Mental retardation (MR) is a generalized disorder, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors with onset before the age of 18. It has historically been defined as an Intelligence Quotient score under 70.^[1] 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.

The term "mental retardation" is a diagnostic term denoting the group of disconnected categories of mental functioning such as "idiot", "imbecile", and "moron" derived from early IQ tests, which acquired pejorative connotations in popular discourse. The term "mental retardation" acquired pejorative and shameful connotations over the last few decades due to the use of "retarded" as an insult. This may have contributed to its replacement with euphemisms such as "mentally challenged" or "intellectual disability". While "developmental disability" may be considered to subsume other disorders (see below), "developmental disability" and "developmental delay" (for people under the age of 18), are generally considered more acceptable terms than "mental retardation".

• In North America mental retardation is subsumed into the broader term developmental disability, which also includes epilepsy, autism, cerebral palsy and other disorders that develop during the developmental period (birth to age 18.) Because service provision is tied to the designation developmental disability, it is used by many parents, direct support professionals, and physicians. In the United States, however, in school-based settings, the more specific term mental retardation is still typically used, and is one of 13 categories of disability under which children may be identified for special education services under Public Law 108-446.
• The phrase intellectual disability is increasingly being used as a synonym for people with significantly below-average cognitive ability.^[2] These terms are sometimes used as a means of separating general intellectual limitations from specific, limited deficits as well as indicating that it is not an emotional or psychological disability. Intellectual disability may also used to describe the outcome of traumatic brain injury or lead poisoning or dementing conditions such as Alzheimer's disease. It is not specific to congenital disorders such as Down syndrome.

The American Association on Mental Retardation continued to use the term mental retardation until 2006.^[3] In June 2006 its members voted to change the name of the organization to the "American Association on Intellectual and Developmental Disabilities," rejecting the options to become the AAID or AADD. Part of the rationale for the double name was that many members worked with people with pervasive developmental disorders, most of whom do not have mental retardation.^[4]

In the UK, "mental handicap" had become the common medical term, replacing "mental subnormality" in Scotland and "mental deficiency" in England and Wales, until Stephen Dorrell, Secretary of State for Health for the United Kingdom from 1995-7, changed the NHS's designation to "learning disability." The new term is not yet widely understood, and is often taken to refer to problems affecting schoolwork (the American usage), which are known in the UK as "learning difficulties." British social workers may use "learning difficulty" to refer to both people with MR and those with conditions such as dyslexia.

In England and Wales between 1983 and 2008 the Mental Health Act 1983 defined "mental impairment" and "severe mental impairment" as "a state of arrested or incomplete development of mind which includes significant/severe impairment of intelligence and social functioning and is associated with abnormally aggressive or seriously irresponsible conduct on the part of the person concerned."^[5] As behavior was involved, these were not necessarily permanent conditions: they were defined for the purpose of authorizing detention in hospital or guardianship. The term Mental Impairment was removed from the Act in November 2008, but the grounds for detention remained. However, English statute law uses "mental impairment" elsewhere in a less well-defined manner—e.g. to allow exemption from taxes—implying that mental retardation without any behavioral problems is what is meant.

In the UK, it is very important to understand that the verbs 'retardation' and 'retarded' (along with private usage of 'retard') are highly, and completely unacceptable in both professional and personal situations - and it is deemed just as offensive as many other stereotypical pejoratives used in the United States. A BBC poll conducted in the United Kingdom came to the conclusion that 'retard' was the most offensive disability-related word.^[6] On the reverse side of that, when a a contestant on Celebrity Big Brother live used the phrase "walking like a retard", despite complaints from the public and the charity Mencap, the communications regulator Ofcom did not uphold the complaint saying "it was not used in an offensive context [...] and had been used light-heartedly". It was however noted that two previous similar complaints from other shows were upheld.^[7]

The term "mental retardation" is still used in Australia, however "intellectual disability" is now the preferred and more commonly used descriptor.^[8][9]

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 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

The limitations of cognitive functioning will cause a child with mental retardation to learn and develop 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 participating members of the community.

