An article that just appeared in the most recent issue of the American Journal of Agricultural Economics by Yuqing Zheng of RTI and Edward McLaughlin and Harry Kaiser of Cornell University presents some interesting thoughts regarding fat and soda taxes.
Most of the studies on fat and soda taxes use elasticities of demand to simulate the effectiveness of a tax. The elasticity of demand tells us how responsive consumption of a product is to a change in it's price. So, for example, an elasticity of demand of -0.6 would tell us that for every 1% increase in price, consumption would fall by 0.6%
Zheng and colleagues point out, however, that most people don't "see" the tax when they're shopping. It only shows up on the bill when you check out. As such, it is incorrect to directly use the price elasticity of demand to infer how consumption of soda or fatty foods will change after a tax.
They show that an excise tax (which is levied on the supplier rather than the consumer) is probably more effective at reducing soda (or fat) consumption than a retail sales tax, but even excise taxes are likely to to be less effective at reducing consumption than an equivalent price increase.
As a result, researchers need to adjust the demand elasticities before calculating the effects of a soda or fat tax (regardless of whether the tax is excise or retail). Yet, almost no one does this. The researchers show, however, that the required adjustment can be fairly dramatic. In their "baseline" scenario, they show that the elasticity of demand needs to be reduced by a factor of 0.14. So, if the elasticity of demand was -0.6, then we'd project a 1% increase in sales tax would only reduce consumption by 0.6*0.14 = 0.084%. Instead, previous research on this topic has applied the original elasticity (e.g., 0.6) rather than the adjusted elasticity (e.g., 0.084). Clearly, the adjusted elasticity will imply much lower effectiveness of the tax.
The authors conclude:
Therefore, even a large increase in the sales tax rate on food and beverages will only reduce demand by a moderate degree.
and
our analysis of sales and excise taxes offers some explanation (e.g., imperfect tax knowledge, slow learning) on why the impact of sales tax is so difficult to detect, thus bridging the gap between the simulation studies and the empirical findings