Careless use of pesticides in agriculture and fish-farming is shown to have caused intellectual disabilities. Malnutrition has been an accepted cause of intellectual decline since the 1970s. High-yield Green Revolution crops were introduced in poorer countries in the 1960s to overcome famine. But these are now blamed for causing intellectual deficits because they do not take up essential micronutrients. They have also displaced other nutritious indigenous food sources.
Cures are self-defeating.
Poland uses crop varieties that take up less heavy metals from contaminated soils. But these varieties also take up less essential minerals. Resultant deficiencies can cause intellectual impairments directly, and synergistically by increasing the human uptake of toxins from other sources. In the 1970s, Herbert Needleman studied the sub-clinical impacts of lead in 3000 US children. This revealed a strong link between high body-burdens of lead, and behavioural and intellectual difficulties recorded by teachers.