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We study the impact of human capital and the level of education on the pollution-income relationship controlling for income inequality in 17 OECD countries. By applying an innovative approach to country grouping, based on the temporal evolution of income inequality and clustering techniques to feature the annual value of the Gini Index on disposable income from 1987 to 2015, we have estimated panel data models by distinguishing between low and high levels of income inequality country clusters. Robustness checks and endogeneity tests are further performed considering as the discriminant factor the income inequality affecting the countries in the sample. The findings highlight the role of the educational level and years of schooling in validating the EKC hypothesis. We recommend that this variable should not be neglected in future EKC studies. Therefore, any EKC theory should also acknowledge a new EKC model specification that we named the Educational EKC.

This study examines the impact of education on the pollution–income relationship, controlling for income inequality in 17 European OECD countries over the period 1950–2015. We developed a novel two-stage algorithm, whose first step consists in applying clustering techniques to group countries according to the income inequality temporal pattern. In the second step, we estimate the educational-mitigated EKC hypothesis (Educational EKC) by employing panel regression techniques accounting for endogeneity issues. The clustering findings suggest the existence of high variability in income inequality levels across countries and heterogeneous development patterns. Empirical estimates highlight that, for high income inequality countries, the Educational EKC hypothesis holds, and that the emissions–income elasticity appears to decline when including the schooling level. In the low income inequality cluster, these effects are not clear-cut. For these countries, we propose a different specification of the EKC, which substitutes the income per capita term with the years of schooling. The new specification is statistically validated for both high income inequality and low income inequality countries. In conclusion, we can state that education should be addressed as a crucial cornerstone to shaping the EKC curve.

International audience; This paper investigates the environmental Kuznets curves (EKC) for CO2 emissions in a panel of 109 countries during the period 1959-2001. The length of the series makes the application of a heterogeneous estimator suitable from an econometric point of view. The results, based on the hierarchical Bayes estimator, show that different EKC dynamics are associated with the different sub-samples of countries considered. On average, more industrialized countries show evidence of EKC in quadratic specifications, which nevertheless are probably evolving into an N shape based on their cubic specification. Nevertheless, it is worth noting that the EU, and not the Umbrella group led by US, has been driving currently observed EKC-like shapes. The latter is associated to monotonic income-CO2 dynamics. The EU shows a clear EKC shape. Evidence for less developed countries consistently shows that CO2 emissions rise positively with income, though there are some signs of an EKC. Analyses of future performance, nevertheless, favor quadratic specifications, thus supporting EKC evidence for wealthier countries and non-EKC shapes for industrializing regions.