I understood his claim a bit differently, namely as &quot;Lead is an epiphenomenon, and a single variable with a big effect is likely to be an epiphenomenon&quot;.<p>To be sure, I think the logic goes like this:<p>- Lead is positively correlated with crime<p>- Reduction in lead-levels is associated with multiple factors that are in turn associated with gentrification<p>- Thus, these gentrification factors are the true cause, and lead-levels just happen to correlate with them.