While the lead theory is interesting and may lend some insight to behavior, it can't necessarily be used that way and still be considered a scientific study. It's pure correlation. Unless an increase in lead can be proven, somehow, to make a criminal out of someone that wasn't otherwise a criminal, it's not scientific. The scientific method exists for a reason and there is no control in these studies.
Also, if we're to believe that lead is the culprit for crime and it's based on a 17 year (17 on the chart, 23 in the text) lag, you can't use that to explain why some demographics are huuuuuuugely disproportionately responsible for crime. The crime rate in 17-23 year olds is always found to be higher than that of older people. How can you explain that if it's a chemical reaction?
So it could have an impact just as there likely is a such thing as attention deficit disorder. The difference is some people will feel the struggles of a child with ADD and give up while some people will work harder to overcome the disability and succeed. There is still other factors - even if you consider a predisposition to being a turd.