This paper attempts to present a simple Cournot model to determine the optimal discriminatory tariff rates, when the international market is a homogeneous (heterogeneous) product market. In the model, there are two foreign firms located in two different countries competing for sales in a local market. When the firms sell a homogeneous product and the home country can impose differential tariffs on imports from different countries, then the tariff on the low-cost producer should be higher. In particular, with constant marginal costs the tariff difference should equal half the cost difference. On the other hand, if the firms sell differentiated products with symmetric linear demands and technologies, then the weaker the degree of product differentiation, the greater is the tariff difference compared with the cost difference.
The Canadian Journal of Economics 24(3), pp.693-702