A Clever Pupil

This column originally appeared in The Observer magazine/Guardian online

What’s the link between intelligence and pupils? It’s that clever children do better at school, right? Well, perhaps so, but a fascinating new study suggests another link that has probably never even crossed your mind: that between intelligence and the size of your pupils. That’s right: the eye kind.

So, let’s give it a try. You need to be in a room that is well-lit, but not overly bright (since this causes the pupils to shrink), and not to be thinking about anything in particular (since thinking through difficult problems, or even just holding things in memory, causes them to grow). If you want to try to recreate the conditions of the relevant study as accurately as possible, look at a small white cross on a black background, about 70cm away from your face. Now have a friend measure your pupils – carefully! – using a clear ruler. If you’re doing in correctly, the diameter of each should be somewhere between 4mm and 8mm.

Bearing in mind that these are just averages, and there are exceptions in both directions, the larger your pupil size, the higher your fluid intelligence (as measured, for example, by tests in which participants select the next number in a sequence). The average is around 6mm, with larger or smaller pupils linked with above-average and below-average intelligence respectively. In fairness, since measuring your pupils with a ruler is tricky, and so many other factors influence pupil size, you should take the absolute measurements with a pinch of salt. That said, as the authors of the original study point out, a difference of about 1mm is “likely observable to the naked eye”, meaning that whether – in general – your pupils seem to be larger or smaller than those around you is pretty good rough-and-ready measure.

Why should pupil size be linked with intelligence? One possibility is that the brain systems which regulate pupil size are those responsible for switching the brain out of the resting “default mode” and into action; something that intelligent people are better at.

A fully referenced version of this article is available at benambridge.com. Order Psy-Q by Ben Ambridge (Profile Books, £8.99) for £6.99 at bookshop.theguardian.com

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