Intellectual disability

• 1. Introduction

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  Intellectual disability is a generalized neurodevelopmental disorder that first shows itself in childhood. It is marked by significant impairment in both intellectual functioning and the everyday skills a person needs to live. The numbers give it scale. It touches about two to three percent of the general population, and cases of unknown cause alone affect roughly 95 million people. Most people who have it have the mild form, somewhere between 75 and 90 percent. But a single label hides a long and uneasy story. Who decides where the line falls between a low test score and a disability? Why has the name for this condition changed so many times that schools, hospitals and journals have had to keep renaming themselves? How did societies move from drowning infants in the Tiber to ratifying a global rights convention? And what does a person with this diagnosis actually learn to do, across a whole life, with the right support around them? Those are the questions ahead.

• 2. Drawing The Line

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  An IQ below 70 is the number most people associate with intellectual disability, but the diagnosis was never meant to rest on that figure alone. Three criteria must be met. There must be significant limitation in general mental abilities. There must be significant limitation in one or more areas of adaptive behavior across multiple environments. And the limitations must have become apparent in childhood or adolescence, during the developmental period. That third criterion does real work. It is what separates intellectual disability from a traumatic brain injury or a dementia like Alzheimer's disease, which can arrive at any age. Adaptive behavior is the half people forget. It covers the social, developmental and practical skills a person uses to get through a day. The DSM-5 lists what intellectual functions mean too. Reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience. A person with a somewhat higher IQ but severe impairment in adaptive functioning can still meet the threshold, because clinical discretion is built into the process. The disorder splits two ways. Syndromic intellectual disability comes bundled with other medical and behavioral signs, as in Down syndrome and fragile X syndrome. Non-syndromic disability arrives without other abnormalities, and those idiopathic cases make up between 30 and 50 percent of all cases. About a quarter of cases trace to a genetic disorder. Only about 5 percent are inherited. For one-third to one-half of cases in children, the cause is simply unknown.

• 3. Degrees Of Support

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  Mild intellectual disability, with an IQ between 50 and 69, may not be obvious until a child starts school. Even then, expert assessment may be needed to tell it apart from a specific learning disability or an emotional disorder. People with mild ID can learn reading and mathematics to roughly the level of a typical child aged nine to twelve. They learn self-care and practical skills, like cooking or using the local transit system, and many reach adulthood living independently in gainful employment. About 85 percent of people with ID have the mild form. Moderate ID, with an IQ between 35 and 49, is almost always apparent within the first years of life, with speech delays a particularly common sign. These individuals need considerable support at school, at home and in the community. As adults they may live with parents, in a supportive group home, or semi-independently with services that help them manage finances, and some work in a sheltered workshop. About 10 percent of people with ID fall here. Severe ID, with an IQ of 20 to 34, accounts for 3.5 percent of people with the condition. Profound ID, at an IQ of 19 or below, accounts for 1.5 percent. People with profound ID are completely dependent on others for all activities of daily living and for their physical health and safety. A diagnosis is considered severe or profound when a person cannot care for themselves without ongoing significant assistance from a caregiver throughout adulthood.

• 4. What Travels With It

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  Roughly 30 percent of people with autism spectrum disorder also have intellectual disability, and the overlap between the two can cause confusion in diagnosis. Both conditions require shortfalls in communication and social awareness as defining criteria, which is part of why they blur together. A 2016 study surveying 2,816 cases found the features that best separate them. Impaired non-verbal social behavior, lack of social reciprocity, restricted interests, strict adherence to routines, stereotyped and repetitive motor mannerisms, and preoccupation with parts of objects. People with ASD tend to show more deficits in body language and reading social cues. A 2008 study of 336 individuals with varying levels of ID found that those with ID display fewer repetitive or ritualistic behaviors, and that people with ASD were more likely to isolate themselves and make less eye contact. The two conditions are even classified differently. ID has a standardized assessment, the Supports Intensity Scale, that measures severity by how much support a person needs, while ASD has no standard assessment and leaves severity to clinical judgment. Epilepsy is another frequent companion, affecting about 22 percent of individuals with ID and about half of those with profound ID. Management is hard here. Around 70 percent of people with ID are thought to be pharmaco-resistant, yet only about 10 percent are prescribed anti-seizure medications licensed for that resistance. Research shows that levetiracetam and brivaracetam work about as well in people with ID as in people without it.

