The Mexican fat-tax experiment isn’t bolstered by payouts of pesos. But it’s probably still worth a try, and for three reasons:

First, the Danish failure is not dispositive. Denmark is a small place with a lively tradition of cross-border shopping. From Copenhagen, it is an easy 35-minute drive to Malmo, Sweden’s third-largest city. Southern Denmark conveniently adjoins shopping in northern Germany. Mexico’s population is concentrated in and around the capital city, far from any border.

Second, while incentives may work better than penalties in controlled experiments, it seems administratively unfeasible to operate such schemes on any large scale. It’s one thing to weigh and pay 204 people, a very different thing to weigh and pay millions of them.

Finally Mexico’s obesity problem is much worse than Denmark’s — and trending fast in the wrong direction. Even marginal improvements are worth pursuing there. In particular, simply reducing Mexican soda consumption could yield substantial benefits. When things are bad enough, policymakers have more reason to say: “What the heck, it’s worth a try.”