The Kuznets curve () expresses a hypothesis advanced by economist Simon Kuznets in the 1950s and 1960s. According to this hypothesis, as an economy develops, market forces first increase and then decrease economic inequality. The Kuznets curve appeared to be consistent with experience at the time it was proposed. However, since the 1960s, inequality has risen in the US and other developed countries, which invalidates the hypothesis.
Kuznets ratio and Kuznets curve
The Kuznets ratio is a measurement of the ratio of income going to the highest-earning households (usually defined by the upper 20%) to income going to the lowest-earning households, which is commonly measured by either the lowest 20% or lowest 40% of income. Comparing 20% to 20%, a completely even distribution is expressed as 1; 20% to 40% changes this value to 0.5.
Kuznets curve diagrams show an inverted U curve, although variables along the axes are often mixed and matched, with inequality or the Gini coefficient on the Y axis and economic development, time or per-capita incomes on the X axis.
Explanations
One explanation of such a progression suggests that early in development, investment opportunities for those who have money multiply, while an influx of cheap rural labor to the cities holds down wages. Whereas in mature economies, human capital accrual (an estimate of income that has been achieved but not yet consumed) takes the place of physical capital accrual as the main source of growth; and inequality slows growth by lowering education levels because poorer, disadvantaged people lack finance for their education in imperfect credit-markets.
The Kuznets curve implies that as a nation undergoes industrialization – and especially the mechanization of agriculture – the center of the nation's economy will shift to the cities. As internal migration by farmers looking for better-paying jobs in urban hubs causes a significant rural-urban inequality gap (the owners of firms would be profiting, while laborers from those industries would see their incomes rise at a much slower rate and agricultural workers would possibly see their incomes decrease), rural populations decrease as urban populations increase. Inequality is then expected to decrease when a certain level of average income is reached and the processes of industrialization – democratization and the rise of the welfare state – allow for the benefits from rapid growth, and increase the per-capita income. Kuznets believed that inequality would follow an inverted "U" shape as it rises and then falls again with the increase of income per-capita. Kuznets had two similar explanations for this historical phenomenon: workers migrated from agriculture to industry; and rural workers moved to urban jobs. In both explanations, inequality will decrease after 50% of the shift force switches over to the higher paying sector.
Evidence
Inequality has risen in most developed countries since the 1960s, so that graphs of inequality over time no longer display a Kuznets curve. Piketty has argued that the decline in inequality over the first half of the 20th century was a once-off effect due to the destruction of large concentrations of wealth by war and economic depression.
The Kuznets curve and development economics
Critics of the Kuznets curve theory argue that its U-shape comes not from progression in the development of individual countries, but rather from historical differences between countries. For instance, many of the middle income countries used in Kuznets' data set were in Latin America, a region with historically high levels of inequality. When controlling for this variable, the U-shape of the curve tends to disappear (e.g. Deininger and Squire, 1998). Regarding the empirical evidence, based on large panels of countries or time series approaches, Fields (2001) considers the Kuznets hypothesis refuted.