What was an effect of the massive fires that hit several cities in the early twentieth century?

In the second half of the nineteenth century, cities and towns across the United States built and expanded municipal waterworks. The number of waterworks grew more than tenfold between 1870 and 1896, with more than one thousand new systems added from 1890 to 1896 alone (Cutler and Miller 2006, p. 169). Sanitation engineers, city officials, and urban boosters alike greeted the new water systems with enthusiasm. The presence of waterworks signaled that city dwellers lived in a “respectable community” (Melosi 2000, p. 82). Urban residents across the country witnessed “an immense change in the standard of living” (Baker, 1897). The introduction of new waterworks eliminated the time and labor required to draw well water and improved fire protection. It also marked the abandonment of badly polluted surface wells (Baker, 1897, Troesken, 2006). Contemporaries observed reductions in typhoid that recent scholarship has shown were causally linked to the introduction of waterworks (Cutler and Miller, 2005, Alsan and Goldin, 2014, Troesken, 1999, Melosi, 2000). These benefits, however, did not accrue evenly across cities. In many cities, the very pipes installed to improve urban life and health leached noxious particulates into the local water supply.

A growing body of evidence in the social and medical sciences traces high crime rates to high rates of lead exposure. Scholars have shown that lead exposure and crime are positively correlated using data on individuals, cities, counties, states, and nations. Reyes (2007) exploits state-specific reductions in lead exposure due to the Clean Air Act to estimate the effect of lead emissions from gasoline on violent crime. She reports that reductions in childhood lead exposure in the 1970s and 1980s accounted for more than half of the violent crime decline of the 1990s.2 Stretesky and Lynch (2001) estimate that, from 1989 to 1991, counties with air lead levels equivalent to .17 μg/m3 had homicide rates four times as high as counties with air lead levels equivalent to 0 μg/m3. Mielke and Zahran (2012) show that air lead and aggravated assault rates were strongly associated in a panel of U.S. cities. Longitudinal studies of individuals document a positive relationship between pre- and post-natal lead exposure and delinquency (Dietrich et al., 2001) and arrests for violent offenses (Wright et al., 2008). Cross-sectional research on individuals (Denno, 1990, Needleman et al., 1996, Needleman et al., 2002) and counties (Stretesky and Lynch, 2004), studies using cross-national panel data (Nevin, 2007), and analyses of national time-series (Nevin, 2000) have yielded similar results.

To date, the strength of the literature on lead exposure and crime lies in the fact that its findings have been replicated at several scales. However, with the exception of Reyes, 2007, Reyes, 2015, few previous studies report estimates that can be considered causal, as researchers for obvious ethical reasons cannot randomly expose humans to lead.3 Credible sources of exogenous variation in lead exposure, meanwhile, are difficult to find. In this article, we set previous estimates of the lead–crime relationship on firmer causal footing by exploiting exogenous variation in the historical distribution of lead water pipes and the acidity of city water.

Because the mechanisms linking lead exposure and crime are biochemical, lead's effects should be observable not only in the late twentieth and early twenty-first centuries, but in earlier periods as well. Reassuringly, scholars studying the historical effects of lead exposure on outcomes other than crime have found evidence supporting this claim. Army enlistees who lived in cities whose water absorbed high levels of lead in 1930, for instance, scored comparatively low on the Army General Classification Test (Ferrie et al., 2012). Cities and towns with high concentrations of water lead at the turn of the twentieth century also had higher infant mortality rates than otherwise similar cities (Troesken, 2008, Clay et al., 2014).

We study the lead–crime relationship using historical data on the water supply of U.S. cities in the late nineteenth century and data on homicide between 1921 and 1936, when the first generation of children exposed to lead through water had reached adulthood. Lead exposure has the most detrimental effects on developing children, whose gut absorbs more lead than the adult gut and whose central nervous system is more sensitive to toxicants than the mature central nervous system (Silbergeld, 1997, Lidsky and Schneider, 2003, Needleman, 2004). For this reason, Mielke and Zahran (2012), Nevin (2000), Nevin (2007), and Reyes, 2007, Reyes, 2015, study the relationship between lead exposure and crime rates roughly 20 years later. The plurality of water systems in our data whose years of construction we know were installed in the 1880s and the 1890s. Thus, in most cities, the first children to have suffered the consequences of early lead exposure would have entered adulthood in roughly the first and second decades of the twentieth century. Water lead was the primary source of lead exposure in the early 20th century (Troesken, 2006, Clay et al., 2014) and service pipes “were the primary source of lead in drinking water” (Troesken 2008, p. 555).

Studying the historical effects of lead exposure has three advantages. First, lead levels in city water were determined by two plausibly exogenous sources: a city's rail distance from the nearest lead refinery and the acidity of its water. The dangers of lead were not widely understood in the period we study, and there is little evidence that city officials used information about lead's effects on health or crime to decide whether to use lead pipes (Troesken, 2006, Clay et al., 2014).4 Second, city-dwellers were exposed to much higher doses of lead historically than they are today (Troesken, 2006). Many of the cities we study had historical lead levels far exceeding the current Environmental Protection Agency (EPA) standard for water (Troesken, 2006, Clay et al., 2014).5 The historical effects of lead consumed in drinking water contaminated by inflowing service pipes should consequently be larger than the contemporary effects of ingested lead paint chips or inhaled gasoline exhaust. Troesken (2008, p. 555) finds that the use of lead water pipes in cities with acidic water increased infant mortality rates three- to fourfold. “Such estimates are quite plausible,” he concludes, “when one considers how much lead could be dissolved into household tap water as a result of lead service lines.”

