Assessing the impact of lead exposure is difficult if individuals select on the basis of their characteristics into environments with different exposure levels. We address this issue with data from when the dangers of lead exposure were still largely unknown, using new evidence on intelligence test scores for male World War Two U.S. Army enlistees linked to the households where they resided in 1930. Higher exposure to water-borne lead (proxied by urban residence and low water pH levels) was associated with lower test scores: going from pH 6 to pH 5.5, scores fell 5 points (1/4 standard deviation). A longer time exposed led to a more severe effect. The ubiquity of lead in urban water systems at this time and uncertainty regarding its impact mean these effects are unlikely to have resulted from selection into locations with different levels of exposure.
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