Lead-crime hypothesis

The lead-crime hypothesis proposes that crime rates in the United States (and even elsewhere) 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 anymore, and the part that people call the "pencil lead" has never contained actual lead — chemical-element symbol "Pb"). No, it's not necessarily less lead-paint eating (though that reduction can only help, and in older US cities such as Providence, Rhode Island, lead paint remains a serious public-health problem). The main driver of the lower lead levels is elimination of leaded gasoline.

This hypothesis stands as an alternative or supplement to other explanations — including: gun proliferation, the United States war on drugs, the legalization of 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 both for the increase in crime rates that began in the 1960s and for the drop that began in the mid-1990s and continues to the present.

Background
Lead, a poisonous heavy metal, has been used since antiquity, and even the ancient Greeks recognized its adverse health effects. In modern times, it has been 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 children do. Childhood exposure is key to screwing someone over for life.

Crime
US crime rates, expressed as crimes per 100,000 people, have dropped since the 1990s:
 * 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 by almost 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 decreasing, remained relatively 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 varies 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.

Socioeconomic
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 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 America, 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? One response is that 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. Another response is that total vehicle numbers in the US increased dramatically after 1945, from about 35 million in 1945 to 170 million in 1985, and 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.

Alternate explanations
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.

Gun ownership
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.

Gun control
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.

Incarceration
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. The largest 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 Roe, but the UK had a surge in crime after the United States' peak (though only by a couple of years). Contrarily, Canada experienced tighter restrictions in legalized abortions from 1969-1988 but had a crime wave similar in duration, peak, and decline to 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 1980s, but the crime waves and their decline continued steadily. The has been rising monotonically for the United States, but crime has continued to plummet after peaking in the 1990s.

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 timespan, 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.

A study that compared equally-poor neighborhoods with different levels of lead exposure found that those children with higher lead levels had worse upward mobility.

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 downward, which is what we have seen. Unfortunately, for the few countries that have only recently phased out leaded gasoline, 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 African Americans 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.