Score contribution per author:
α: calibrated so average coauthorship-adjusted count equals average raw count
This paper proposes a conditional directional distance function estimator in order to examine the link between regional environmental efficiency and GDP per capita levels. As an illustrative example we apply our model to US regional data revealing an inverted ‘U’ shape relationship between regional environmental efficiency and per capita income. The results derived from a non-parametric regression indicate a turning point at 49,000 dollars.