Last updated: 26 November 2025
“Words are never precise: the variegated cloud of meanings they carry about with them is their expressive power. But it also generates confusion, ‘cause you know sometimes words have two meanings’”
Carlo Rovelli. Helgoland. Information. p.90
Defining disability is important, even if only for legal reasons. Governments grant rights and benefits to those classified as disabled, and many treatises have been written on the subject. However, the label of being disabled is also linked to stigmatisation and various social disadvantages. As Evans et al. have pointed out, beyond these reasons, there are also practical and philosophical issues. The World Health Organisation’s introduction of the concept of activity limitations may have clarified matters somewhat. Nevertheless, the term is widely used, and I will discuss these issues in this paper. I conclude by outlining the core idea of disability, which remains uncontested.
Table of Contents
Introduction
The Oxford English Dictionary [OED] inadvertently illustrates the challenges of defining disability. Disability is assigned two meanings:
- “a physical or mental condition that limits a person’s movements, senses, or activities”, and
- “a disadvantage or handicap, especially one imposed or recognized by the law.”
These definitions of disability, or more precisely, these descriptions of the linguistic meanings of disability and its usage, raise many issues.
The first definition implies that disability causes limitations, viewing it as a condition that restricts activities. This contradicts the usual interpretation, which sees impairments as limiting activities. It also suggests that disability can restrict movement and senses. In contrast, the World Health Organisation’s International Classification of Functioning (WHO ICF) states the opposite: impairments of movement or senses lead to disability. It is accurate that physical or mental conditions, namely impairments, are the primary direct causes of disability.
The second definition suggests that disability is a restriction on social participation (as it is now termed); the term ‘handicap’ was used in the WHO International Classification of Impairments, Disabilities, and Handicaps (ICIDH) to describe a change in social roles. In the biopsychosocial model of illness, which underlies the WHO ICF, disability results in limitations in social participation (i.e., handicap); it is not identical to it.
This page will explore the definition of disability as a term referring to an individual’s interactions with their physical environment.
Context
Any change in the structure or function of a body part can impact overall health or cause unusual or uncomfortable sensations. These changes can occur at any level, such as biochemical (including genetic), cellular, organ, or whole-body levels; sometimes they remain within the normal range for similar individuals, but more often they cause the structure or function to fall outside typical parameters. If the changes are sufficient to lead the person to notice something unusual, the resultant state is a malady; I have explained this in a post on malady.
The following words are associated with the changes at different levels. Changes at any level below the whole body are referred to as disease or pathology. Changes at the whole-body level are called impairments (of a bodily structure or function) and are usually referred to as symptoms or signs.
The interaction between a person and their physical environment is called goal-directed behaviour, and changes are referred to as a ‘limitation of activities’, but they are mostly called a disability. The interaction between a person and others in social situations has several descriptive phrases, such as social role performance and social activities, and changes are referred to as limitations on social participation; the term ‘handicap’ is old and inappropriate.
This seems to present a very straightforward linear progression from disease to its social consequences; however, it is far from a simple chain of relationships. People have long recognised that personal and environmental factors affect the course of the disease. George Engel explicitly articulated this in his article on the biopsychosocial model of illness, and it was initially systematised in 1980 within the World Health Organisation’s International Classification of Impairments, Disabilities, and Handicaps (WHO ICIDH), which was later improved in 2001 to the International Classification of Functioning, Disability and Health (WHO ICF).
Furthermore, not all illnesses originate from internal structural damage. For example, post-traumatic stress disorder originates from external stressors. Conditions such as Huntington’s disease arise from faulty genetic material that leads to neuronal damage, but it is part of that person’s nature rather than externally caused. The complex multifactorial nature of most illnesses is obvious; the holistic biopsychosocial model of illness is widely used, and I have explored this on this site.
The issues discussed so far relate to the nature of disability. However, there are also many social and particularly legal consequences of being ‘disabled’ as part of any illness, and these have both positive and negative effects. On the positive side, the person with a disability can access many resources provided by society, usually free of charge, such as healthcare services, additional financial and personal support, and specialist equipment.
On the negative side, some individuals may exaggerate or feign disability to access resources or attain a role and status linked to disability, such as participating in the Paralympics or serving in governmental committees as a representative. For most people, the benefits seem somewhat smaller but are not necessarily less meaningful to them.
In summary, there are two critical contextual matters to consider:
- the healthcare and scientific understanding of disability, and
- the person’s society, their immediate family, friends and people in their broader social networks.
Some published definitions.
I have already discussed the definitions given in the Oxford English Dictionary.
A legal definition in England and Wales is given in the Equality Act 2010: “You’re disabled under the Equality Act 2010 if you have a physical or mental impairment that has a ‘substantial’ and ‘long-term’ negative effect on your ability to do normal daily activities.” The UK General Medical Council has produced a helpful flowchart that expands on this to help managers interpret the Act.
