Commodity taxes and welfare under endogenous market conduct

We consider consumption taxes in a model of endogenous Cournot versus Bertrand competition. It is argued that when the choice of unit versus ad valorem taxes affects longer-term decisions beyond the customary price or quantity decisions, the mix of the two taxes co-determines market conduct. This gives ad valorem taxes an anti-competitive effect that harms ad valorem taxes’ efficiency in comparison with unit taxes. We show that a mix of the taxes—or a unit tax alone if we compare one or the other of the taxes—is sometimes welfare superior on account of consumer-price and tax revenue effects. A practical implication of our findings is that pass-through rates are only sometimes useful guides for policy. In fact, we show when the proper response to demand for higher revenue is a higher unit tax rate and a lower ad valorem tax rate.