This article is a first attempt to systematically examine policy design and its influence on policyeffectiveness in a comparative perspective. We begin by providing a novel concept and measure ofpolicy design. Our Average Instrument Diversity (AID) index captures whether governments tendto reuse the same policy instruments and instrument combinations or produce policy solutions that arecarefully tailored to the policy problem at hand. Second, we demonstrate that our AID index is a valid andreliable measure of policy design quality with a strong explanatory power for the outcome variables tested.Analyzing the composition of environmental policy portfolios in 21OECDcountries, we show that higherlevels of AID are positively associated with a country’s policy effectiveness in environmental matters.Based on this finding, we analyze, in a third step, the factors that lead countries to adopt more or lessdiverse policy portfolios. We find that the policy design quality is significantly improved when policymakers are not bound b