The UK Advisory, Conciliation and Arbitration Service (ACAS) is involved in disputes, including those related to the Equality Act 2010. They note that some diagnoses are automatically classed as disabilities with no reference to function; they include cancer and visual impairment, but not hearing impairment. They also provide guidance on how severe the disability must be before the Act applies.
The United States Centre for Disease Control and Prevention (US CDC) defines disability thus: “A disability is any condition of the body or mind (impairment) that makes it more difficult for the person with the condition to do certain activities (activity limitation) and interact with the world around them (participation restrictions).” The website was updated in April 2025 to broaden the meaning of disability: “Disability has three dimensions: impairment, activity limitation, and participation restrictions.”
The Americans with Disabilities Act (1990) was modified in 2009 by the Americans with Disabilities Act Amendments Act. (See: What is the ADA?) The Act is transparent about the definition used, emphasising that the definition is for legal use:
“It is important to remember that in the context of the ADA, “disability” is a legal term rather than a medical one. Because it has a legal definition, the ADA’s definition of disability is different from how disability is defined under some other laws, such as for Social Security Disability related benefits.
The ADA defines a person with a disability as a person who has a physical or mental impairment that substantially limits one or more major life activity. This includes people who have a record of such an impairment, even if they do not currently have a disability. It also includes individuals who do not have a disability but are regarded as having a disability. The ADA also makes it unlawful to discriminate against a person based on that person’s association with a person with a disability.”
The World Health Organisation states, “Disability results from the interaction between individuals with a health condition, such as cerebral palsy, Down syndrome and depression, with personal and environmental factors including negative attitudes, inaccessible transportation and public buildings, and limited social support.”
However, the WHO ICF also states, “Disability has three dimensions:
- Impairment in a person’s body structure or function, or mental functioning; examples of impairments include loss of a limb, loss of vision or memory loss.
- Activity limitations, such as difficulty seeing, hearing, walking, or problem-solving.
- Participation restrictions in normal daily activities, such as working, engaging in social and recreational activities, and obtaining health care and preventive services.”
The United Nations Convention on the Rights of Persons with Disabilities, in Article one on Purpose, defines their scope as: “Persons with disabilities include those who have long-term physical, mental, intellectual or sensory impairments which in interaction with various barriers may hinder their full and effective participation in society on an equal basis with others.”
These definitions demonstrate that disability:
- Is used for several reasons, such as defining access to legal rights and social resources, as well as a clinical term
- Is used as an umbrella term for the consequences of bodily changes and disease, as well as a clinical term
- Is generally considered a health condition
- Is concerned with functional activities that are affected by physical or mental disorders
- Is influenced by external physical or social factors
There is no uniformity among the definitions.
Defining disability
In 2006, Leonardi and her colleagues considered a definition of disability in an article, “The definition of disability: what is in a name?”. In it, they suggested that “a definition of disability should be applicable to all people, without segregation into groups such as “the visually impaired” or “wheelchair users” or those with a chronic illness, and be able to describe the experience of disability across many areas of functioning.”
Moreover, they suggested that: “The definition should
- allow comparison of severity across different types of disability,
- be flexible enough for different applications (e.g., statistical or clinical use),
- describe all types of disability,
- recognise the effects of the environment on a person’s disability …
- not include stipulations about the causes of any disability.”
The definition they were commenting on was developed by the “Measuring Health and Disability in Europe: Supporting Policy Development (MHADIE)” consortium on behalf of the WHO: “Disability is a difficulty in functioning at the body, person, or societal levels, in one or more life domains, as experienced by an individual with a health condition in interaction with contextual factors.”
They felt it was important to distinguish “objective descriptions of the disability experience” (not otherwise explained) from subjective (i.e. patient) appraisals. They suggested the primary purpose of a definition was to allow “many issues in health and social policy to be tackled”.
Self-identification of disability.
As I have argued for assessing the quality of life, an alternative approach is to let the person decide whether they have a disability.
In a study on Who Self-Identifies as Disabled? An Examination of Impairment and Contextual Predictors, involving 750 people with a variety of diagnoses, Kathleen Bogart et al. (2017) identified four factors associated with self-identification as being disabled:
- severity, measured as the total number of impairments;
- age (older age being more likely to be disabled);
- income (lower income being more disabled); and
- stigma (people who had higher stigmatisation score).
This was considered evidence for the validity of the WHO ICF model. There was no independent measure of disability, so how close the relationship was between self-assessment and an external assessment is unknown.
In a study, Naming and Claiming: The Tension Between Institutional and Self-Identification of Disability, Gillian Parekh and Robert Brown discovered that less than a third of students identified by the educational establishment as being disabled self-reported their disability.
