“…These variables are frequently found to be integrated of order one (I(1)). Hence, before assessing the existence of any nexus between metal use and economic growth, it is imperative to individually test for each country (i) whether (log) GDP per capita is an I(1) process using unit-root tests and, if this holds true, (ii) whether the error term in Equation 1 is stationary using, e.g., tests for nonlinear cointegration developed by Wagner (2013) and Wagner and Hong (2016), rather than standard cointegration tests. In contrast to these tests for nonlinear cointegration, classical cointegration tests are based on the assumption that all stochastic regressors appearing in Equation 1, i. e. y t and y 2 t , as well as the dependent variable m t , are I(1).…”