## Sinister Statistics

Thursday, September 12th, 2013

Alex Tabarrok examines some sinister statistics:

Over the 20th century, left handers have increased as a fraction of the population. Left handedness may be relatively fixed as a genetic matter but in the earlier decades of the 20th century children were strongly discouraged from exhibiting left-handedness. As a result, many “natural” lefties learned right-handed behavior and identified as right-handed adults. Over time, however, the cultural suppression of left-handedness declined and the proportion of adults exhibiting left-handedness increased, as the figure, at left, indicates.

Now suppose you take a random sample of people who died in 1990. In this sample, some people will have died old and some young. Among those those who died old, however, fewer people will be identified as left-handed because the old grew up in a time when left-handedness was suppressed. As a result, the old deaths in your sample will tend to be have more right-handed people and the young deaths will tend to have more left-handed people causing you to incorrectly conclude that left-handed people die younger. Studies show that this statistical artifact can easily explain a 9 year difference in apparent mortality rates.

To make this crystal clear consider the following thought experiment (offered by Chris McManus). Imagine you take a sample of people who died recently and asked their surviving family members, Did the deceased ever read the Harry Potter novels? One would clearly find in such a sample that those who died tragically young (at age 12 let’s say) would have been much more likely to have read Harry Potter than those who died in their 90s. Despite what some might argue, however, we should not conclude that Harry Potter kills.