‘When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’

Lewis Carroll, Through the Looking Glass

I’m neurodivergent. It’s easy for me to say that, knowing that I’m autistic and that autism is invariably listed as an example of neurodivergence. But can we describe it better than by compiling a checklist of medical diagnoses? It’s tempting to go with Humpty Dumpty and say it means what I choose it to mean, but this doesn’t help when trying to communicate the idea to others.

One of the aims of the Neurodiversity Movement is to move beyond a view of the rich variations found in the human mind that is dominated by the medical model and its diagnostic lists of symptoms and traits framed as deficits and impairments to be treated and fixed. To that end, the Movement has embraced the social model–based on the social model of disability–that focuses on the needs of the individual and the barriers they face.

While the medical model is flawed because its perspective is of a person having things “wrong” with them needing to be fixed, there remains a need for diagnosis and medical treatment: I’m certainly not suggesting that we abandon medicine. But it should never be a goal to “normalise” neurodivergent people, to “fix” or hide their traits for the comfort of others around them who see those traits as wrong or broken. This is significantly different in terms of motivation from an individual seeking treatments and/or accommodations to help them overcome barriers in their life.

Individuals do not exist in isolation. If they did there would be no need for concepts such as neurodivergent which exist to define a subset of people in wider society. Neurodivergent defines a relationship between a given individual and the society in which they live, a relationship based on how that person experiences the world, and how they respond to and interact with their environment including other people.

If it were possible to quantify the differences in thought and sensation, reduce that complex experience to a number and plot a graph including everybody’s scores, then you would find that most people cluster around the middle. But you would also see that a significant minority fall outside that cluster, tailing off in each direction: it would resemble the chart below.

At each end are the scores that diverge from the majority, and in the middle are the ones which are typical. But where is the line that separates divergent from typical? Well, there isn’t one: there is only a progression from more typical to less typical; from less divergent to more divergent, each morphing seamlessly into the other. There is no cut-off point; there is not even a meaningful boundary.

Neurodiversity is similar to this simple score-based model, except we have no way to put a number on a person’s degree of neurodivergence. So how can it work if we can’t measure how neurodivergent somebody is?

The simple answer is that in the majority of cases either a person recognises that they are different from most others around them or people around them recognise that that particular person is different. Where the difference or differences correspond to neurological functioning–differences in learning, language, sensory processing, personality and many other traits–that is neurodivergence.

Some of these differences correspond to medically-defined conditions including autism, ADHD, epilepsy, Tourette Syndrome: these have been identified and described medically because they are instances of divergence. This doesn’t mean that any diagnosis is required for somebody to be neurodivergent: something does not need to be considered medically significant for it to be a significant neurodivergence.

Deciding what is significant defines the limits of what we consider to be neurodivergence. This is where the answer becomes less simple, but more important. More significant, if you like. These are what’s often referred to as the edge or corner cases. The majority of neurodivergence is pretty easy to define in terms of recognised conditions and that’s how most people view it, but it’s important not to restrict our definition to that and only that.

If I take autism as an example, there are traits that might differ in degree and not meet the arbitrary, subjective levels required in a diagnostic situation (I say arbitrary and subjective because the requirements differ between diagnostic manuals and between practitioners) but that do affect how a person experiences the world: while it’s not true to say that everybody is “a bit autistic”, it is true that some are in the fuzzy area that lies between neurotypical and autistic. There’s no clear cut-off between autistic and non-autistic, but at the same time the majority of people are generally identifiable as one or the other.

Many autistic people are able to identify their own autistic nature and that of other autistic people with or without a formal diagnosis because of their awareness of the traits of autism and because they can relate that to observations of the people around them. This is reflected in the wide acceptance of self-diagnosis within the autistic community, although it’s rather less widely accepted outside of those spaces.

A recognition of the validity of self-identity or identification by one’s peers is important when diagnosis may be based on incomplete or biased medical definitions of conditions, when diagnosis may be inaccurate, or when access to diagnosis is not universal.

What I’ve described in the case of autism applies more generally to all instances of neurodivergence. While the core may be well- and even objectively defined, at the edges–out of necessity–we fall back on subjective assessment and judgement of what qualifies as significantly neurodivergent. Just as with formal, clinical diagnosis there will be questionable, debatable decisions.

In conclusion, neurodivergence encompasses conditions including but not limited to autism, ADHD, dyslexia, Tourette’s Syndrome, epilepsy and PTSD. It includes anything relating to neurological function that results in one or more significant differences from the typical range of functioning in the wider population. What counts as significant may be decided by the individual or by consensus within society, and cannot be defined objectively.