A recent and interesting article at the investigative MotherJones magazine discusses links between crime rates and the prevalence of lead emissions, touting the latter as America's Real Criminal Element. A follow-up also noted interesting connections involving lead paint.
"growing body of research linking lead exposure in small children with a whole raft of complications later in life, including lower IQ, hyperactivity, behavioral problems, and learning disabilities."
The same study used MRI-scanning to determine the effect of lead poisoning on the whole brain. Neuronal loss was found in various areas such as the cognitive and emotional areas of the anterior cingulate cortex, regions associated with "general intellectual and executive functioning, antisocial behaviors, and attention deficit hyperactivity disorder (ADHD)". Poor scores were found in physical movement tests too, but by far the largest and most serious finding related to a permanent reduction in grey matter, specifically in the medial part of the pre-frontal cortex. According to one of the researchers, Kim Cecil, lead poisoning affected precisely the brain areas "that make us most human", as the pre-frontal cortex is associated with the brain's executive functions; emotion regulation, impulse control, attention, verbal reasoning and mental flexibility.
"does it trump drugs, poverty, urban gang warfare, education, and other such issues to the point that they account for a bare ten percent of the crime statistics? That's a harder case to make, partly because as Drum himself notes correlation is not causation: the fact, for instance, that falling crime follows a pattern of falling lead exposure doesn't rule out many other influences."