The studies I've read seem to indicate that lead exposure erodes impulse control which leads to people making very bad decisions they'd otherwise not make. So

The studies I've read seem to indicate that lead exposure erodes impulse control which leads to people making very bad decisions they'd otherwise not make. So it probably tilts things towards people committing more brazen crimes which lead to higher arrest rates, but also more crime and violence in general. The "all-else-being-equal" studies tend to focus on the primary source of lead: Leaded fuel. As many housing projects were built next to highways, people living there were exposed to significantly higher levels of atmospheric lead than in other more remote parts. When lead was banned from fuel, crime rates dropped, and you can see this play out in similar neighborhoods in different states where the phasing out happened at different times.