Alfred Marshall, Knut Wicksell, and John Maynard Keynes
Marshall was motivated to study economics observing poverty among labourers in the East End of London. In chapter 1 of his Principles of Economics (1890), he states his object of studying economics.
He found the primary cause of poverty of labourers in the low quality of their labour. Theoretically, this means a low productivity of labour, and the wage rate will become equal to the marginal productivity of labour. Usually it is assumed that the marginal productivity of labour is decreasing. In this view of distribution of income, a low wage rate is caused by low quality of labour or excessive supply of labour. His way to better the standard of life of labourers was to raise the productivity of labourers and to restrain population growth. He proposed to give them education, more leisure time and better living conditions (better food, better houses, a better environment, and so on), in short, to make the labourers more educated and more healthy.Knut Wicksell in Sweden had a keen interest in the problem of poverty. He saw the main cause of poverty in an excessive population. He ardently supported neoMalthusianism. In his main work, Lectures on Political Economy (original Swedish version, 1901), chapter 1 begins by discussing the population problem, although that chapter was not included in the English (1934) version.
As is well known, J.M. Keynes elaborated his theory of employment to show the fundamental cause of mass unemployment in the 1930s. Unemployment is one of the most serious causes of poverty in affluent societies (The General Theory of Employment, Interest and Money, 1936). He also had a profound interest in the population problem, because he believed that an excessive increase of population would lead to the decline of the standard of living of the people of the country by making the terms of trade in international trade worse for Britain. In his youth, Keynes was a neo-Malthusian and participated in the movement to promote the use of artificial birth control, but in the 1930s he changed his mind and became concerned with the decrease of population after there appeared forecasts of a future decline of the population in Britain (see Toye 2000).