Lead can be absorbed into bones, teeth and blood. It causes kidney damage, inhibits body growth, causes abdominal pain, anemia and can damage the nervous system. Studies have also shown it causes headaches, lowers IQ and appears to affect impulsivity, attention and personal self-control.

But does it cause violent crime?

It is well documented that there was an unexpected decline in violent crime in industrial societies that began in the 1990s. Scholars have hypothesized that the diminished presence of lead after the 1970s is a major cause of this decline. It was in the 1990s that the first group of lead free kids reached adulthood.

Not everyone accepts this theory. Many argue that correlation does not equal cause. Proponents of the theory acknowledge this concern and clarify that while lead is not the only explanation for the fall in violent crime, it is certainly the most persuasive. Even though the theory will never be proved (nobody is going to poison children with lead to see if they grow to be criminals later in life) it does have practical implications and police and policymakers have taken notice.