If there was causation (due to the neurological effects of long-term lead exposure during childhood years), then you would observe delayed correlation (at a 20 year scale), not simultaneous correlation.
"There is a substantial causal relationship," she says.
"I can see it in the state-to-state variations. States
that experienced particularly early or particularly sharp
declines in lead experienced particularly early or
particularly sharp declines in violent crime 20 years later."
The other group of people we find this in is those suffering from childhood lead exposure, and we know that the regions of the brain most affected are those which occur from lead exposure.
While not going so far as to imply everyone with this pattern becomes a criminal, it certainly increases the statistical likelihood that they are, and studies on people with extreme issues - i.e. psychopaths - tend to show that's its a pattern of life experiences which determine whether you learn to function in society, or go another path. Notable is the neuroscientist who was studying psychopaths and discovered he has the same sort of brain structure as one - but, is a scientist, married and has children.
Since we can't really control an individuals life experiences through policy, but they roughly sort out statistically based on socioeconomic status, it seems pretty clear that exposure to brain development inhibiting neurotoxins is going to give you a higher then expected fraction of criminally-inclined young adults.