Any condition that affects normal development of the brain can cause mental deficiency. Physical damage to the brain of lack of oxygen (anoxia) during birth can result in intellectual impairment. Some types of mental deficiency are caused by specific genetic defects that alter the body metabolism.
PKU, which we discussed earlier, is one such defect, but it is relatively rare. Somewhat more common is Down’s syndrome (mongolism), a severe form of mental deficiency accompanied by physical abnormalities.
Some children with Down’s syndrome learn to speak a little and to master simple chores. For most, however, speech is absent or confined to a few hoarsely spoken sounds and skills beyond those basic to elementary self-care are seldom attained. Mongoloid children tend to be affectionate and docile.