… is from page 13 of Douglas Irwin’s new, vital 2017 volume, Clashing Over Commerce (footnote excluded; links added):
The interdependence of exports and imports has long been recognized. A proposition known as the Lerner Symmetry Theorem holds that a tax on imports is equivalent to a tax on exports. In effect, by levying a tax to restrict imports, policymakers are also levying a tax that restricts exports.
DBx: Pictured here is Abba Lerner (who, although a man of the left, studied at the L.S.E. under Hayek.)