Do Left-Handers Die Younger?
A famous 1991 study reported that left-handed people die younger than right-handed people. The explanation lies in how left-handedness became far more socially accepted across the twentieth century: people who died elderly grew up when many left-handers were forced to switch hands, while people who died young grew up in a more permissive era. Rebuild the argument yourself and see how the illusion arises from that shift alone.
Introduction
In 1991, Halpern and Coren analysed obituaries and found left-handed people died younger on average than right-handed people, and the finding made headlines. Left-handedness was suppressed by schools and parents earlier in the twentieth century, so people who died at an old age were less likely to have been allowed to stay left-handed as children, while people who died young grew up in a more permissive era. This cohort effect can produce exactly the age gap the 1991 study reported, without any effect of handedness on lifespan, and it can be uncovered with the same statistical tools that first missed it.
Guiding Questions
- State the claim precisely: left-handers die at a younger average age than right-handers. What data would you need to test it?
- Find or estimate the rate of left-handedness across different age groups today. What pattern do you see, and why might it exist regardless of any effect on lifespan?
- Build a contingency table of age group against handedness and run a chi-squared test for independence. Does the pattern in your data match a cohort effect?
- If you only had obituary data (ages at death, with handedness recorded), how would a cohort effect create the appearance of a lifespan difference even if none exists?
- What data collection design would avoid this bias entirely?