President Biden’s proposed American Jobs Plan includes $45 billion to replace all lead pipes and service lines in the country — an enormous investment that some worry is still not enough. A growing body of research shows that exposure to lead in childhood has lasting, detrimental effects. Exposure to this toxin affects brain development in a way that reduces academic achievement and even increases criminal behavior.
Below, I summarize the latest evidence on the effects of lead exposure on criminal behavior. Given the tremendous cost of crime to society, investing more in lead remediation to protect children from the dangerous effects of this toxin would be an extremely cost-effective strategy to improve public safety, and one that deserves bipartisan support.
Investigating the lead-crime link
In recent decades there has been growing interest in whether environmental toxins affect behavior. Scientific evidence suggests that exposure to lead and similar substances in childhood can alter brain development, with lasting detrimental effects on behavior. But it has been difficult to quantify the effects of such exposure in the real world. (It’s possible that education and health care during childhood and early adulthood can mitigate any early impacts so that there are no long-term consequences.) The reason it has been so difficult to measure the causal effects of such exposure is that lead isn’t randomly distributed around the United States. Those with high exposure to lead also tend to live in poorer communities, with lower-quality schools and less access to health care. It is thus difficult to tell if worse outcomes are due to lead exposure or to these other factors.
Several recent studies use innovative methods to isolate the causal effects of lead exposure on delinquent and criminal behavior. These studies are exciting because they provide rigorous evidence that exposure to lead and similar toxins causes criminal behavior later in life – and that reducing such exposure would make our communities safer.
Lead in drinking water increased homicide rates in the early 20th century
An early example of the lead-crime link comes from the use of lead pipes during the late 19th century. James Feigenbaum and Christopher Muller consider the effects of these lead pipes on homicide rates between 1921 and 1936 – when the kids who were exposed to lead pipes were adults. (Of course, the dangers of lead were not understood at the time and so did not affect which places adopted lead pipes.)
Comparing homicide rates in places with and without lead pipes could be misleading if those places are different in other ways. For instance, people in cities with lead pipes tended to be wealthier and better educated. These factors could independently affect crime rates, leading to correlations that do not reflect a causal relationship between lead and crime. Feigenbaum and Muller use two empirical strategies to get around this problem.
First, they used cities’ proximity to lead refineries to predict the likelihood that a city would install lead pipes. Because transportation of those pipes was expensive, proximity mattered. The researchers effectively compare homicide rates in places closer to lead refineries with homicide rates in places that were farther away. This strategy avoids the independent effect of factors like wealth and education on a city’s decision to install lead pipes.
Second, they exploit an additional fact about how lead pipes cause lead exposure: The lead only leaches into the water if the water is more acidic. This gives the researchers another way to isolate the causal effect of lead exposure. Since lead only seeps into acidic water, they consider places with lead pipes and acidic water as ‘treated’ by lead exposure; all other places are not treated. They then adjust for other features of those places by using cities with lead pipes and non-acidic water and cities with acidic water but no lead pipes as control groups.
Based on both strategies, the researchers found that more lead exposure increased cities’ homicide rates. Cities that used lead pipes had homicide rates 24 percent higher than in cities without lead pipes, on average; places with more acidic water saw bigger increases.
Removing lead from gasoline reduced violent crime
Evidence from the mid- to late-20th century also supports the hypothesis that lead exposure increases criminal behavior. During those years, the primary source of exposure to lead was from leaded gasoline; the lead was released into the air with car exhaust and then settled into soil near the roads.
Jessica Reyes used the removal of lead from gasoline due to the Clean Air Act as a natural experiment. Those regulations were implemented in a way that created substantial variation in lead exposure across states and over time; Reyes used this variation to measure the effect of childhood lead exposure on crime rates when those kids were older.
She found that the reduction in environmental lead exposure during the late 1970s and early 1980s caused substantial declines in violent crime in the 1990s. In a subsequent paper using the same empirical strategy and data from the National Longitudinal Survey of Youth, Reyes found that lead exposure also increased self-reports of antisocial and risky behaviors among children and teenagers.
Exposure to lead as a child increased school suspensions and juvenile delinquency
Finally, more recent evidence using individual data on school kids in the 1990s and 2000s shows that even lower levels of lead exposure matter.
Anna Aizer and Janet Currie consider the effects of lead exposure on antisocial behavior in school and juvenile delinquency, using data from Rhode Island. This state is ideal for studying the effects of lead exposure because most children in the state have lead screenings (about 70 percent are screened, relative to a national average of 22-30 percent).
The researchers compare otherwise-similar children who happened to live in areas with high or low lead levels in the soil, using the location of busy roads as a natural experiment. Since car exhaust used to contain lead, soil near those roads is still contaminated with higher lead levels than is soil in other nearby areas. In addition, because lead contamination decreased over time, young children who lived near the busy roads in the 1990s were more exposed to lead in the soil than were those who lived in the same places in the 2000s. Controlling for what happens to this second group of kids (those born later) helps account for any other differences between the types of kids who live near busy versus less-busy roads. And controlling for what happens to kids living near less-busy roads at the same time controls for other neighborhood factors that might typically be correlated with lead exposure – things like school quality, access to social services, and levels of air pollution.
Aizer and Currie found that, as expected, kids who lived near busier roads during the 1990s had higher blood-lead-levels (BLLs) in preschool. In turn, those higher BLLs led to more disciplinary infractions and juvenile delinquency in school. The average BLL in their sample was 3.8 micrograms per deciliter (µg/dl); poorer children (those eligible for free school lunch) had average BLLs of 4.5 µg/dl compared with 3.0 µg/dl for students who were not eligible for free lunch. The researchers found that this seemingly small difference was large enough to have a big impact on antisocial behavior. For boys, just a 1-unit increase in BLLs increased the likelihood of a school suspension by 6 percent, and the likelihood of being detained in a juvenile correctional facility (a relatively rare event) by 57 percent. There was no significant effect on girls.
