The largest study, of 1000 black children in Philadelphia, showed that both lead levels and anaemia were predictors of the number of juvenile offences, the seriousness of juvenile offences and the number of adult offences, for males. Here Masters cites a wealth of studies showing how lead and manganese cause changes in the development of the brain, and in the functioning of neurotransmitters in the brain. Lead in the brain damages glia, a kind of cell associated with inhibition and detoxification. Manganese lowers levels of serotonin and dopamine, which are neurotransmitters associated with impulse control and planning. Masters emphasises that children who are raised from birth on infant formula and are not breast fed will absorb five times as much manganese as breast-fed infants.
Calcium deficiency increases the absorption of manganese.
Masters says toxic metals affect individuals in complex ways. For example, because lead diminishes a person's normal ability to detoxify poisons, lead may heighten the effects of alcohol and drugs. For the neurotoxicity hypothesis to hold up, individuals must receive doses of toxic metals sufficient to be associated with violent behaviour. Masters argues that, despite recent significant decreases in lead in the environment, in neighbourhoods where automobile traffic has historically been high, and in towns where industries have released large quantities of toxic metals for years, many local soils still contain toxic quantities of lead, cadmium and manganese sufficient to poison children who play in the dirt. He also says that ageing water delivery systems very likely contribute lead and manganese.