Recent protests from beverage distributors over Philadelphia's new sugary drink tax may have obscured one of its key goals: reducing consumption of sugary beverages.

The tax is, after all, a "sin tax," meant to discourage harmful behavior. So if not by looking at sales, how do we judge the success or failure of a sin tax? That subject is taken up in a new study co-authored by Wharton Professor Benjamin Lockwood, who spoke with NewsWorks Tonight Host Dave Heller.