Rules of thumb are dumb

Some rules of thumb are derived from experience, accurate or otherwise, (eg: the praecox effect) and some from hard research (eg: people with memory problems don’t admit to them), but all have a common flaw: even if true, they are both generalisations across a population and specific to the circumstances of their origin.

In an undergraduate lecture over twenty years ago, a senior Clinical Psychologist described the “praecox effect” (as in dementia praecox, or schizophrenia):

if, after having spoken to someone for half an hour, you have no idea what they’re on about, they’re schizophrenic

To an undergraduate struggling to cope with the complexities of psychology, psychiatry and mental health, any simple rule was welcome. Reading the literature on the high rate of psychotic diagnoses in immigrant populations, it didn’t take me too long to see the problems with this rule of thumb.

Later, while administering a lengthy but popular cognitive test (with over 100 items), I found a study which indicated that, in testing for organic brain damage, the test was no more sensitive or specific than a single question

do you have memory problems?

People who said “no” usually had some form of dementia, those who said “yes” were more likely to be depressed. If a single question is as sensitive and specific as the full test battery, why subject the client (and an underpaid, overworked Assistant Psychologist) to the arduous testing process?

Rules of thumb abound:

  • people you can’t understand are mad
  • people who admit to memory problems are actually depressed / anxious
  • alcoholics under represent their alcohol consumption
  • child sexual abuse is a male problem (so investigate Dad, not Mum)
  • stroke patients show little improvement after the first 18 months since their stroke
  • folded arms and crossed legs indicate a lack of receptivity, if not outright rejection

While the praecox effect can be dismissed as dangerously biased, the memory rule requires further examination. The rule did seem to be valid…in the majority of older adults causing sufficient concern to themselves and their families for their GPs to refer them to a Clinical Psychology department for psychometric testing. Bolting this rider onto the rule makes it less snappy and less attractive as a rule of thumb, as it is then much harder to generalise to, say, 30 year old cardiac patients.

Therapists have so much to remember that rules of thumb offer us a great temptation: simplicity and universality, compressing all the subtle variations across clients and settings into a single sentence. While this is one of the goals of reductionist science, rules of thumb are rarely subject to the same scrutiny as physical laws before they pass into general use.

The quest for rules of thumb continues, most recently in the journal Laterality, which reports that “patients suffering from depression or anxiety are more likely to choose to sit to their left when visiting the GP”. The BPS Research Digest reports the study fairly accurately (the finding was true for 59% of depressed or anxious right-handed patients in one GP’s consulting room) but references a previous report that depressed mothers cradle their babies on the right: following the link reveals that this was a non-significant trend in a sample of only 19 depressed mothers. A rule of thumb is on the way.

The recent drive in the NHS towards evidence-based practice has highlighted just how much clinical practice is based upon rules of thumb rather than hard research. Clinical guidelines have been created but, especially in the field of mental health, the basis of the guidelines remains expert opinion… and it was an expert who described the praecox feeling for us.

Consider that, in fifteen years of practice, having seen over two thousand individuals and their families, I have treated three football referees but never an estate agent. What does this say about the relative sanity of these two groups?

Nothing at all, of course.

Leave a Reply

Your email address will not be published. Required fields are marked *