Lerner symmetry theorem

The Lerner symmetry theorem is a result used in international trade theory, which states that, based on an assumption of a zero balance of trade (that is, the value of exported goods equals the value of imported goods for a given country), an ad valorem import tariff (a percentage of value or an amount per unit) will have the same effects as an export tax. The theorem is based on the observation that the effect on relative prices is the same regardless of which policy (ad valorem tariffs or export taxes) is applied.