Crazy as it seems, it looks as if lead poisoning could be the major cause of the rise and fall of violent crime. Lead is so toxic that it is unsafe at any level and in Europe it has been banned from many products, including paint and petrol, since the 1990s. Lead poisoning in infancy, even at very low levels, impairs the development of those parts of the brain that regulate behaviour and mood. The effect is stronger in boys than in girls. Lead poisoning is associated with attention deficit disorder, impulsiveness, aggression and, some think, psychopathy. Studies between cities, states and nations show that the rise and fall in violent crime follows, with a roughly 20-year lag, the rise and fall in the exposure of infants to trace quantities of lead. Virtually all studies agree there is a strong correlation.