A list of duties or customs to be paid on imports. Tariffs may be imposed to increase the cost of imported goods in relation to domestic production, thereby reducing the volume of imports and keeping the balance of payments in credit, or to protect domestic industry from foreign competition. Preferential tariffs reduce import duties on products of a certain type or origin. The impact of tariffs may be less than expected: Behrens (2006) Papers Reg. Sci. 85, 3 shows that decreasing transport costs are more likely to trigger agglomeration than decreasing tariffs, and Medvedev (2006) World Bank Policy Research Working Paper 4038 finds that preferential trade agreements do not much increase trade flows. In a highly technical paper, Mai et al. (2006) CIRJE-F-435 show that, when the transport cost is sufficiently small, tariff competition coupled with firm migration leads to a core–periphery economy.