Intellectual Impairment is a term for a pattern of persistently slow learning of basic motor and language skills ("milestones") during childhood, and a significantly below-normal global intellectual capacity as an adult. One common criterion for diagnosis of mental retardation is a tested intelligence quotient (IQ) of 70 or below. People may be described as having developmental disabilities, global developmental delay, or learning difficulties.

Children with developmental disabilities may learn to sit up, to crawl, or to walk later than other children, or they may learn to talk later.

Both adults and children with intellectual disabilities may
• have trouble speaking
• find it hard to remember things
• have trouble understanding social rules
• have trouble discerning cause and effect
• have trouble solving problems
• have trouble thinking logically.

CAUSES

Down syndrome, fetal alcohol syndrome and Fragile X syndrome the three most common inborn causes. However, doctors have found many other causes. The most common are:

Genetic conditions - Sometimes disability is caused by abnormal genes inherited from parents, errors when genes combine, or other reasons. Examples of genetic conditions include Down syndrome, Fragile X syndrome and phenylketonuria (PKU).
Problems during pregnancy. Mental disability can result when the fetus does not develop inside the mother properly. For example, there may be a problem with the way the fetus's cells divide as it grows. A woman who drinks alcohol (see fetal alcohol syndrome) or gets an infection like rubella during pregnancy may also have a baby with mental disability.

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.

Health problems - Diseases like whooping cough, measles, or meningitis can cause mental disability. It can also be caused by not getting enough medical care, or by being exposed to poisons like lead or mercury.

Iodine deficiency, affecting approximately 2 billion people worldwide, is the leading preventable cause of mental 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 retardation 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 outstanding, 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 begun taking action, while Russia has not.
Malnutrition is a common cause of reduced intelligence in parts of the world affected by famine such as Ethiopia.

The use of forceps during birth can lead to intellectual disability in an otherwise normal child. They can fracture the skull and cause brain damage.

Institutionalization at a young age can cause intellectual disability in normal children.

Sensory deprivation in the form of severe environmental restrictions (such as being locked in a basement), prolonged isolation, or severe atypical parent-child interactions.

Psycho-social disadvantage - Contributing factors are lack of reading material, use of language not common in that community, poor diet, poor health practices, and poor housing.