Sunday, December 29, 2013

Left-handedness is not a major cause of death

From Sinister Statistics: Do Left Handed People Die Young? by Alex Tabarrok. A wonderful example of the importance of understanding context when interpreting statistics. The critical thing is to ensure that you are comparing apples-to-apples and yet there are all sorts of obscure and historical contextual issues that stand in the way of making a like-to-like comparison.
In 1991 Halpern and Coren published a famous study in the New England Journal of Medicine which appears to show that left handed people die at much younger ages than right-handed people. Halpern and Coren had obtained records on 987 deaths in Southern California–we can stipulate that this was a random sample of deaths in that time period–and had then asked family members whether the deceased was right or left-handed. What they found was stunning, left handers in their sample had died at an average age of 66 compared to 75 for right handers. If true, left handedness would be on the same order of deadliness as a lifetime of smoking. Halpern and Coren argued that this was due mostly to unnatural deaths such as industrial and driving accidents caused by left-handers living in a right-handed world. The study was widely reported at the time and continues to be regularly cited in popular accounts of left handedness (e.g. Buzzfeed, Cracked).

What is less well known is that the conclusions of the Halpern-Coren study are almost certainly wrong, left-handedness is not a major cause of death. Rather than dramatically lower life expectancy, a more plausible explanation of the HC findings is a subtle and interesting statistical artifact. The problem was pointed out as early as the letters to the editor in the next issue of the NEJM (see Strang letter) and was also recently pointed out in an article by Hannah Barnes in the BBC News (kudos to the BBC!) but is much less well known.
It is also a good example of cognitive pollution. The initial study came out in 1991 and the statistical flaw in the conclusions was identified in the very next issue. Yet, here we are twenty-two years later, the false conclusion is still being reported as fact. Why? Probably because it fits a preferred narrative (left handers are different and prone). The compatibility of the conclusion with the assumed narrative overrides the actual facts.

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