Liu 2014. JAMA Pediatr Blood Lead Concentrations and Children’s Behavioral and Emotional Problems A Cohort Study (Subscription required)
- The mean (SD) blood lead concentration was 6.4 (2.6) µg/dL, with the 75th and 90th percentiles being 7.5 and 9.4 µg/dL, respectively. General linear modeling showed significant associations between blood lead concentrations and increased scores for teacher-reported behavioral problems. A 1-µg/dL increase in the blood lead concentration resulted in a 0.322 (95% CI, 0.058 to 0.587), 0.253 (95% CI, 0.016 to 0.500), and 0.303 (95% CI, 0.046 to 0.560) increase of teacher-reported behavior scores on emotional reactivity, anxiety problems, and pervasive developmental problems, respectively (P < .05), with adjustment for parental and child variables. Spline modeling showed that mean teacher-reported behavior scores increased with blood lead concentrations, particularly for older girls.
- Kangas 1996. Myth: The black/white IQ gap is 15 points and growing
- Dickens & Flynn 2006 Psychological Science Black Americans Reduce the Racial IQ Gap: Evidence from Standardization Samples
- Nevin, 2012 Lead Poisoning and the Bell Curve.