In early childhood mild mental retardation (IQ 50–69) 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. 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. Children with moderate mental retardation will require considerable supports in school, at home, and in the community in order to participate fully. 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.

A person with a more severe mental retardation will need more intensive support and supervision his or her entire life.

According to the latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV),^[10] 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.

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 deviatons 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:

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.

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 structured interviews, with which they systematically elicit information about persons' functioning in the community from people who know them well. There are many adaptive behavior scales, and accurate assessment of the quality of someone's adaptive behavior requires clinical judgment as well. Certain skills are important to adaptive behavior, such as:

• daily living skills, such as getting dressed, using the bathroom, and feeding oneself;
• communication skills, such as understanding what is said and being able to answer;
• social skills with peers, family members, spouses, adults, and others.

This third condition is used to distinguish it from dementing conditions such as Alzheimer's disease or due to traumatic injuries with attendant brain damage.

Down syndrome, fetal alcohol syndrome and Fragile X 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,^[11] and Siderius type X-linked mental retardation (Template:OMIM6) as caused by mutations in the PHF8 gene (Template:OMIM6).^[12][13] 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.^[14]
• Malnutrition is a common cause of reduced intelligence in parts of the world affected by famine, such as Ethiopia.^[15]
• Absence of in the brain of the arcuate fasciculus.^[16]

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

There are thousands of agencies in the United States that provide assistance for people with developmental disabilities. They include state-run, for-profit, and non-profit, privately run agencies. Within one agency there could be departments that include fully staffed residential homes, day rehabilitation programs that approximate schools, workshops wherein people with disabilities can obtain jobs, programs that assist people with developmental disabilities in obtaining jobs in the community, programs that provide support for people with developmental disabilities who have their own apartments, programs that assist them with raising their children, and many more. The Burton Blatt Institute at Syracuse University works to advance the civic, economic, and social participation of people with disabilities. There are also many agencies and programs for parents of children with developmental disabilities.

Beyond that there are specific programs that people with developmental disabilities can take part in wherein they learn basic life skills. These "goals" may take a much longer amount of time for them to accomplish, but the ultimate goal is independence. This may be anything from independence in tooth brushing to an independent residence. People with developmental disabilities learn throughout their lives and can obtain many new skills even late in life with the help of their families, caregivers, clinicians and the people who coordinate the efforts of all of these people.

Although there is no specific medication for mental retardation, many people with developmental disabilities have further medical complications and may take several medications. For example autisic children with developmental delay may utilize anti-psychotics or mood stabilizers to help with behavior. Use of psychotropic medications such as benzodiazepines in people with mental retardation requires monitoring and vigilance as side effects occur commonly and are often misdiagnosed as behavioural and psychiatric problems.^[17]

Several traditional terms denoting varying degrees of mental deficiency long predate psychiatry, but have since been subject to the euphemism treadmill. In common usage they are simple forms of abuse. Their now-obsolete use as psychiatric technical definitions is of purely historical interest. They are often encountered in old documents such as books, academic papers, and census forms (for example, the British census of 1901 has a column heading including the terms imbecile and feeble-minded).

There have been some efforts made among mental health professionals to discourage use of these terms. Nevertheless their use persists. In addition to the terms below, the abbreviation retard or tard is still used as a generic insult. A BBC survey in 2003 ranked retard as the most offensive disability-related word, ahead of terms such as spastic (not considered offensive in America^[18]) and mong.^[19]

