Joel Nigg of Oregon Health & Science University in Portland points to lead as an ideal candidate to trigger the developmental disorder called ADHD.

In his article, published in Current Directions in Psychological Science, Nigg offers a causal model in which lead attaches to sites in the brain's striatum and frontal cortex, and acts on the genes in these regions -- causing them to turn on or remain inactive. This disruption in brain activity alters cognitive control, and in turn results in hyperactivity and lack of vigilance.