Wander into, say, a Walgreens drugstore, and you will find a 100 tablet package of 325 mg Bayer Aspirin selling for $7.99. Nearby, a 100 tablet package of Walgreens’ 325 mg private-label aspirin sells for $4.99.

There is no difference between the two products, other than the brand name. The dosages are the same, as is the active ingredient, acetylsalicylic acid. It’s hard to make economic sense of why people would buy anything but the less expensive generic. And yet, as evidenced by the space Walgreens gives to Bayer on its shelf, there are plenty of people willing to pay up.

Indeed, a new analysis of shopping data collected by the Nielsen Co. shows that 24% of aspirin sales by volume, and 59% by expenditure, is devoted to Bayer. Consumers who continue to buy branded over-the-counter drugs rather than generics are leaving a lot of money on the table, according to economists Jean-Pierre Dubé, Matthew Gentzkow and Jesse Shapiro at the University of Chicago Booth School of Business, with Bart Bronnenberg at Tilburg University. Singling out headache remedies like aspirin, ibuprofen and acetaminophen for which differences between branded and generic products are negligible, they calculate that if all American consumers switched to generics, spending would be reduced by $410 million.

But some consumers are far less likely to pay up for brand-name drugs than other.