In the second half of the nineteenth century, many American cities built water systems using lead or iron service pipes. Municipal water systems generated significant public health improvements, but these improvements may have been partially offset by the damaging effects of lead exposure through lead water pipes. We study the effect of cities’ use of lead pipes on homicide between 1921 and 1936. Lead water pipes exposed entire city populations to much higher doses of lead than have previously been studied in relation to crime. Our estimates suggest that cities’ use of lead service pipes considerably increased city-level homicide rates.
That’s from Feigenbaum and Muller in Lead Exposure and Violent Crime in the Early Twentieth Century. Lead, it ain’t just about Flint.