This paper contributes to the environmental literature by (i) demonstrating that the estimated coefficients and the statistical significance of the non-leading terms in quadratic, cubic, and quartic logarithmic environmental Kuznets curve (EKC) specifications are arbitrary and should therefore not be used to choose the preferred specification and (ii) detailing a proposed general-to-specific type methodology for choosing the appropriate specifications when attempting to estimate higher-order polynomials such as cubic and quartic logarithmic EKC relationships. Testing for the existence and shape of the well-known EKC phenomenon is a hot topic in the environmental economics literature. The conventional approach widely employs quadratic and cubic specifications and more recently also the quartic specification, where the variables are in logarithmic form. However, it is important that researchers understand whether the estimated EKC coefficients, turning points, and elasticities are statistically acceptable, economically interpretable, and comparable. In addition, it is vital that researchers have a clear structured non-arbitrary methodology for determining the preferred specification and hence shape of the estimated EKC. We therefore show mathematically and empirically the arbitrary nature of estimated non-leading coefficients in quadratic, cubic, and quartic logarithmic EKC specifications, being dependent upon the units of measurement chosen for the independent variables (e.g. dependent upon a rescaling of the variables such as moving from $m to $bn). Consequently, the practice followed in many previously papers, whereby the estimates of the non-leading terms are used in the decision to choose the preferred specification of an estimated EKC relationship, is incorrect and should not be followed since it potentially could lead to misleading conclusions. Instead, it should be based upon the sign and statistical significance of the estimated coefficients of the leading terms, the location of turning point(s), and the sign and statistical significance of the estimated elasticities. Furthermore, we suggest that researchers should follow a proposed general-to-specific type methodology for choosing the appropriate order of polynomials when attempting to estimate higher-order polynomial logarithmic EKCs.

Research Fellow Fakhri is a research fellow leading the KAPSARC Global Energy Macroeconometric Model (KGEMM) project. Previously, he was an associate professor… Fakhri is a research fellow leading the KAPSARC Global Energy Macroeconometric Model (KGEMM) project. Previously, he was an associate professor and director of the Center for Socio-Economic Research at Qafqaz University, Azerbaijan. He has served as a deputy director of the Research Institute at the Ministry of Economic Development, and a senior economist at the Research Department of the Central Bank of Azerbaijan Republic. He received a Fulbright Post-Doctoral Scholarship and conducted a research on building and applying a macroeconometric model for policy analysis at the George Washington University. Fakhri is a member of the research program on forecasting at the George Washington University and the editorial board of the Asian Journal of Business and Management Sciences. His research interests and experience span econometric modeling and forecasting, building and applying macroeconometric models for policy purposes, energy economics with a particular focus on natural resource-rich countries.
Expertise
- Applied macroeconometrics
- Energy economics
- Time series modeling and forecasting
Publications See all Fakhri Hasanov’s publications
Your Mileage May Vary: Have Road-fuel Demand Elasticities Changed Over Time in Middle-income Countries?
This paper contributes to the environmental literature by (i) demonstrating that the estimated coefficients and…
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This paper contributes to the environmental literature by (i) demonstrating that the estimated coefficients and…
11th September 2022