mental disability

Arrested or incomplete development of mental capacities. It can be very mild, but in more severe cases is associated with social problems and difficulties in living independently. A person may be born with a mental disability (for example, Down's syndrome) or may acquire it through brain damage. Between 90 and 130 million people in the world suffer from such disabilities.

Clinically, mental disability is graded as profound, severe, moderate, or mild, roughly according to IQ and the sufferer's ability to cope with everyday tasks. Among its many causes are genetic defect (phenylketonuria), chromosomal errors (Down's syndrome), infection before birth (rubella) or in infancy (meningitis), trauma (brain damage at birth or later), respiratory difficulties at the time of birth, toxins (lead poisoning), physical deprivation (lack of, or defective, thyroid tissue, as in cretinism), and gross psychological deprivation. No clear cause of disability can be established for more than half of individuals with an IQ of less than 70.