The division of labour is the specialization of cooperating individuals who perform specific tasks and roles. Because of the large amount of labour saved by giving workers specialized tasks in Industrial Revolution-era factories, some classical economists as well as some mechanical engineers such as Charles Babbage were proponents of division of labour. Also, having workers perform single or limited tasks eliminated the long training period required to train craftsmen, who were replaced with lesser paid but more productive unskilled workers. Historically, an increasingly complex division of labour is associated with the growth of total output and trade, the rise of capitalism, and of the complexity of industrialised processes. The concept and implementation of division of labour has been observed in ancient Sumerian (Mesopotamian) culture, where assignment of jobs in some cities coincided with an increase in trade and economic interdependence. In addition to trade and economic interdependence, division of labour generally increases both producer and individual worker productivity.