Previously, we’ve talked about how enlarged pupils can signify attention or arousal. If someone’s pupils dilate when you talk to them, conventional wisdom went, that means they’re very interested in what you have to say. It’s been speculated that this is why restaurants go in for mood lighting: the low light helps create the illusion that your dining companion is riveted, and thus increases the odds you’ll stay long enough to linger over that slice of cheesecake or extra glass of wine. However, it’s not so simple as looking into your date’s eyeball and reading their mind.
In a 2017 study, researchers from the University of Amsterdam and the University Medical Centre Hamburg-Eppendorf found that enlarged pupils can also be a sign of indecision.
Here’s how they support this conclusion: in a recent study, 27 subjects sat in a dark room in front of a computer screen, which displayed a series of problems. In each sequence, the participant had to make a judgement about dots moving across this screen, with varying difficulty levels. An infrared camera was there to measure pupil dilation as the subjects arrived at each choice—and waited three seconds to learn the correct result.
A clear pattern emerged: the greater the uncertainty about a given problem, the greater the pupil dilation. Those enlarged pupils were also more likely to show up when the participant gave a wrong answer, suggesting that dilation in this context is not a safeguard against making mistakes, but more a side effect of experiencing doubt.
When you’re very focused on something, this can trigger a flood of modulatory neurotransmitters, like serotonin, dopamine, and noradrenaline. These chemical messengers influence communication between your neurons. However, this cocktail doesn’t just show up when your attention is riveted by a fascinating conversation; it also may be sparked by simple indecision.
This is not altogether shocking. Writer and psychologist Daniel Kahneman had already written about the link between pupil dilation and mental strain in his landmark book Thinking Fast and Slow. Give someone an easy multiplication problem and their pupils enlarge ever so slightly. Make them really go to town on a doozy of an equation and those pupils widen far more. Kahneman went so far as to write that, simply by watching a subject’s eyes, it was possible to predict the moment at which they’d give up on a math problem.
If we consider indecision to be another form of mental strain, then the 2017 findings square with what we already knew—although, of course, the experiment would need to be replicated with a sample size larger than 27 before we could positively affirm it.
Still, it’s something to think about the next time you’re out to dinner, eyes locked on your dining companion. Maybe their pupils are speaking the language of love, or maybe they’re simply pondering whether or not to order that cheesecake…