In a separate paper with co-authors Peter Simon and Patrick Vivier, Aizer and Currie find that lead exposure also reduces children’s future test scores.
Another recent study, by Hans Grönqvist, J. Peter Nilsson, and Per-Olof Robling, uses rich administrative data from Sweden to investigate the effects of removing lead from gasoline in a relatively low-lead-exposure environment (similar to the present-day United States). It finds that even BLLs above 5 µg/dl have long-term detrimental effects on boys’ academic and crime outcomes. It also finds that the detrimental effects of lead operate primarily by degrading “noncognitive skills” such as the ability to remain focused, tolerate stress, and take responsibility for one’s actions.
These findings suggest that lead exposure could have detrimental effects on longer-run outcomes such as academic achievement and employment – which could, in turn, increase criminal activity further.
What about other environmental toxins?
Exposure to lead appears to have long-term, detrimental effects that lead to more criminal behavior down the road. This raises the question of whether there are other, similar toxins that have such effects. Indeed, there is growing evidence that exposure to a variety of environmental toxins in everyday air pollution affects criminal behavior. Perhaps most striking, such exposure is detrimental enough that it affects behavior in real time. Studies that exploit changes in wind direction to measure the effect of exposure find increases in violent crime in the places where the wind is blowing pollution (for instance, from a highway or factory) on a given day. These effects are in addition to whatever longer-run effects air pollution has on behavior through such mechanisms as brain development.
What can we do to reduce these effects?
Investing in removing lead from such sources as topsoil, paint in older homes, and drinking water would have a variety of benefits. The evidence summarized above shows that such investment would likely yield large reductions in violent crime in the future. There would be other benefits too, such as increases in cognitive function and academic achievement, reductions in mortality, and increases in fertility.
Lead remediation is costly. But violent crime is costly, too. Stephen Billings and Kevin Schnepel consider the effects of lead remediation on house prices. They estimate that each $1 spent on lead remediation generates $2.60 in benefits. While house prices incorporate the private benefits (to the homeowner) of lead remediation, they may not incorporate public benefits, such as a reduction in the social costs to potential crime victims and a reduction in the level of law enforcement that would be required if criminal behavior falls. For this reason, the benefits of lead remediation are likely much larger than what Billings and Schnepel estimate.
There are also ways to mitigate the negative effects for kids who have already been exposed to lead. In a different paper, Billings and Schnepel consider the effects of a CDC-recommended intervention for children with high BLLs – above 10 µg/dl during the period they were considering, when interventions were offered to children born between 1990 and 1997. (CDC guidance has since been adjusted to clarify there is no level of lead exposure that is considered safe; its latest intervention recommendations are available here.) The intervention studied by Billings and Schnepel included education for caregivers (including nutritional advice and information about reducing lead exposure in the home), a voluntary home environment investigation, and a referral to lead remediation services.
It turns out that BLL tests are quite noisy, due in part to the type of test used. In order to qualify for this intervention, kids who tested above 10 µg/dl were asked to take a second, confirmatory test. Only those who tested above 10 µg/dl twice were eligible for the program. Billings and Schnepel use this noise to their advantage: Kids who tested above 10 µg/dl once, but just under that threshold in the second test, were probably very similar to those who scored just above 10 µg/dl in both tests. If anything, the second group may have had slightly higher true levels of lead exposure (biasing their results toward finding worse outcomes for that group), but those differences are probably negligible. The researchers compare outcomes for these two groups – those with one test just over 10 µg/dl and one just under, versus those with two tests just over 10 µg/dl – to measure the effect of eligibility for the CDC program on longer-run outcomes.
They find that this intervention had big impacts! Eligibility for the intervention reduced antisocial behavior (including school suspensions, school-reported crimes, and adolescent criminal arrests); it may also have increased primary and middle school educational achievement. Their estimates suggest that the effects of lead exposure on antisocial behavior were essentially reversed by the intervention. For instance, eligibility for the program reduced the number of arrests for violent offenses by 14 percent, lowering the risk of “treated” kids to that of kids with BLLs under 3 µg/dl. This program is cost-effective, but less cost-effective than lead remediation: Each $1 invested in the program yielded a return of at least $1.40.
Lead exposure continues to be a threat in communities across the United States. A crisis of lead-contaminated drinking water in Flint, Michigan, has made headlines for years. And the New York City Housing Authority recently revealed that 9,000 of its apartments housing young children are contaminated by lead paint.
There is a growing body of high-quality evidence showing that such exposure creates big problems. Kids’ exposure to lead in topsoil, paint, and drinking water leads to large increases in antisocial and criminal behavior in the future; these behavioral effects are costly to those individuals and to their communities. Lead remediation is therefore a highly cost-effective crime-reduction strategy. It would also likely yield large benefits in terms of increasing academic achievement and reducing mortality. While we are working on lead remediation, investing more resources in mitigation strategies such as the CDC-recommended intervention for children with high BLLs would also be a smart investment that would yield lower crime rates down the road.
Some have argued that removal of lead from gasoline caused the large decline in violent crime rates in the U.S. during the 1990s. The jury is still out on whether that is the case – my hunch is that lead was one of many contributing factors. But regardless of what drove crime trends three decades ago, we know from this growing body of research that lead exposure is an important driver of criminal and delinquent behavior today.
Data on lead levels in drinking water are available from the EPA, provided in an easy-to-access format by researchers at the Rockefeller Foundation: https://public.tableau.com/profile/rf.data#!/vizhome/IllustrativeLeadDashExtracted/DataWithThumbnail.