Defining disability

“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’”

Defining disability is essential, if only for legal reasons. Governments assign rights and benefits to people classified as disabled, and long treatises are written about this. However, the label of being disabled is also associated with stigmatisation and many other social disadvantages. As Evans et al. have highlighted, these reasons aside, there are also practical and philosophical problems. The World Health Organisation’s introduction of the concept of activity limitations may have clarified matters a little. Nonetheless, the word is universally used, and I will explore these issues in the post.

Table of Contents


Structural damage to the body will often lead to difficulties with bodily function. If the damage is not reversed, the physical function may also be affected in the long term. The bodily dysfunction may affect internal and physiological functioning and behavioural and social functioning. The first is referred to as symptoms and signs, or impairment in the jargon of rehabilitation. The second is usually termed disability or, in rehabilitation jargon, limitation on activities, and the last has no universally-accepted name. Still, standard terms include reduced social participation and reduction in social activities.

This seems a very straightforward linear progression from disease to its social consequences; in reality, it is anything but a linear set of relationships. People have realised for many years that personal and environmental factors influence the effects of the disease. George Engel made this explicit in his article on the biopsychosocial model of illness, and was systemised in the Word Health Organisation International Classification of Functioning, Disability and Health (WHO ICF).

Furthermore, not all illnesses originate from internal structural damage to the body. 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 of most illnesses is obvious, and I have explored this on this site and elsewhere.

The issues discussed so far concern the nature of disability. However, there are also many social and specifically legal consequences of being ‘disabled’ as part of any illness, and these have both positive and negative influences. On the positive side, the person with a disability may access healthcare services, additional support, both financial and personal, and specialist equipment. On the negative side, some people may exaggerate or feign disability to access resources or achieve a role and status associated with disability, for example, participating in the Paralympics or participating in governmental committees as a representative. For most people, the gains are somewhat less but not necessarily less important.

In summary, there are two critical contextual matters to consider: the context of healthcare and scientific understanding and the context of society, immediate family and friends and the broader community.


Some published definitions

There are existing definitions.

The Oxford English Dictionary gives two: “a physical or mental condition that limits a person’s movements, senses, or activities”, which is a description, and “a disadvantage or handicap, especially one imposed or recognised by the law”, which relates more to the consequences.

A legal definition in England and Wales is “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.”. This is used by the Equalities Act 2010.

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 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:

  1. 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.
  2. Activity limitations, such as difficulty seeing, hearing, walking, or problem-solving.
  3. Participation restrictions in normal daily activities, such as working, engaging in social and recreational activities, and obtaining health care and preventive services.”

Defining disability

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. 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, and recognise the effects of the environment on a person’s disability. Finally, the definition should 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 involving 750 people with a variety of diagnoses, Kathleen Bogart et al. 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.

On a relatively brief search using Google, I could not find any studies comparing self-assessment of disability with external assessment. This surprised me.

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 considering this approach and these examples further, I will introduce Edwin Jesudason, a British Rehabilitation specialist who has very recently commented on Evans et al. His first point, perhaps reflecting an increasing concern with Mental Capacity in the UK, is that a Capability Space measuring what a person can do in their context would be a better phrase. He then makes 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 will have a different position within the standard distribution curve and fluctuate to a greater or lesser degree.  Is someone disabled when one of these attributes moves out of the average area? There is always some measurement error, and someone’s measure may move into or out of the normal distribution secondary to error, not real 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 a linguistic and philosophical basis by pointing out that there is more to disability than simply describing a variety of capabilities (or other measurable constructs).

His final point is that we must always consider the reason for the definition “Who is asking who is disabled and why?” and highlights that one powerful reason is judging who may have access to resources and how much help they might access. 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.


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 and 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 significant issues is whether a disability is defined, usually externally, by considering what some could do, their capability, or whether it is defined by the patient as referring to what they do, their performance. While the two may often be identical, and some people appear to exceed their capacity as judged by professional clinicians, a discrepancy is present in many people.

Some of the discrepancies between capability and performance can be attributed to context because of the physical, social and situational environments influencing performance. More of the difference 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 whether it is determined by the patient’s actual observed or reported performance.

Criterion: population, past, predicted?

A second big issue, already discussed in part, is how is disability 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 a suitable sample of the population. It is not easy to decide on the selection. People without any health problems? Matched on age, gender and other demographic factors? A random sample of the population?  There is no satisfactory answer to these questions, and all responses are quickly 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 also has significant problems, 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 people do not learn to drive until the age of 25 years, but if an accident at 24 precludes driving, they will be disabled by their inability to learn.

A third criterion is an expectation. This helps when considering people with conditions starting at birth or before the age of 16 years. Still, the question remains, whose expectation: the person, the parents, or do you use Society norms with all the problems that follow. A subset of this criterion is the patient’s desired level of performance.


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 more resources offered are given on a categorical basis, they usually will be classified as disabled even if they could work for, let us say, 2-4 days a week most of the time.


I conclude that there is no means to define disability that can be applied across the population, to all conditions, and for all purposes. I also suppose that criteria for accessing social support could be developed but would have poor 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, choosing a few relevant to the person’s situation with a description of how much fluctuation is seen. Decisions on support needed should be made by experts in rehabilitation who can make clinical judgements on the potential to reduce the need for support through, for example, undertaking changes in the environment. 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)


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 what he wants, but enjoys watching Manchester United play football”.

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