• 5. Where It Comes From

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  Down syndrome is the most common genetic cause of intellectual disability, followed by DiGeorge syndrome and fetal alcohol spectrum disorders. The list of genetic conditions runs long. Klinefelter syndrome, fragile X syndrome which is common among boys, neurofibromatosis, congenital hypothyroidism, Williams syndrome, phenylketonuria, and Prader-Willi syndrome. Rarer entries include Phelan-McDermid syndrome, Mowat-Wilson syndrome, and Siderius type X-linked intellectual disability caused by mutations in the PHF8 gene. Some causes have nothing to do with genes. A pregnant woman who drinks alcohol or catches an infection like rubella may have a baby with the disability, and a baby deprived of oxygen during labor may suffer brain damage. Diseases like whooping cough, measles or meningitis can cause it when care is delayed, and exposure to lead or mercury can damage mental ability. The single largest preventable cause sits in the developing world. Iodine deficiency affects approximately 2 billion people worldwide, and it is the leading preventable cause of intellectual disability where it is endemic. India alone has 500 million people with a deficiency, 54 million with goiter, and 2 million with congenital iodine deficiency. China and Kazakhstan have run widespread salt iodization programs, but as of 2006 Russia had not. Malnutrition does similar damage in places struck by famine, such as Ethiopia, and in nations where extended warfare disrupts the growing and distribution of food.

• 6. A History Of Names And Walls

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  Hippocrates, writing in the late fifth century BCE, gave the oldest physiological view of intellectual disability, blaming an imbalance in the four humors of the brain. The treatment of people themselves was far harsher. In ancient Rome they were considered property, could be kept as slaves by their father, and could not marry, hold office, or raise children. Many were killed in childhood and dumped into the Tiber to spare society the burden, though Roman law exempted them from crimes. Caliph Al-Walid, who reigned from 705 to 715, built one of the first care homes for people with intellectual disabilities and assigned each individual a caregiver. In the 13th century, England declared such people incapable of managing their own affairs and created guardianships to take over their finances. In the 17th century, Thomas Willis offered the first description of the condition as a disease caused by structural problems in the brain. The asylum model rose through the 18th and 19th centuries, removing people from their families, usually in infancy, into large institutions often kept self-sufficient by the labor of the residents. A survey taken in 1891 in Cape Town, South Africa, counted 2,046 persons, with 1,281 in private dwellings, 120 in jails, and 645 in asylums. Then came eugenics. After Charles Darwin's On the Origin of Species, Francis Galton proposed selective breeding of humans to reduce intellectual disability. The movement spread worldwide, drove forced sterilization and marriage bans, and was later used by Adolf Hitler as a rationale for mass murder during the Holocaust. The turning point in policy came in 1969, with Wolf Wolfensberger's work The Origin and Nature of Our Institutional Models, which argued that segregated institutions ignored the productive contributions all people can make.

• 7. The Euphemism Treadmill

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  Mental retardation is the term most people of a certain age will remember, and it became popular in the middle of the 20th century to replace an older set of words. Imbecile, idiot, feeble-minded, and moron were all once clinical, and all are now offensive. The pattern has a name, the euphemism treadmill. Each replacement word eventually curdles into an insult and has to be replaced again. By the end of the 20th century, retard had become widely seen as disparaging and ableist. A 2008 BBC poll in the United Kingdom found it the most offensive disability-related word. The churn forces institutions to keep renaming themselves. The Midlands Institute of Mental Sub-normality became the British Institute of Mental Handicap and is now the British Institute of Learning Disability. In the United States, the American Association on Mental Retardation changed its name to the American Association on Intellectual and Developmental Disabilities in 2007. The terms also fracture by country. In the UK, Stephen Dorrell, Secretary of State for Health from 1995 to 1997, changed the NHS designation to learning disability, a phrase often confused with the schoolwork problems that Britons call learning difficulties. The DSM-5 of 2013 settled on intellectual disability, while the ICD-11 chose disorders of intellectual development. Yet mental retardation survives in some governmental aid and health insurance paperwork, where it is specifically covered and the newer term is not. In a 2012 survey of 101 Canadian healthcare professionals, 78 percent said they would use developmental delay with parents, against only 8 percent for intellectual disability.

• 8. Lives And Rights

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  About 8 percent of people with intellectual disabilities currently live in an institution or a group home, a figure that would have been almost unimaginable before deinstitutionalization. There is no cure for an established disability, but with support and teaching most individuals can learn to do many things, and some causes like congenital hypothyroidism can be treated early to prevent the disability forming. The Carolina Abecedarian Project showed what early intervention can do. Conducted with over 100 children from low socioeconomic status families from infancy through pre-school years, it produced children who by age 2 scored higher than controls and stayed about 5 points higher a decade after the program ended, with better educational and employment outcomes in adulthood. Reading outcomes stay mixed. About a third of people with ID never learn to read, a third learn to recognize sight words and sound words out, and a third learn to read fairly well. The costs are measurable. In the United States, the average lifetime cost of a person with an intellectual disability came to $223,000 in direct costs and $771,000 in indirect costs, in 2003 dollars, with 76 percent of the total tied to reduced productivity and shortened lifespans. Those shortened lifespans are stark. Studies from the United Kingdom show people there with intellectual disability live on average 16 years less than the general population. Rights have moved the other way. The UN Convention on the Rights of Persons with Disabilities has been ratified by more than 180 countries, and in Alajos Kiss v. Hungary in 2010 the European Court of Human Rights ruled that Hungary cannot strip voting rights based only on guardianship for a psychosocial disability.