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 donâ€™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 â€œpraecox effectâ€ (as in dementia praecox, or schizophrenia):
if, after having spoken to someone for half an hour, you have no idea what theyâ€™re on about, theyâ€™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 didnâ€™t take me too long to see the problems with this rule of thumb.