On the surface the logic of a soda tax seems simple: raise the price of an unhealthy food, people consume less, and public health improves. But, as I've pointed out again and again on this blog, the story is much less simple than it first appears.
First, even if we believe people suffer from various behavioral biases, higher prices almost certainly make people worse off. Second, when we raise the price of one unhealthy thing, people might substitute to consume other unhealthy things. Third, if the tax is just added at the checkout counter and not on the shelf display, it may not have nearly the effect on purchase behavior as assumed. Forth, if people know the reason for the tax, some may "protest" and buy more instead. Fifth, the projected weight loss from such taxes often relies on unreasonable rules of thumb like 3500kcal=1lb. Six, even when taxes have an effect, the causal impact may arise more from an "information effect" rather than a "price effect." Seventh, such taxes may induce unanticipated effects because of how sellers respond to the policy. Finally, soda taxes are regressive - having a proportionally larger effect on on lower income households (see also my co-authored paper on effects of "unhealthy" food taxes more generally).
Now, comes this new paper in the American Journal of Agricultural Economics by Emily Wang, Christian Rojas, and Francesca Colantuoni, which incorporates the insight that some households are more likely to respond to promotions and to store. The abstract: