Left handedness and life expectancy: Causal inferences cannot be trusted

BMJ 1994; 308 doi: http://dx.doi.org/10.1136/bmj.308.6925.408 (Published 5 February 1994)
Cite this as: BMJ 1994;308:408.1

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  1. R Peto

    EDITOR, - Bryan S Turner was entirely wrong to conclude, from the evidence presented, that left handed individuals have a substantially lower life expectancy than otherwise similar right handers. Almost always in epidemiology, causal inferences cannot be trusted if they depend importantly on comparisons between the “average ages at death” of people with different characteristics. This is because the prevalences of so many human characteristics vary strongly with age. (What should instead be compared is the death rates among people who differ only in the characteristic of interest, and not in age.)

    Several decades ago, left handedness used to be strongly discouraged in schools, but nowadays this is much less the case. At present, therefore, the proportion of adults who remain left handed is larger among the young than among the old. Hence, in 1994 the proportion of left handers will likewise be larger among those who die at 40 than among those who die at 80. This is turn implies that the “average age at death” for those who die in 1994 will be lower for left handers than for right handers. But, as a left handed statistician, I do not find this sinister.

    References

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    Studies mislead on age at death

    1. A Burgess

      EDITOR, - In his editorial on left handedness Bryan S Turner …

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