I think the point they're making is that, once everyone is exposed to less lead, the crime rate should flatten out. If the crime rate continues to decrease eve

I think the point they're making is that, once everyone is exposed to less lead, the crime rate should flatten out. If the crime rate continues to decrease even though the relative number of young people with high lead intake doesn't change (because there aren't many of them) then there is likely to be an alternative explanation.