Uncertainty, a powerful stress producer

A new study has found that uncertainty creates more stress than knowing for definite that something unpleasant or painful is about to happen.

The UCL study found that situations in which subjects had a 50 percent chance of receiving a shock were the most stressful while 0 percent and 100 percent chances were the least stressful. People whose stress levels tracked uncertainty more closely were better at guessing whether or not they would receive a shock, suggesting that stress may inform judgements of risk.

The experiment involved 45 volunteers who played a computer game in which they turned over rocks that might have snakes under them. They had to guess whether or not there would be a snake, and when there was they received a mildly painful electric shock on the hand.

Participants' uncertainty that any individual rock would have a snake under it was estimated from their guesses using a sophisticated computational model of learning. This uncertainty matched the stress levels reported by participants, which was also tracked using measurements of pupil dilation and perspiration.

Lead author Archy de Berker said that the experiment allows them to draw conclusions about the effect of uncertainty on stress. It turns out that it's much worse not knowing you are going to get a shock than knowing you definitely will or won't. "We saw exactly the same effects in our physiological measures, people sweat more and their pupils get bigger when they are more uncertain."

This is the first time that the effect of uncertainty on stress has been quantified, but the concept is likely to be familiar to many people.

Co-author Dr Robb Rutledge said, "The most stressful scenario is when you really don't know. It's the uncertainty that makes us anxious. The same is likely to apply in many familiar situations, whether it's waiting for medical results or information on train delays."

Senior author Dr Sven Bestmann said that appropriate stress responses might be useful for learning about uncertain, dangerous things in the environment. Modern life comes with many potential sources of uncertainty and stress, but it has also introduced ways of addressing them.

The study is published in Nature Communications.