If childhood lead exposure really did produce criminal behavior in adults, you’d expect that in states where consumption of leaded gasoline declined slowly, crime would decline slowly too. Conversely, in states where it declined quickly, crime would decline quickly. And that’s exactly what she found.
Drum’s whole article is well worth reading, although I wonder if this debate will go the way of other economists’ unconventional explanations of crime.
Update: Tyler Cowen links to the other side of the debate.