Mental retardation

Mental retardation (also called mental handicap and the UK Mental Health Act (1983) defines mental impairment and severe mental 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) below 70.

The term mental retardation has gradually acquired pejorative and shameful connotations over the last few decades, as is demonstrated frequently by the vandalizing responses of casual readers to this article. In North America, the term developmental delay has become an increasingly preferred synonym for mental retardation among parents and physicians, but in other contexts as well. Some people object to the term developmental delay as less precise, sometimes being used for specific rather than global delay, and less clearly denoting an expected continued cognitive impairment as an adult. Developmental disability is also preferred by some, but even more often is used to refer to specific rather than global disabilities as well. Intellectual disability is occasionally used as a synonym for mental retardation for the same reasons but also lacks specificity as it also applies to dementing conditions such as Alzheimer's disease. The American Association on Mental Retardation continues to use the term mental retardation [1] (

Although it can be defined objectively, mental retardation does not represent a single condition. Some of the difficulties of characterizing mental retardation more precisely are illustrated by comparing mental retardation to the condition of short stature, with which it shares all of the following characteristics:

  • Diagnostic criteria are defined statistically and arbitrarily.
  • There are many subgroups with distinguishable developmental patterns.
  • It is not a single, homogeneous disease; there are many known causes, both inherent and environmental, and congenital and acquired.
  • Different diagnostic criteria are used for different purposes.
  • More than one factor may contribute to retardation for any one person.
  • New conditions and causes are discovered or better understood each year.
  • Treatments can be very effective, marginally beneficial, or ineffective, varying by cause and age of intervention.
  • For a significant proportion of affected people, a cause cannot be determined.

The limitations of cognitive function will cause a child to learn and develop more slowly than a typical child. Children who have retardation may take longer to learn to speak, walk, and take care of their personal needs such as dressing or eating. They will have trouble learning in school. Learning will take them longer, require more repetition, and there may be some things they cannot learn. The extent of the limits of learning is a function of the severity of the retardation.

There are three criteria before a person is considered to have mental retardation: an IQ below 70, significant limitations in two or more areas of adaptive behavior (i.e., ability to function at age level in an ordinary environment), and evidence that the limitations became apparent in childhood. Down syndrome, fetal alcohol syndrome and fragile X are the three most common inborn causes of mental retardation.

By most definitions, mental retardation is more accurately considered a disability rather than a disease. Mental retardation can be distinguished in many ways from mental illness, such as schizophrenia or depression. There is no "cure" for established mental retardation, though with appropriate support and teaching most individuals with retardation can learn to do many things.


Signs of mental retardation

There are many signs of mental retardation. For example, retarded children may:

  • sit up, crawl, or walk later than other children;
  • learn to talk later, or have trouble speaking,
  • find it hard to remember things,
  • not understand how to pay for things,
  • have trouble understanding social rules,
  • have trouble seeing the consequences of their actions,
  • have trouble solving problems, and/or
  • have trouble thinking logically.

About 87 percent of mentally retarded people will only be a little slower than average in learning new information and skills. In early childhood, mild retardation (IQ 60-75) may not be obvious, and may not be diagnosed until they begin school. Even when poor academic performance is recognized, it may take expert assessment to distinguish mild mental retardation from learning disability or behavior problems. As they become adults, many people with mild retardation can live independently and may be considered by others in their community as "slow" rather than "retarded".

Moderate retardation (IQ 50-60) is nearly always obvious within the first years of life. These people will encounter difficulty in school, at home, and in the community. In many cases they will need to join specially separated classes, but they can still progress to become functioning members of society.

The remaining 13 percent of individuals with mental retardation score below 50 on IQ tests. A person with more severe retardation will need more intensive support and supervision his or her entire life.

Nevertheless, every retarded child is able to learn, develop, and grow to some extent. The capacity to experience joy and human relationships is not IQ-dependent.

How is mental retardation diagnosed?

Mental retardation is formally diagnosed by professional assessment of intelligence and adaptive behavior.

Intelligence is the ability of a person to learn, think, solve problems, and make sense of the world (called IQ or intellectual functioning). Intellectual functioning, or IQ, is usually measured by a test called an IQ test. The average score is 100. People scoring below 75 are often -- but not always -- mentally retarded. Since factors other than mental ability (depression, anxiety, lack of adequate effort, etc.)can yieled low IQ scores, it is important for the evaluator to rule them out prior to concluding that measured intelligence is "significantly below average".

Adaptive behavior, or adaptive functioning refers to the skills needed to live independently (or at the minimally acceptable level for age). To assess adaptive behavior, professionals compare the functional abilities of a child to those of other children of similar age. To measure adaptive behavior, professionals use instruments that are actually structured interviews, with which they systematically elicit information about the person's functioning in the community from someone that knows them well. There are many adaptive behavior scales, and accurate assessment of the quality of someone's adaptive behavior requires clinical judgement as well. Examples of the kinds of behaviors that are assessed by adaptive behavior scales Certain skills are important to adaptive behavior. These are:

Ranges of IQ

The following data based on the Wechsler Adult Intelligence Scale (WAIS) was used in 1958:

Today the following ranges are in standard use:

Profound mental retardationbelow 20
Severe mental retardation20-34
Moderate mental retardation35-49
Mild mental retardation50-69
Borderline deficiency70-79

Causes of mental retardation

Doctors have found many causes of mental retardation. The most common are:

  • Genetic conditions. Sometimes mental retardation 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 retardation can result when the baby does not develop inside the mother properly. For example, there may be a problem with the way the baby'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 retardation.
  • Problems at birth. If a baby has problems during labor and birth, such as not getting enough oxygen, he or she may have mental retardation.
  • Health problems. Diseases like whooping cough, the measles, or meningitis can cause mental retardation. Mental retardation can also be caused by extreme malnutrition (not eating right), not getting enough medical care, or by being exposed to poisons like lead or mercury.
  • Iodine deficiency is the leading, preventable cause of mental retardation in areas of the developing world where iodine deficiency is endemic.

Traditional terms

The three traditional terms denoting varying degrees of mental deficiency long predate psychiatry. They were originally used in English as simple forms of abuse, and this is still the main usage. Their now obsolete use as psychiatric technical definitions is of purely historical interest. There have been some efforts made among mental health professionals to discourage use of these terms. Note that the term retard or tard is still used as a generic insult, especially among children.

  • Idiot indicated the greatest degree of mental deficiency, where the mental age is 2 years or less, and the person cannot guard himself against common physical dangers. The term was gradually replaced by the term profound mental retardation.
  • Imbecile indicated a mental deficiency less extreme than idiocy and not necessarily inherited. It is now usually subdivided into two categories, known as severe mental retardation and moderate mental retardation.
  • Moron was defined by the American Association for the Study of the Feeble-Minded in 1910, following work by Henry H. Goddard, as the term for an adult with a mental age between eight and twelve; mild mental retardation is now the term for this condition. Alternative definitions of these terms based on IQ were also used.


  • American Association on Mental Retardation. "Mental Retardation. Definition, Classification, and Systems of Support, 10th Edition" (2002). Washington, D.C., AAMR.
  • Kaufman, Alan S. & Lichtenberger, Elizabeth O. "Assessing Adolescent and Adult Intelligence" (2002), Boston, Allyn & Bacon.
  • Wechsler, David The Measurement of Adult Intelligence (1944), Baltimore, The Williams & Wilkins Company.

External links

  • Mental Deficiency Theories ( - from a criminology course, but provides a good overview

fr:Retard mental ja:知的障害 pl:Obniżenie poziomu rozwoju intelektualnego


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