"Third, a city-level analysis such as ours does not suffer from a common problem with individual-level studies, namely that individuals exposed to lead typically come from poor neighborhoods with both low-quality housing and high crime rates. Common confounders of the lead-crime relationship in individual-level studies pose less of a problem in our city-level study, where in most cases the entire city population was exposed to lead through water."
Paper's here: http://scholar.harvard.edu/files/jfeigenbaum/files/feigenbau...