• Cretin is the oldest and comes from a dialectal French word for Christian.^[20] The implication was that people with significant intellectual or developmental disabilities were "still human" (or "still Christian") and deserved to be treated with basic human dignity. Individuals with condition were considered to be incapable of sinning, thus "christ-like" in their disposition. This term is not used in scientific endeavors since the middle of the 20th century and is generally considered a term of abuse: notably, in the 1964 movie Becket, King Henry II calls his son and heir a "cretin." "Cretinism" is also used as an obsolescent term to refer to the condition of congenital hypothyroidism, in which there is some degree of mental retardation.
• Amentia has a long history, mostly associated with dementia. The difference between amentia and dementia was originally defined by time of onset. Amentia was the term used to describe an individual who developed deficits in mental functioning early in life, while dementia described individuals who develop mental deficiencies as adults. During the 1890s, amentia was used to describe someone who was born with mental deficiencies. By 1912, ament was a classification lumping "idiots, imbeciles, and feeble minded" individuals in a category separate from a dement classification, in which the onset is later in life.^[21]
• Idiot indicated the greatest degree of intellectual disability, where the mental age is two years or less, and the person cannot guard himself or herself against common physical dangers. The term was gradually replaced by the term profound mental retardation.
• Imbecile indicated an intellectual disability 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. This group was known in UK law from 1911 to 1959/60 as "feeble-minded".
• Mongolism was a medical term used to identify someone with Down syndrome. For obvious reasons, the Mongolian People's Republic requested that the medical community cease use of the term as a description of mental retardation. Their request was granted in the 1960s, when the World Health Organization agreed that the term should cease being used within the medical community.^[21]
• In the field of special education, educable (or "educable mentally retarded") refers to MR students with IQs of approximately 50-75 who can progress academically to a late elementary level. Trainable (or "trainable mentally retarded") refers to students whose IQs fall below 50 but who are still capable of learning personal hygiene and other living skills in a sheltered setting, such as a group home. In many areas, these terms have been replaced by use of "severe" and "moderate" mental retardation. While the names change, the meaning stays roughly the same in practice.
• Retarded comes from the Latin retardare, "to make slow, delay, keep back, or hinder." The term was recorded in 1426 as a "fact or action of making slower in movement or time." The first record of retarded in relation to being mentally slow was in 1895. The term retarded was used to replace terms like idiot, moron, and imbecile because it was not a derogatory term. By the 1960s, however, the term had taken on a partially derogatory meaning as well.^[21] The noun "retard" is particularly seen as pejorative; as of 2010, the Special Olympics, Best Buddies and over 100 other organizations are striving to help eliminate the use of the "r-word" (analogous to the "n-word") in everyday conversation.^[22][23]

Perhaps the negative connotations associated with these numerous terms for mental retardation reflect society's attitude about the condition. There are competing desires among elements of society, some of whom seek neutral medical terms, and others who want to use such terms as weapons with which to abuse people.^[21]

Today, the term "retarded" is slowly being replaced by new words like "special" or "challenged." The term "developmental delay" is rapidly gaining popularity among caretakers and parents of individuals with mental retardation. Using the word "delay" is preferred over "disability" by many people, because that term (delay) encapsulates the core deficit that creates mental retardation in the first place. Delay suggests that a person has been held back from their potential, rather than someone who has been disabled. ^{[citation needed]}

Usage has changed over the years, and differed from country to country, which needs to be borne in mind when looking at older books and papers. For example, "mental retardation" in some contexts covers the whole field, but previously applied to what is now the mild MR group. "Feeble-minded" used to mean mild MR in the UK, and once applied in the US to the whole field. "Borderline MR" is not currently defined, but the term may be used to apply to people with IQs in the 70s. People with IQs of 70 to 85 used to be eligible for special consideration in the US public education system on grounds of mental retardation. ^{[citation needed]}

Along with the changes in terminology, and the downward drift in acceptability of the old terms, institutions of all kinds have had to repeatedly change their names. This affects the names of schools, hospitals, societies, government departments, and academic journals. For example, the Midlands Institute of Mental Subnormality became the British Institute of Mental Handicap and is now the British Institute of Learning Disability. This phenomenon is shared with mental health and motor disabilities, and seen to a lesser degree in sensory disabilities.^{[citation needed]}

History has not been kind to those with developmental delays. Throughout history, people with developmental disabilities have been viewed as incapable and incompetent in their capacity for decision-making and development. Until the Enlightenment in Europe, care and asylum was provided by families and the church (in monasteries and other religious communities), focusing on the provision of basic physical needs such as food, shelter and clothing. Stereotypes such as the dimwitted yokel, and potentially harmful characterizations (such as demonic possession for people with epilepsy) were prominent in social attitudes of the time. During the Renaissance communities would sometimes send away those who were developmentally delayed in boats. These ship of fools as they would call it, would then show up at another harbour only to be sent away to the next community.

