Humiliation, the soda tax, and deadweight loss

Catherine Rampell’s excellent column considers the case for a soda tax in Britain.  Here is one bit:

Why not just target the output, rather than some random subset of inputs? We could tax obesity if we wanted to. Or if we want to seem less punitive, we could award tax credits to obese people who lose weight. A tax directly pegged to reduced obesity would certainly be a much more efficient way to achieve the stated policy goal of reducing obesity.

Of course, “fat taxes,” even when framed as weight-loss tax credits, seem pretty loathsome. Why is . . . unclear.

We tax soda instead, even though that is less effective, for instance because soda drinkers may substitute into other sugary beverages.  We are unwilling to humiliate the obese by taxing them directly, and so our chosen policies do less to help…the obese.  (That’s assuming that attempting to shift their consumption behavior helps them at all, which is debatable.)  As Robin Hanson has told us many times, politics isn’t about policy…