The Two Types of Mental Disorder

Published by timdean on

Here’s a hypothesis: there are two broad types of mental disorder, and distinguishing between them might aid in understanding their causes as well as in diagnosis and treatment. The idea springs from my thoughts on the evolutionary forces that have shaped our psychology, particularly in selecting for a diversity of personality types and cognitive styles.

First up, I define mental disorder along pretty broad functional lines as being any lasting psychological condition that hampers an individual’s ability to pursue their interests. I prefer the functional approach (as I do in many cases) rather than trying to dig around to find some essential core or set of features that characterises all mental disorders, but that doesn’t preclude other modes of defining mental disorder.

The benefit of the functional approach is in seeing the mind as an evolved tool, the function of which is to produce behaviours that serve to satisfy our interests (defining what these interests are is another important issue, but I’ll leave that to one side for now).

So, in this light, I propose that there are two broad types of mental disorder:

The first is caused by what might be called a ‘malfunction’ from the normal function of the brain. This might include things like schizophrenia, autism, aphasia, amnesia, Tourette’s and the kinds of things Oliver Sacks writes about.

These can be understood by starting with the normal function of the brain – again defined in functional terms as those functions that enable us to behave in a way that serves our interests – and then looking for specific malfunctions that prevent that from happening.

For example, schizophrenia may be caused in part by a malfunction of the brain structures responsible for the internal monologue, disassociating the inner voice from the sense of self, thus producing a separate voice that can influence behaviour. It might also involve malfunctions of the regions of the brain that process perceptual information before they’re reacted to by higher cognition. Whatever they are, they appear to involve areas of the brain that are not functioning ‘normally’ in the evolutionary functional sense.

Contrasting this could be a second broad type of mental disorder which is simply an extreme version of natural variation in some psychological trait or cognitive style. This might include things like Asperger’s, obsessive compulsive disorder, depression, many affective and mood disorders, phobias, anxiety etc.

Rather than being caused by an outright malfunction, they might be caused by extremes of natural variation. For example, negative emotions serve a functional role of negative reinforcement of the stimuli and demotivating behaviour that might cause those stimuli from reoccurring. There is likely a natural variation in terms of how sensitive we are to negative emotions, how intense we feel them, and in concert with experience, what triggers them and how intensely. Someone who suffers from depression may just be experiencing the normal negative affect, only at a more extreme intensity than is functionally normal, thus impairing behaviour.

Likewise for things like Asperger’s, which may be caused by an extreme of the systematising cognitive style, as defined by Simon Baron-Cohen.

Obsessive-compulsive disorder might be taking our normal concern over order and cleanliness etc and pushing it to an extreme.

In some sense there may be some ‘malfunction’ in these variation-based disorders, but it’s not a fundamental break in the normal function, but instead pushing the normal function to an extreme, thus interfering with normal behaviour.

This is conjectural at this stage. And I profess that psychopathology isn’t my area of expertise. But I am interested in the function of minds, and that raises the prospect of accounting for malfunction. So thought I’d put this out there and see what people think.


Matthew Hammerton · 9th February 2012 at 4:53 pm

Hi Tim, interesting distinction, I find it prima facie plausible. I also think your definition of ‘mental disorder’ is pretty good, but here is a counterexample: being inclined to consider distant future consequences of our actions when deliberating on what action to take is a lasting psychological condition that many people have (e.g. I may ask myself ‘what are the consequences for me in 40 years time of if I fail to use sun cream?’). In certain circumstances this lasting psychological condition hampers an individual’s ability to pursue her interests. For example, if I am destined to die in the near future (perhaps I have an undiagnosed medical condition that will kill me before I reach 30) then considering distant future consequences will be against my interest on any plausible account of interest (e.g. hedonism, preference satisfaction, objective list theory). Yet it seems wrong to say that considering distant future consequences is a mental disorder. We usually consider it a mental virtue, and even in the case where it works against ones interest we intuitively don’t want to say that it constitutes a mental disorder as it seems of a totally different kind to archetypical mental disorders.

A reverse of this counterexample is cases where archetype mental disorders like schizophrenia or amnesia actually serve the interest of the person who suffers them. We probably want to claim that they are still mental disorders even in cases where they happen to benefit the person who possesses them.

Tim Dean · 10th February 2012 at 2:01 pm

Hey Matt. I hear what you’re saying but I think discounting could resolve that issue with trading off future and present preferences. We already discount automatically, but rational deliberation can probably do so more accurately, taking into account the probability of premature death etc.

Then there just needs to be a vague line where if someone sacrifices too much of their present interests such that it hampers their ability to function could be considered a mental disorder. For example, there’s a baseline of present interests that need to be served in order for me to live long enough to satisfy future interests.

And sure, some mental disorders can serve some interests better, but I’d suggest they would all come at a cost of harming other interests more.

Also, I should flag that I’m not terribly convinced by any of the “plausible” accounts of interest. In short, my account of interests (which I need to articulate on this blog in more detail some day) suggests interests start with our evolved biology, including basic interests like sustenance, health, bodily integrity, psychological wellbeing etc. I’d call these ‘ultimate interests’.

However, knowledge of these interests is elusive. We more often follow our ‘proximate interests’ that are generally heuristics that steer our behaviour to serve our ultimate interests. Pleasure and pain, our sweet tooth, our desire for social interaction etc are the heuristics that serve our ultimate interests, although they can also harm them, such as through overeating or status anxiety.

Then we have our ‘reflective interests’, which are those interests we think we have. These are the interests that most people cite when asked what motivates their behaviour, but they may or may not be pegged to their ultimate or proximate interests, and can often be confused or contradictory.

It’s for this reason I think the “plausible” accounts of interests are generally wrong. They tend to focus exclusively on the proximate or reflective interests and ignore the importance of ultimate interests. And I’d suggest that most people are actually pursuing their ultimate interests, but doing so poorly by chasing proximate interests.

David Duffy · 13th February 2012 at 12:42 pm

It sounds like neurosis v psychosis. However, things are never that simple eg


…does not resolve the question of whether an OCD patient who completely lacks insight should be diagnosed with a psychotic disorder instead of OCD.

This is just as true of major depression, where one can get psychotic features.

The converse example is personality disorders, such as schizotypal personality. That is, all mental disorders can be seen as lying on a spectrum blurring into normality.

David Moss · 16th February 2012 at 6:29 am

I’d second DD’s point. It seems like there isn’t too clear a distinction given that some of the examples you cite in the different groups are themselves considered to merely be two ends of a continuum (e.g. Aspergers/high functioning autism and plain (lower functioning) autism).

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