The variables producing lowered birthweight include prematurity, maternal genetic tendencies, low prepregnancy weight, low weight gain during pregnancy, pathologies of pregnancy, multiple births, maternal malnutrition, and intrauterine growth retardation. Reductions in intelligence related to low birthweight are distributed equally among different socioeconomic groups. However, when mental subnormality is used as the criterion of impairment, low socioeconomic status is a better predictor of poor outcome than low birthweight. Even when general intelligence is not impaired, low birthweight children show specific cognitive, perceptual and behavioral signs of organic brain damage. In the absence of frank congenital abnormality, absolute birthweight is a better predictor of impaired intelligence than intrauterine growth retardation. Maternal malnutrition during the period of pregnancy does not influence offspring intelligence. Malnutrition throughout the maternal and offspring lifespan does impair intelligence, especially when surrounding social conditions are counterproductive to its development.