<P> Among children, the cause of intellectual disability is unknown for one - third to one - half of cases . About 5% of cases are inherited from a person's parents . Genetic defects that cause intellectual disability but are not inherited can be caused by accidents or mutations in genetic development . Examples of such accidents are development of an extra chromosome 18 (trisomy 18) and Down syndrome, which is the most common genetic cause . Velocardiofacial syndrome and fetal alcohol spectrum disorders are the two next most common causes . However, doctors have found many other causes . The most common are: </P> <Ul> <Li> Genetic conditions . Sometimes disability is caused by abnormal genes inherited from parents, errors when genes combine, or other reasons . The most prevalent genetic conditions include Down syndrome, Klinefelter syndrome, Fragile X syndrome (common among boys), neurofibromatosis, congenital hypothyroidism, Williams syndrome, phenylketonuria (PKU), and Prader--Willi syndrome . Other genetic conditions include Phelan - McDermid syndrome (22q13del), Mowat--Wilson syndrome, genetic ciliopathy, and Siderius type X-linked intellectual disability (OMIM 300263) as caused by mutations in the PHF8 gene (OMIM 300560). In the rarest of cases, abnormalities with the X or Y chromosome may also cause disability . 48, XXXX and 49, XXXXX syndrome affect a small number of girls worldwide, while boys may be affected by 49, XXXXY, or 49, XYYYY. 47, XYY is not associated with significantly lowered IQ though affected individuals may have slightly lower IQs than non-affected siblings on average . </Li> <Li> Problems during pregnancy . Intellectual disability can result when the fetus does not develop properly . For example, there may be a problem with the way the fetus' cells divide as it grows . A pregnant person who drinks alcohol (see fetal alcohol spectrum disorder) or gets an infection like rubella during pregnancy may also have a baby with intellectual disability . </Li> <Li> Problems at birth . If a baby has problems during labor and birth, such as not getting enough oxygen, he or she may have developmental disability due to brain damage . </Li> <Li> Exposure to certain types of disease or toxins . Diseases like whooping cough, measles, or meningitis can cause intellectual disability if medical care is delayed or inadequate . Exposure to poisons like lead or mercury may also affect mental ability . </Li> <Li> Iodine deficiency, affecting approximately 2 billion people worldwide, is the leading preventable cause of intellectual disability in areas of the developing world where iodine deficiency is endemic . Iodine deficiency also causes goiter, an enlargement of the thyroid gland . More common than full - fledged cretinism, as intellectual disability caused by severe iodine deficiency is called, is mild impairment of intelligence . Certain areas of the world due to natural deficiency and governmental inaction are severely affected . India is the most notorious, with 500 million suffering from deficiency, 54 million from goiter, and 2 million from cretinism . Among other nations affected by iodine deficiency, China and Kazakhstan have instituted widespread salt iodization programs, whereas, as of 2006, Russia had not . </Li> <Li> Malnutrition is a common cause of reduced intelligence in parts of the world affected by famine, such as Ethiopia . </Li> <Li> Absence of the arcuate fasciculus . </Li> </Ul> <Li> Genetic conditions . Sometimes disability is caused by abnormal genes inherited from parents, errors when genes combine, or other reasons . The most prevalent genetic conditions include Down syndrome, Klinefelter syndrome, Fragile X syndrome (common among boys), neurofibromatosis, congenital hypothyroidism, Williams syndrome, phenylketonuria (PKU), and Prader--Willi syndrome . Other genetic conditions include Phelan - McDermid syndrome (22q13del), Mowat--Wilson syndrome, genetic ciliopathy, and Siderius type X-linked intellectual disability (OMIM 300263) as caused by mutations in the PHF8 gene (OMIM 300560). In the rarest of cases, abnormalities with the X or Y chromosome may also cause disability . 48, XXXX and 49, XXXXX syndrome affect a small number of girls worldwide, while boys may be affected by 49, XXXXY, or 49, XYYYY. 47, XYY is not associated with significantly lowered IQ though affected individuals may have slightly lower IQs than non-affected siblings on average . </Li> <Li> Problems during pregnancy . Intellectual disability can result when the fetus does not develop properly . For example, there may be a problem with the way the fetus' cells divide as it grows . A pregnant person who drinks alcohol (see fetal alcohol spectrum disorder) or gets an infection like rubella during pregnancy may also have a baby with intellectual disability . </Li>

A genetic disorder that causes a moderate to severe mental disability is called