Early in the twentieth century the eugenics movement became popular throughout the world. This lead to the forced sterilization and prohibition of marriage in most of the developed world and later used by Hitler as rationale for the mass murder of mentally challenged individuals during the holocaust. The eugenics movement was later proven to be seriously flawed and in violation of human rights and the practice of forced sterilization and prohibition from marriage was discontinued by most of the developed world by the mid 20th century.

The movement towards individualism in the 18th and 19th centuries, and the opportunities afforded by the Industrial Revolution, led to housing and care using the asylum model. People were placed by, or removed from, their families (usually in infancy) and housed in large institutions (of up to 3,000 people, although some institutions were home to many more, such as the Philadelphia State Hospital in Pennsylvania which housed 7,000 people through the 1960s), many of which were self-sufficient through the labor of the residents. Some of these institutions provided a very basic level of education (such as differentiation between colors and basic word recognition and numeracy), but most continued to focus solely on the provision of basic needs. Conditions in such institutions varied widely, but the support provided was generally non-individualized, with aberrant behavior and low levels of economic productivity regarded as a burden to society. Heavy tranquilization and assembly line methods of support (such as "birdfeeding" and cattle herding) were the norm, and the medical model of disability prevailed. Services were provided based on the relative ease to the provider, not based on the human needs of the individual.

Ignoring the prevailing attitude, Civitans adopted service to the developmentally disabled as a major organizational emphasis in 1952. Their earliest efforts included workshops for special education teachers and daycamps for disabled children, all at a time when such training and programs were almost nonexistent.^[24] The segregation of people with developmental disabilities wasn't widely questioned by academics or policy-makers until the 1969 publication of Wolf Wolfensberger's seminal work "The Origin and Nature of Our Institutional Models",^[25] drawing on some of the ideas proposed by SG Howe 100 years earlier. This book posited that society characterizes people with disabilities as deviant, sub-human and burdens of charity, resulting in the adoption of that "deviant" role. Wolfensberger argued that this dehumanization, and the segregated institutions that result from it, ignored the potential productive contributions that all people can make to society. He pushed for a shift in policy and practice that recognized the human needs of "retardates" and provided the same basic human rights as for the rest of the population.

The publication of this book may be regarded as the first move towards the widespread adoption of the social model of disability in regard to these types of disabilities, and was the impetus for the development of government strategies for desegregation. Successful lawsuits against governments and an increasing awareness of human rights and self-advocacy also contributed to this process, resulting in the passing in the U.S. of the Civil Rights of Institutionalized Persons Act in 1980.

From the 1960s to the present, most states have moved towards the elimination of segregated institutions. Along with the work of Wolfensberger and others including Gunnar and Rosemary Dybwad,^[26] a number of scandalous revelations around the horrific conditions within state institutions created public outrage that led to change to a more community-based method of providing services.^[27] By the mid-1970s, most governments had committed to de-institutionalization, and had started preparing for the wholesale movement of people into the general community, in line with the principles of normalization. In most countries, this was essentially complete by the late 1990s, although the debate over whether or not to close institutions persists in some states, including Massachusetts.^[28]

It could be argued that we still have a very long way to go before people with such disabilities are seen as full citizens of society. Person Centered Planning and Person Centered Approaches are seen as methods of addressing the continued labelling and exclusion of socially devalued people, such as people with a developmental disability label, encouraging a focus on the person as someone with capacities and gifts, as well as support needs.

• Secondary handicap

• Mental Retardation Research Center
• National Center on Birth Defects and Developmental Disabilities
• Mental Health Care,Inc.
• The Arc
• Special Olympics