What do a gym membership, a bottle of prescription pills and a holiday gift card have in common? Each of them is a thing that is bought and then often goes unused. In their recent paper “Paying Not to Go to the Gym,” the economists Stefano DellaVigna and Ulrike Malmendier showed that people who buy an annual membership to a health club overestimate by more than 70 percent how much they’ll actually use it. Many people, therefore, would be better off buying monthly or daily passes. ...As for gift cards — well, let’s just say there is good reason that...