I know that hookworm played a significantly role in the US south, and after its eradication things improved every single year since. Hookworm is still a major problem in most of Africa, though. Infection with hookworm in childhood results in permanent IQ loss, regardless of whether it is later treated, the damage remains.
My personal opinion is that we should not necessarily expect the actual monetary scale of the difference to accurately predict the magnitude of the violent crime. Most likely, the economic disparity is a proxy for something else that does not scale the same as the monetary values do. Namely control. Economic disparity always coexists with a small group exerting control upon a larger one to extract the value they create and take it for themselves. Humans always respond negatively to control - both being controlled and exerting control on others. They react pretty predictably, too. They assert control over themselves and their own bodies, then seek to establish status by being seen as dominant in their peer group, then seek to establish an alternative power hierarchy from the overaching order, then engage in plays for dominance between those alternative hierarchies (gangs), then eventually the gangs merge and become an army who goes to war against the overarching order. It happens in families, in schools, in prisons, in militaries, in nations, everywhere. Different scales, and different manifestations, but control is a poison to humans.
