At the same time as Cohen was working on Karl Marx's Theory of History, American economist John Roemer was employing neoclassical economics in order to try to defend the Marxist concepts of exploitation and class. In his A General Theory of Exploitation and Class (1982), Roemer employed rational choice and game theory in order to demonstrate how exploitation and class relations may arise in the development of a market for labour. Roemer would go on to reject the idea that the labour theory of value was necessary for explaining exploitation and class.