Translating "phenotypically diverse"

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Michael Marshall, "The hidden links between mental disorders", Nature 5/5/2020:

Perhaps there are several dimensions of mental illness — so, depending on how a person scores on each dimension, they might be more prone to some disorders than to others. An alternative, more radical idea is that there is a single factor that makes people prone to mental illness in general: which disorder they develop is then determined by other factors. Both ideas are being taken seriously, although the concept of multiple dimensions is more widely accepted by researchers.

The details are still fuzzy, but most psychiatrists agree that one thing is clear: the old system of categorizing mental disorders into neat boxes does not work.

This discussion deals with "disorders" like autism spectrum disorder, obsession-compulsive disorder, attention deficit/hyperactivity disorder, bipolar disorder, schizophrenia, and many others, which are what clinical researchers call "phenotypically diverse" — in each case, exhibiting some subset of a wide range of symptoms. Medical culture has always assumed that this diverse array of behavioral manifestations must somehow be grouped into a hierarchy of natural kinds, like species of mushrooms, even if it takes experience and expertise to recognize which box a particular specimen belongs in. And modern medical insurance amplifies this prejudice. But as the cited Nature article notes, this boxology doesn't really work very well in the area of behaviorally-defined mental disorders.

A friend of mine jokes that in this domain , "phenotypically diverse" is the Greek translation of "we have no fucking clue".  His perspective is that there's a complex multidimensional space that we all live in, but don't understand very well. Some corners of this space interfere with people's lives to the extent that they're medicalized. This requires the creation of diagnostic boxes, even if the criteria are things like "three or more of the following eight symptoms", and both the boxes and the decision criteria change from edition to edition of the DSM.

This view is represented in the Nature article:

Some psychiatrists are already trying to reimagine their discipline with dimensions in mind. In the early 2010s, there was a push to eliminate disorder categories from the DSM-5 in favour of a ‘dimensional’ approach based on individual symptoms. However, this attempt failed — partly because health-care funding and patient care has been built up around the DSM’s categories. However, other catalogues of disorders have shifted towards dimensionality. In 2019, the World Health Assembly endorsed the latest International Classification of Diseases (called ICD-11), in which some psychopathologies were newly broken down using dimensional symptoms rather than categories.

The challenge for the dimensionality hypothesis is obvious: how many dimensions are there, and what are they?

The article lays out some research aiming at finding answers to that question. I have two reactions:

  1. The relatively small amount of available data, and its relatively poor quality in most respects, is a big problem. It's like trying to analyze a language based on a sample of a few hundred sentences. Clinical research is several decades behind the Big Data curve, partly due to valid concerns for privacy and confidentiality, but also simply for cultural reasons, especially researchers' possessive attitude about "their" data. Finding ways to solve this problem is a key issue — maybe THE key issue.
  2. The dimensions resulting from PCA or t-SNE (or any other dimensionality-reduction method applied to a collection of multi-dimensional observations) may be interesting and even useful, but it's a serious mistake to believe too quickly that they're real. This is a trap that IQ researchers fell into many years ago, and we're still paying the price.



  1. MattF said,

    May 12, 2020 @ 9:49 am

    Both 'dimensional' and 'boxing' methods will have underlying models, and these models have to make sense. If your dimensions turn out to be 'horse', 'bunny', and 'socket wrench'… what have you learned?

  2. Gregoire said,

    May 12, 2020 @ 10:39 am

    The idea that mental illnesses are just extremities in personality or behavior is neither novel or promising. The whole field understands that mental illness is not something that fits into neat categories, but I really fail to see how a patient with symptoms like "suffers auditory hallucinations", "is tortured by intrusive thoughts", or "has violent mood swings" is going to be better served by a personalized spider chart. And I have to balk at "phenotypically diverse", we're not talking about mushrooms with different shaped caps, we're talking about illnesses that have serious and often life-threatening impacts.

  3. Stephen J said,

    May 12, 2020 @ 7:47 pm

    I see they did not consider the idea that having fallen into the hands of the profession, you will receive more and more labels simply because they scrutinise you more and more. The likelihood of having more than one "disorder" may be a consequence of the profession's approach to diagnosis and not reflective of any underlying reality.

  4. Jon said,

    May 12, 2020 @ 9:43 pm

    I recall an attempt more than 20 years ago to treat all diseases this way. The idea was that each symptom was a vector in multi-dimensional space. You would add all of a patient's vectors, which would take you to a location in multi-dimensional space. Each disease would appear as a cloud in this space. Calculating the distance from the patient's location to the centre of each cloud would indicate which disease the patient was most likely to be suffering from.
    It did not carry any implication that different diseases had any connection between them. I don't know what happened to this idea.

  5. bks said,

    May 13, 2020 @ 7:35 am

    As if creating a taxonomy of mushrooms is straightforward!
    "Species and Speciation in Mushrooms: Development of a species concept poses difficulties"

    In physics, all electrons are identical. In biology every cell is unique.

  6. anonymouse said,

    May 14, 2020 @ 1:38 am

    " cultural reasons". Actually, real "cultural reasons" complicate this issue immensely. It is well known that different biases and "folk understandings" of mental and biological function change how people report their symptoms of mental illness and how much of an impact those symptoms have on daily functioning. It matters whether "intrusive auditory hallucinations" are seen as a message that you're irreversibly broken and need to be shunned or whether it's a sign of potential connection with the divine. These things feed on themselves; it would not surprise me if the perception of whether the symptoms are threatening or welcome influences the severity and tenor of the hallucinations themselves.

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