We've known that lead exposure as a young child hinders brain development, resulting in reduced IQs and developmental problems that last a lifetime. Did the introduction of tetraethyl lead to gasoline in the 40s cause an increase in violent crimes over the last half of the 20th century? As always, correlation isn't causation, but there is a fair amount of research done during the last ten years shows a correlation between lead exposure and increased crime rates on local, regional, and global levels.