The Importance of Definitions

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In the winter of 1916, the Massachusetts State Board of Insanity called a meeting. The purpose of this meeting was to discuss a law passed in 1911, which established a “department of defective delinquents” responsible for imprisoning anyone deemed to be “mentally defective” and “immoral” as determined by a doctor. The problem was that no precise definition of “mentally defective” had been agreed upon, and this made carrying out the law challenging.

Dr. Walter E. Fernald, a major proponent of this law, reportedly responded that a definition was not important, because all medical providers had a general sense of what it meant. The question was legal, not medical. And his contemporaries agreed—Herman Adler, an Assistant Professor of Psychiatry at Harvard Medical School, wrote:

“This represents fairly the position of nearly everyone who has to deal with this subject. We all recognize the term, and in a good many instances agree in the diagnosis. We each of us, however, have our own ideas and prejudices in regard to delinquency and nobody wants to set up a hard and fast definition… this is an eminently scientific attitude”. 

What followed was decades of imprisonment of individuals whose greatest crime was their disability. It set the stage for the establishment of Bridgewater State Hospital, a current state facility housing the “criminally insane” that is reportedly still mistreating patients, possibly to a criminal extent3. And the ambiguity of the definitions only bolstered the power of this law to do harm.

In my personal experience, I have found that people are often hesitant to talk about intellectual disability. I don’t see ill intentions behind this hesitance, but instead fear—fear of saying the wrong thing, using the wrong word, of being ignorant or accidentally unkind, of being asked a question they can’t answer or caught in a situation they can’t comfortably escape. These fears emerge from a long history of ambiguous definitions and systematic removal of disabled people from the public. Fear of intellectual disability continues to haunt and harm all of us, but particularly those living with an intellectual disability.

With this in mind, I want to put forward some definitions and statistics. I do not believe that “we get the general idea” is an eminently scientific attitude. It’s hard to have a conversation about any topic without agreed upon definitions of important terms, and given the history of the weaponized use of language against intellectual disability, it’s even more important here.

What the DSM-V says about ID

The DSM-V definition of intellectual disability has three basic requirements.

1. The individual must present with deficits in skills such as reasoning, logic, academic learning, abstract thinking, and problem solving.
2. The individual must present with deficits in adaptive functioning across multiple environments. This basically means that they require ongoing support in their daily life for their disability.
3. These deficits must onset during the developmental period. This excludes people who acquire these deficits later in life, through a traumatic brain injury, for example.

Of course, the DSM-V is not the ultimate authority on what intellectual disability is and isn’t. The definition has changed before, and it will certainly change again. It’s merely lines in the sand around a cluster of people who have similar challenges. But for the most part, when I’m talking about intellectual disability, I’m using this definition.

Classifications of ID

The DSM-V breaks down this definition further into four main categories: mild, moderate, severe, and profound, which describe the spectrum of challenges people with intellectual disabilities face.

People with severe or profound intellectual disabilities need significant support managing basic life tasks. Examples of support needs are active assistance with eating, bathing, and dressing.

People with moderate intellectual disabilities are typically able to achieve independence in most day-to-day tasks, although they may need ongoing support and reminders. They may benefit from additional help learning skills like cooking, self care, and organization.

People with mild intellectual disabilities are usually able to perform most self-care tasks independently, but may need support in managing things like finances, transportation, and finding a job.

Demographics

Estimates of the prevalence of intellectual disability are not particularly precise, ranging anywhere from 1% to 3% of the total population, depending on who you ask4. Regardless, it’s likely more common than well-known diagnoses and disabilities such as OCD, Schizophrenia, hearing loss, and vision loss.

It is surprising to many people—it was certainly surprising to me—to learn that mild intellectual disability is much more common than moderate, severe, or profound, making up around 85% of all people with intellectual disabilities. Moderate intellectual disability makes up around 10%, severe 4%, and profound 1%. These numbers provide important context for discussions about how intellectual disability is perceived, how our system can be better designed to support people with intellectual disabilities, and the role that stigma plays in the lives of those with intellectual disabilities.

Other demographics to consider are race and gender. Recent CDC data shows boys are around twice as likely as girls to be diagnosed with intellectual disability. Black children are the group most likely to be diagnosed with ID—approaching 3%—while non-Hispanic Asian children are the least likely, at under 1%. In my opinion, these differences are most likely a result of diagnostic bias rather than a reflection of the actual population with ID. This doesn’t mean that the person diagnosing is necessarily biased, but that certain cultural expectations, as well as class demographics and access to education, may result in the over-diagnosis of some groups and under-diagnosis of others.

What causes ID?

Intellectual disability has no single root cause. Some genetic conditions can cause ID, including Down Syndrome (specifically Trisomy 21), Fragile X Syndrome, and Williams Syndrome. Other recognized causes are problems during birth, exposure to toxins, and metabolic disorders. But many other cases of ID have no discernable cause. I’ve seen the figure 2/3 of cases have no determinate cause, and that seems correct to me, although I can’t find the original source. It’s also important to note that those with mild intellectual disability are the least likely to know the cause of their disability. This may mean they have more difficulty accessing resources and connecting with communities of other intellectually disabled people, particularly because major risk factors for mild ID include low socioeconomic status, malnutrition, maternal education, and access to healthcare5—all of which make managing a disability more challenging.

These data are meant as an introduction to a larger conversation on intellectual disability, not a perfect or complete description of it means to have an intellectual disability. I encourage kind and thoughtful discussion and feedback—I always want to work under an assumption of good intentions, and I hope you will extend the same courtesy to me and to each other.

1. Acts and Resolves passed by the General Court of Massachusetts in the year 1911. See this link for a digitized copy.
2. 2. Adler, Herman M. “A Psychiatric Contribution to the Study of Delinquency” Journal of the American Institute of Criminal Law and Criminology, vol. 8, no. 1, May 1917. Open access here.
3. Becker, “Mistreatment persists at Bridgewater State Hospital, report says” WBUR, 13 March 2024. Accessed here 7/9/24.
4. McKenzie, Katherine et al. “Systematic Review of the Prevalence and Incidence of Intellectual Disabilities: Current Trends and Issues”, Curr Dev Disord Rep 3, 104–115 (2016). Link here.
5. Patel, Dilip R. “A clinical primer on Intellectual Disability”, Translational Pediatrics, Feb. 2020. Link here.