A brief search of Google Scholar for ‘Self-identification of disability’ produced no other papers.
Defining disability from first principles.
Evans et al. have recently approached the identification of disability from a different perspective.
They started from the premise that measurable aspects of human abilities (capacity) and activities (performance) will all be on a spectrum. Therefore, it was essential to consider both disability and capacity enhancement and performance. Their insight that both ends of the range should be considered is, as far as I know, unique.
They then suggest considering a person’s capacity space, “the dynamic relationship between an individual person and their social and environmental milieu,” and then suggest using this measure to compare a person with a reference cohort, “that group against which a given individual is compared in the context of evidence-based research in the life sciences.” Last, they assume that the distribution on all measures will be statistically normal and that disability and enhancement will be defined by comparison with the standard bell curve.
Their first example is of people who move, by comparison with their cohort, out of the standard curve, either down (i.e. becoming disabled) or up, becoming enhanced. They ask whether someone moving down by a similar amount but remaining within the curve can be considered still normal, or have they become disabled? If your IQ dropped from 118 to 88, have you become cognitively disabled?
Their next thought experiment assumes that the person remains stationary while the comparative cohort improves. For example, a child developing slowly may still be at the expected level at ten years but not at 20 years, or a healthy adult may be as expected at 70 years but enhanced at 80 years if they keep very fit.
Their following example will be very familiar to most clinicians because it concerns variability. What if someone performs within the normal range one day but below on another occasion, only to return within the normal range on some later occasion. Are they classified as disabled? Further, what about a person who fluctuates by the same amount or even more but always remains within the standard curve?
Their last example concerns progressive loss within the average area of function for the cohort (also adjusted over time).
A creative criticism
Before exploring this approach and these examples further, I will introduce Edwin Jesudason, a British Rehabilitation specialist who wrote a commentary on the approach, ‘Disability: leaning away from the curve.’
Perhaps reflecting a growing concern with Mental Capacity in the UK, he suggests that a ‘Capability Space‘, measuring what an individual can do within their context, would be a better term. He then presents three substantive points.
First, he argues that disability must be considered within the biopsychosocial model, which highlights that disability results from a complex, multifactorial interaction. He thinks this makes their assumption of statistical normality unlikely to be valid.
Second, he points out that every person has a wide range of capabilities, each of which occupies a different position within the standard distribution curve and fluctuates to a greater or lesser extent. Is someone disabled when one of these attributes moves out of the average area? There is always some measurement error, and a person’s measure may shift into or out of the normal distribution due to error, not genuine change. Do they become disabled (or non-disabled) even if they have not changed? What if the capability that changes is trivial? Or of vital importance?
He develops this point on linguistic and philosophical grounds, pointing out that there is more to disability than simply describing a range of capabilities (or other measurable constructs).
His final point is that we must always consider the reason(s) for the proposed definition: “Who is asking who is disabled and why?” He highlights that one powerful reason is the judgment of who may have access to resources and how much help they might receive. He suggests that a definition developed for one purpose may be misused for another. This is the same argument I put forward when considering the definition of rehabilitation and, more recently, when discussing What is a diagnosis?
Synthesis
There are many reasons why a single definition of disability is impossible. Most of the reasons were set out in 1943 (in French) in The Normal and the Pathological by Georges Canguilhem, which discussed the definition of disease and illness, and the logic has not changed.
For example, is normal the statistical normal derived from a relevant population, or is it the ideal which would be observed if all humans had adequate food, medical care etc.? Or is normal what is expected by the doctor, by the person concerned, or by society? When considering blood pressure, does a single observation change someone from being normotensive to hypertensive?
In the case of disability, I will raise three issues that pose problems for any definition.
Capability or performance?
One of the more important issues is whether a disability is defined externally, usually by assessing what some can do or their capability, or internally, by the patient referring to what they do or their performance. While these two may often be the same, and some people seem to surpass their capacity as judged by professional clinicians, a discrepancy exists in many individuals.
Some of the discrepancies between capability and performance can be attributed to context, as the physical, social, and situational environments influence performance. Some of the differences can be attributed to psychological factors such as depression. Many discrepancies can be attributed to the patient’s lack of desire to achieve the goal.
Whatever the explanations may be, it is still crucial to decide whether a disability is, in the final analysis, determined and defined by a healthcare professional or by the patient’s actual observed or reported performance.
Criterion: population, past, predicted?
A second big issue, already discussed in part, is how disability is determined. The word suggests that the current state is less than a comparator state, and the question is, what is recent performance compared with?
One option, already discussed, is selecting a representative sample of the population. However, it is not easy to make a choice. Should it be people without any health issues? Matched on age, gender, and other demographic factors? A random sample of the population? There is no definitive answer to these questions, and all responses are swiftly criticised.
