In summary, a long series of studies have investigated the relationship between income and pollution as defined by the Environmental Kutznets Curve. Papers by Grossman and Krueger (1991), Shafik and Bandyopadhyay (1992) and Selden and Song (1994) presented evidence that some pollutants have historically followed an inverted U-curve with respect to income. Although these and other empirical studies point to a correlation between income and pollution, the causal relation is not observed for all sets of data. That is, there seems to be a highly specific and controlled environment under which the EKC condition.
