To measure stress, the researchers looked at pupil dilation, perspiration and reports by participants.
The higher the levels of uncertainty, say the findings, the more stress people experienced. The most stressful moments were when subjects had a 50% chance of receiving a shock, while a 0% or 100% chance produced the least stress.
People whose stress levels correlated closely with their uncertainty levels were better at guessing whether or not they would receive a shock, suggesting that stress may help us to judge how risky something is.
Lead author Archy de Berker comments:
"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."
While many people will find the concept familiar, this is the first time for research to quantify the effect of uncertainty on stress.