Knowing that there is a small chance of getting a painful electric shock can lead to significantly more stress than knowing that you will definitely be shocked. A new 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 judgments of risk.