Until a vaccine is widely available, face masks and other nonpharmaceutical interventions (NPIs) will continue to be the frontline defense against Covid-19 in developing countries. But their effectiveness depends critically on compliance by young adults, who are most likely both to become...

Anecdotal, empirical, and experimental evidence suggests that offering extrinsic rewards for certain activities can reduce people's willingness to engage in those activities voluntarily. We propose a simple rationale for this 'crowding out' phenomenon, using standard economic arguments. The...