The use of face masks has been a key response to the COVID-19 pandemic in almost every country. However, despite widespread use of masks policies in classrooms and offices around the world, almost nothing is known about their effects on cognitive performance. Here I show that mandatory mask-wearing has a negative causal effect on the cognitive performance of competitive chess players. I analyzed the quality of almost 3 million chess moves played by 8,531 individuals (ages 5-98) in 18 countries before and during the pandemic. Wearing a mask decreased the quality of players’ decisions – a measure of their cognitive performance – by approximately one-third of a standard deviation. However, the disruptive effect of masks is relatively short-lived, gradually weakening such that there is no measurable disadvantage from wearing a mask after roughly four hours of play. Moreover, the overall decrease is driven by a large, negative effect on experts, with little to no change in performance at lower levels. The results shed light on an important factor in the assessment of mask policies as the world moves into the phase of ‘living with COVID’.

zcarol2@ceibs.edu