Perhaps another study should be done following individuals. It seems a decent attempt at disproving the hypothesis.
Take say 50 specific individuals ... actually look at where they lived each year of their lives, and estimate as accurately as possible their lead exposure. If they move cross country, "follow" them and estimate the change in lead exposure accordingly.
Once you have an assessment of the lifetime lead exposure of the 50 people, see if it predicts their levels of criminal behavior or aggression.
Crime is rare enough that you wouldn't get any sort of conclusive result from the variations in lead level among 50 people.
(Example: suppose 0.1% of people become murders in their lives, and lead is 100% responsible for every murder, and you had 2 groups of 50 people who had either tons of lead or no lead exposure at all; what will the result be? Likely, no murderers in your total group of 100 at all and certainly no difference between the two groups - despite extremely generous assumptions!)
Take say 50 specific individuals ... actually look at where they lived each year of their lives, and estimate as accurately as possible their lead exposure. If they move cross country, "follow" them and estimate the change in lead exposure accordingly.
Once you have an assessment of the lifetime lead exposure of the 50 people, see if it predicts their levels of criminal behavior or aggression.