Comparative Statics with Consumption Externalities

We consider the comparative statics of consumer demand when there are consumption externalities in one commodity between two individuals. We show that the externality can switch goods which would naturally be normal into inferior goods and as a result th e externality can also lead to Giffen goods. In addition the externality can transform complementarity relations between goods. Thus substitutes can become complements or vice versa once the feedback effects of the externality are taken into account. Next we consider the effect of externalities on Slutsky symmetry and negativity restrictions With consumption externalities there are generalised forms of such restrictions. We derive these both for the two individual case and for cases in which either there are two individuals but all goods may cause externalities or there is a single externality good but H individuals. We relate the generalised symmetry restrictions to the rank conditions of Browning and Chiappori. Finally we consider the effects of consumption externalities on consumer surplus analysis.