Sweating as a predictor of suicide? Seriously? (maybe a little)

This week’s surprisingly well sourced weird-ass science fact from cracked.com is “Among depressed people, sweating is heavily predictive of suicide.”  According to them, 97% subjects getting treatment for depression who sweated less in response to loud noises went on to commit suicide, compared with 2% of people with normal sweat response.  Those numbers are astonishing- and a sample size of 800, big for a psych study.  Not to doubt the website that brought us “6 Fictional Alcoholic Beverages That Actually Get You Drunk“, but I wanted to check the tape on this one.

1. This was actually a metastudy, combining the results of several other studies.  Five different studies with a total of n participants are not nearly as good as a single study with n participants, first because there’s more opportunities to throw out data, second because the experiments rarely have exactly the same set up.  A person who scored high-sweating in one set up might score low-sweating in another.
2. The total of 783 people includes people with bipolar disorder (126), depression  (540), and other, which meant either dysthemia (mild depression), or depression AND a personality or adjustment disorder  (118).  Those are different things.  Bipolar patients are at significantly higher risk of suicide than unipolar patients, because they’re more likely to simultaneously have the desire to end their life and the will to act on it.  And Other will include people with borderline personality disorder, which is really its own issue.
3. Only 36 people total completed suicide.  The researchers claim for specificity is for completed suicide (violent or not) AND violent attempts, which may occur before or after the study took place. They excluded non-violent suicide attempts out of the belief that non-violent attempts were more likely to have been cries for help.  9% of completed suicides were non-violent where 55% of attempts were, so that part seems fair. On the other hand, predicting the past is not nearly as impressive or useful as predicting the future.
4. In the discussion section, the authors acknowledge that many of the subjects were selected from mental hospitals where they had been admitted due to a past or feared suicide attempt.  This is what’s known as a biased sample.  The data is still suggestive of underlying physiological processes, but the specificity/sensitivity numbers can’t be translated to the general population without further work, and in particular I wouldn’t recommend using this as a diagnostic criteria for whether hospital admittance is a good idea.

So it looks like cracked was severely exaggerating the claims of this research, which is too bad.  But there does appear to be some effect, which could help us figure out the physiology of depression, which would be extremely useful.