What's the point of this comment? I wasn't trying to make a point I was answering the GP's question (which was a good question). The lead hypothesis is common and was examined in the article in more detail. The main reason I added the second link (which included the lead hypothesis) was because it contained the most data I could find - it went back to 1876. The fact it was graphed with lead exposure wasn't at all important, the murder rate data going back to the 1800s was the important part and it answered the question.