Mental Retardation

Mental retardation (MR)

Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors. It has historically been defined as an Intelligence Quotient score under 70. Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environment. As a result, a person with a below-average intelligence quotient (BAIQ) may not be considered mentally retarded. Syndromic mental retardation is intellectual deficits associated with other medical and behavioral signs and symptoms. Non-syndromic mental retardation refers to intellectual deficits that appear without other abnormalities.

Mental retardation is a subtype of intellectual disability, although that term is now preferred by most advocates in most English-speaking countries as a euphemism for MR. However, intellectual disability is a broader concept and includes intellectual deficits that are too mild to properly qualify as mental retardation, too specific (as in specific learning disability), or acquired later in life, through acquired brain injuries or neurodegenerative diseases like dementia. Intellectual disabilities may appear at any age. Developmental disability is any disability that is due to problems with growth and development. This term encompasses many congenital medical conditions that have no mental or intellectual components, although it, too, is sometimes used as a euphemism for MR.



Signs and symptoms

The signs and symptoms of mental retardation are all behavioral. Most people with mental retardation do not look like they have any type of intellectual disability, especially if the disability is caused by environmental factors such as malnutrition or lead poisoning. The so-called "typical appearance" ascribed to people with mental retardation is only present in a minority of cases, all of which involve syndromic mental retardation.
Children with mental retardation 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 mental retardation may also exhibit some or all of the following characteristics:

      • Delays in oral language development
      • Deficits in memory skills
      • Difficulty learning social rules
      • Difficulty with problem solving skills
      • Delays in the development of adaptive behaviors such as self-help or self-care skills
      • Lack of social inhibitors

Children with mental retardation learn more slowly than a typical child. Children may take longer to learn language, develop social skills, and take care of their personal needs, such as dressing or eating. Learning will take them longer, require more repetition, and skills may need to be adapted to their learning level. Nevertheless, virtually every child is able to learn, develop and become a participating member of the community.
In early childhood, mild mental retardation (IQ 50–69, a cognitive ability about half to two-thirds of standard) may not be obvious, and may not be identified until children begin school. Even when poor academic performance is recognized, it may take expert assessment to distinguish mild mental retardation from learning disability or emotional/behavioral disorders. People with mild MR are capable of learning reading and mathematics skills to approximately the level of a typical child aged 9 to 12. They can learn self-care and practical skills, such as cooking or using the local mass transit system. As individuals with mild mental retardation reach adulthood, many learn to live independently and maintain gainful employment.

Moderate mental retardation (IQ 35–49) is nearly always apparent within the first years of life. Speech delays are particularly common signs of moderate MR. People with moderate mental retardation need considerable supports in school, at home, and in the community in order to participate fully. While their academic potential is limited, they can learn simple health and safety skills and to participate in simple activities. As adults they may live with their parents, in a supportive group home, or even semi-independently with significant supportive services to help them, for example, manage their finances. As adults, they may work in a sheltered workshop.
A person with severe or profound mental retardation will need more intensive support and supervision his or her entire life. They may learn some activities of daily living. Some will require full-time care by an attendant.


Among children, the cause is unknown for one-third to one-half of cases. Down syndrome, velocariofacial syndrome, and fetal alcohol syndrome are 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. The most prevalent genetic conditions include Down syndrome, Klinefelter's syndrome, Fragile X syndrome, 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 mental retardation (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 47, XYY, 49, XXXXY, or 49, XYYYY.
      • Problems during pregnancy. Mental 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 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.
      • Exposure to certain types of disease or toxins. Diseases like whooping cough, measles, or meningitis can cause mental disability if medical care is delayed or inadequate. Exposure to poisons like lead or mercury may also affect mental ability.
      • 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 instituted widespread iodization programs, whereas, as of 2006, Russia had not.
      • Malnutrition is a common cause of reduced intelligence in parts of the world affected by famine, such as Ethiopia.
      • Absence of the arcuate fasciculus.


According to the latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), three criteria must be met for a diagnosis of mental retardation: an IQ below 70, significant limitations in two or more areas of adaptive behavior (as measured by an adaptive behavior rating scale, i.e. communication, self-help skills, interpersonal skills, and more), and evidence that the limitations became apparent before the age of 18. It is formally diagnosed by professional assessment of intelligence and adaptive behavior.

IQ Below 70

The first English-language IQ test, the Terman-Binet, was adapted from an instrument used to measure potential to achieve developed by Binet in France. Terman translated the test and employed it as a means to measure intellectual capacity based on oral language, vocabulary, numerical reasoning, memory, motor speed and analysis skills. The mean score on the currently available IQ tests is 100, with a standard deviation of 15 (WAIS/WISC-IV) or 16 (Stanford-Binet). Sub-average intelligence is generally considered to be present when an individual scores two standard deviations below the test mean. Factors other than cognitive ability (depression, anxiety, etc.) can contribute to low IQ scores; it is important for the evaluator to rule them out prior to concluding that measured IQ is "significantly below average". The following ranges, based on Standard Scores of intelligence tests, reflect the categories of the American Association of Mental Retardation, the Diagnostic and Statistical Manual of Mental Disorders-IV-TR, and the International Classification of Diseases-10 [citation needed].

     Class                                       IQ

      Profound mental retardation              Below 20
      Severe mental retardation                  20–34
      Moderate mental retardation             35–49
      Mild mental retardation                       50–69
      Borderline intellectual functioning    70–84

Since the diagnosis is not based only on IQ scores, but must also take into consideration a person's adaptive functioning, the diagnosis is not made rigidly. It encompasses intellectual scores, adaptive functioning scores from an adaptive behavior rating scale based on descriptions of known abilities provided by someone familiar with the person, and also the observations of the assessment examiner who is able to find out directly from the person what he or she can understand, communicate, and the like.


Mental retardation affects about 2–3% of people. 75–90% of the affected people have mild retardation. Non-syndromic or idiopathic MR accounts for 30–50% of cases. About a quarter of cases are caused by a genetic disorder.