Surplus labour (German: Mehrarbeit) is a concept used by Karl Marx in his critique of political economy. It means labour performed in excess of the labour necessary to produce the means of livelihood of the worker ("necessary labour"). The "surplus" in this context means the additional labour a worker has to do in his/her job, beyond earning his own keep. According to Marxian economics, surplus labour is usually uncompensated (unpaid) labour. Marxian economics regards surplus labour as the ultimate source of capitalist profits.
Marx explains the origin of surplus labour in the following terms:
"It is only after men have raised themselves above the rank of animals, when therefore their labour has been to some extent socialised, that a state of things arises in which the surplus-labour of the one becomes a condition of existence for the other. At the dawn of civilisation the productiveness acquired by labour is small, but so too are the wants which develop with and by the means of satisfying them. Further, at that early period, the portion of society that lives on the labour of others is infinitely small compared with the mass of direct producers. Along with the progress in the productiveness of labour, that small portion of society increases both absolutely and relatively. Besides, capital with its accompanying relations springs up from an economic soil that is the product of a long process of development. The productiveness of labour that serves as its foundation and starting-point, is a gift, not of nature, but of a history embracing thousands of centuries."
The historical emergence of surplus labour is, according to Marx, also closely associated with the growth of trade (the economic exchange of goods and services) and with the emergence of a society divided into social classes. As soon as a permanent surplus product can be produced, the moral-political question arises as to how it should be distributed, and for whose benefit surplus-labour should be performed. The strong defeat the weak, and it becomes possible for a social elite to gain control over the surplus-labour and surplus product of the working population; they can live off the labour of others.
Labour which is sufficiently productive so that it can perform surplus labour is, in a cash economy, the material foundation for the appropriation of surplus-value from that labour. How exactly this appropriation will occur, is determined by the prevailing relations of production and the balance of power between social classes.