An Essay on the Principle of Population
Malthus' claim in "An Essay on the Principle of Population" (1798) that population growth was the primary cause of subsistence level wages for laborers provoked Marx to develop an alternative theory of wage determination. Whereas Malthus presented a historical theory of population growth, Marx offered a theory of how a relative surplus population in capitalism tended to push wages to subsistence levels. Marx saw this relative surplus population as coming from economic causes and not from biological causes (as in Malthus). This economic-based theory of surplus population is often labeled as Marx's theory of the reserve army of labour.