A second answer, only applicable to people who start without any health-related disability, is to use performance before the onset of the condition. This approach also faces significant issues, apart from the fact that it cannot be used with people born with a disabling condition. People’s abilities change over time. For example, many do not learn to drive until the age of 25, but if an accident at 24 prevents driving, they will be considered disabled due to their inability to learn.
A third criterion is an expectation. This is useful when considering individuals with conditions beginning at birth or before the age of 16 years. However, the question remains: whose expectation is it — the person’s, the parents’, or society’s norms, with all the associated issues? A subset of this criterion is the patient’s desired level of performance.
Fluctuation
A third problem arises from the desire to categorise people, which assumes that a person’s state will remain unchanged. After an accident, such as breaking the forearm bones, few would debate that the person had a disability. At a particular moment in the first few days or weeks, few would doubt that the person was disabled. But, as a general statement, is this person now one of “the disabled”?
This is not a trivial problem. People with multiple sclerosis or chronic fatigue syndrome fluctuate, sometimes being able to function well, for example, working, but at other times may not be able to work. Can such a person be classified as disabled? Given that most resources are offered on a categorical basis, they are usually classified as disabled even if they could work, let us say, 2-4 days a week most of the time.
Suppose someone has a 40-hour-per-week contract but can consistently work only 35 hours. Should they receive no support (i.e., they are not classified as having a disability), or 12.5% to reflect the missing 5 hours, or 100% of the available support because they meet the criterion of being disabled?
Summary of synthesis
Disability, like all words, carries many meanings within it, so that the underlying construct cannot be defined precisely to allow categorisation into separate classes. I have mentioned this when discussing rehabilitation interventions. Rather than despair, we should accept that we need to consider each patient individually; they should not be considered ‘disabled’. They should be regarded as “Jeremy, who needs someone within him when walking, and has difficulty explaining anything complex.
What DOES disability mean?
My review of the definition of disability demonstrates that one cannot give a logically coherent single definition of disability to suit all purposes. This should not be a surprise. Long legal judgments examine the meaning of single words – and still cannot be definitive. Words represent concepts, and all concepts have fuzzy borders.
However, words do represent concepts, and they have a central core, the Platonic ideal form. What is disability’s perfect form?
Disability refers to a change or restriction in a person’s interaction with their environment. It involves the individual’s system on one side and their environment on the other.
Interactions occur in two ways: either something in the environment changes, or the person wants to change their environment. These interactions are behaviours, and the behaviours are goal-oriented, aiming to achieve a specific effect or outcome.
In the WHO ICF and the holistic biopsychosocial model of health, the interactions are referred to as activities. Changes in the body’s functional capability may alter or limit the performance of an activity. That is a limitation of an activity.
Therefore, disability refers to a change or restriction in a person’s ability to interact with their environment caused by an alteration or atypical limitation within the individual.
The phrase ‘atypical restriction’ is necessary to recognise that some individuals have a capacity substantially below the population average, requiring support at levels that most others do not.
This description of the core features of disability does not abolish the scope for uncertainty and debate, such as:
- Some environments impose demands exceeding the capacity of many healthy people. For example, the cognitive demands of travelling in the London Underground during rush hour are beyond the capacity of many people. Are they suffering a disability?
- Some people’s capacity will inevitably be less than average, limiting their ability at more complex activities. When does a difficulty arising from a natural lower capacity become disability? For people with learning difficulties, the limit is around the second percentile, but this is itself challenging to measure.
- What is the comparator to determine a limitation or change due to a change or limitation in a person’s capacity? The expected capacity judged against other similar people, their capacity before an event, the capacity they want?
Conclusion
The core idea of disability is a change or restriction in a person’s ability to interact with their environment caused by an alteration or atypical limitation within the individual. This central concept captures the essence of disability. However, it is not possible to determine whether an individual has a disability because the interaction is influenced by both the environment and the person’s natural capacity.
Thus, there is no way to define disability that can be applied across all individuals in the population, to all conditions, or for all purposes. Criteria for determining disability for each specific reason could be achieved, but they would still be imperfect.
For example, a definition suitable for accessing social support for personal care or vocational activities could be developed. However, they would probably lack good discrimination, excluding many people who should have support and including many who do not need it. In practice, it would be better to describe the person in terms of activities, selecting a few relevant ones to the person’s situation, and noting the degree of fluctuation observed. Decisions on the support needed should be made by rehabilitation experts who can make clinical judgements on the potential to reduce the need for support, for instance, through environmental modifications.
Therefore, one must always expand what one means because it is true that:
“Words are never precise: the variegated cloud of meanings they carry about with them is their expressive power. But it also generates confusion, ‘cause you know sometimes words have two meanings’”
Carlo Rovelli. Helgoland. Information (p90)