All human societies exhibit some degree of division of labour by gender. These divisions continue to exist as participation in paid work has increased over time. Gender divisions occur between household tasks, between unpaid and paid work, and within paid work. Economists have explained these divisions through reliance on essentialist arguments and/or the fundamental economic concepts of efficiency of specialization and division of labour, and investment in human capital. However, gender discrimination can also cause division of labour, and the feedback effects of such discrimination make it difficult to untangle the causes of the gender division of labour.