Left-handed people don’t die young

The New Yorker has a rundown of the studies looking for links between left-handedness and various other traits. Contrary to beliefs of 100 years ago, left-handed people aren’t more likely to be criminals or schizophrenic, but we do do better, on average, in certain kinds of cognitive tests. So there.

A long time ago, I used to hear about how, as a left-hander, I should expect to die young and poor. As the New Yorker piece points out, that was debunked a long time ago. Studies showing that left-handers die younger than right-handers reached that conclusion due to a problem well known to astrophysicists: selection bias.

Some stars are giant stars, and some stars aren’t. Suppose that you tried to figure out what percentage of stars were giants. If you do a survey of the nearest stars, you get one answer, but if you do a survey of very distant stars, you get a different answer: in the latter case, you find a higher percentage of giants. You might conclude that something in our local environment stops giant stars from forming. But in fact there’s a different explanation: it’s easier to spot giant stars than small stars. When we’re looking nearby, we see all the stars, but when we’re looking far away, we miss some, and the ones we miss tend not to be giants.

The same thing happened with studies of left-handed people. When we look at young people, we find all the left-handers, but when we look at old people, we miss some, because in the past left-handed people used to be “converted” to right-handedness. (This happened to my uncle, for instance.) So when you look at a sample of old people, you think that a bunch of lefties have gone missing. If you didn’t take into account the fact that lefties used to be converted, you’d think that that meant that lefties die young.

 

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