He argued that nations should specialize in goods they could efficiently produce and trade them for others they could not produce so efficiently. David Ricardo later formulated the classic trade theory based on comparative advantage, which shows that even countries that do not have absolute advantage in the production of any products or services benefit from specialising in the production in which they have a relative production advantage and importing others, no matter the product and no matter the degree of specialisation. Koren and Tenreyro (2007) show that poor countries tend to specialize in sectors with relatively simple production technologies and a narrow range of inputs (as the theory of comparative advantage proclaims) and therefore are more vulnerable to economic shocks.