Third, lead exposure today is not uniformly distributed within cities. In the late twentieth and early twenty-first centuries, poor children were more likely than middle-income or rich children to come into contact with lead (Brooks-Gunn and Duncan, 1997). This empirical regularity makes it difficult to disentangle the effects of lead exposure from the effects of individual or neighborhood poverty (Bellinger, 2008). Studying cities in the early twentieth century enables us to circumvent this problem because in most cases the entire city population was exposed to lead through water (Troesken, 2004, Clay et al., 2014). Our analysis compares cities that used lead water pipes to cities that did not rather than comparing individuals whose exposure to lead might be correlated with other causes of crime. Like Reyes, 2007, Reyes, 2015, we exploit exogenous variation in lead exposure to estimate the effects of lead on aggregate-level crime rates. However, our variation in lead exposure comes from a different source. The cities that used lead water pipes in the late nineteenth century are not the same cities that had the highest concentrations of lead deposited from gasoline in the mid-to-late twentieth century.6

To study the effect of lead exposure on homicide, we use two separate identification strategies.7 First, we report estimates of the effect of cities' use of lead pipes instrumented by their distance by rail from the nearest lead refinery. Because transportation costs were a major consideration in city officials' decisions about whether to use lead or iron service pipes (Clay et al. 2014, p. 460), cities' rail distance from the nearest lead refinery is a strong predictor of whether they used lead. Second, we adopt an identification strategy introduced by Troesken (2006) and Clay et al. (2014) that exploits the fact that more lead will leach into water with a low pH. Among cities using lead pipes, cities with more acidic water should consequently have had higher homicide rates than cities with more basic water. In addition, we show that cities that used lead pipes had higher rates of death from cirrhosis, infant diarrhea, influenza, and scarlet fever, but not other causes of death. Previous scholarship has linked lead exposure to cirrhosis (Troesken 2006, p. 45-46) and infant mortality (Troesken, 2006, Troesken, 2008, Clay et al., 2014), but not scarlet fever and influenza. With the exception of the latter results, these findings show that cities using lead water pipes had higher rates of death only from causes known to be linked to lead exposure, and not from causes associated with poverty, like tuberculosis and typhoid. These three strategies, along with our baseline results, yield a range of estimates of the effect of lead exposure on homicide. Taken together, they suggest that using lead service pipes increased cities' homicide rates considerably. Different estimation strategies and sample restrictions yield substantively similar results.

Lead exposure, cognition, and behavior

Lead exposure impairs brain development and interferes with neurotransmitter systems (Reyes 2015, p. 1583). Previous research has linked the biochemical and neurological effects of lead exposure to impulsivity, behavioral problems, hyperactivity, and impaired cognition, all of which are associated with crime. Cecil et al. (2008) find that childhood blood lead is negatively correlated with adult brain volume in regions associated with executive function, ADHD, and childhood behavioral problems.

Measuring lead exposure and homicide in the early twentieth century

We collect information on the historical pipe metal used by all U.S. cities for which data are available. In prior research, Clay et al. (2014) compiled data on the pipe metals used in a sample of municipal water systems in the late nineteenth century. We supplement these data with additional information on water pipes drawn from The Manual of American Water-Works (Baker, 1897). In total, we have information on the type of metal used in the water pipes of 591 cities. In the results presented

Which cities used lead pipes?

An important concern in a cross-sectional study such as ours is that the cities that installed lead pipes in the nineteenth century might have differed from the cities that did not in unobserved ways that were correlated with their homicide rates. Based on available data on the characteristics of the cities in our sample, we find little evidence that the cities that used lead pipes differed in observable ways from the cities that did not. Although larger cities, denser cities, and cities with

Estimating the effect of lead exposure

We use count data regressions to model annual city-level homicides. Count models such as Poisson and negative binomial models are commonly used in crime and mortality research using non-negative integer data because they are more efficient than incorrectly specified linear models. Although the point estimates from linear models of count data will be consistent, the covariance matrix will not, making inference unreliable (Grogger, 1990).

Both homicide counts and homicide rates are highly

Results

Fig. 6 presents our main results including all covariates. The dots represent point estimates from negative binomial regressions of the number of homicides in each city on a variable indicating whether the city used pipes with some lead. We estimate these regressions separately in every year of our sample. Bars around the estimates represent 95% confidence intervals. Based on the lowest and highest point estimates, cities that used lead pipes had between 14 and 36% higher homicide rates than

Conclusion

In this article, we draw on variation in the use of lead service pipes in the late nineteenth century to estimate the effect of lead exposure from water on city-level homicide rates from 1921 to 1936. Our baseline negative binomial estimates imply that cities that used lead water pipes had homicide rates that were 24% higher than cities that did not. These results are robust to instrumenting a city's use of lead pipes with its rail distance from the nearest lead refinery. We found evidence

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