Nature or Nurture? Judicial Lawmaking in the European Court of Justice and the Andean Tribunal of Justice

Are international courts power-seeking by nature, expanding the reach and scope of international rules and the courts' authority where permissive conditions allow? Or, does expansionist lawmaking require special nurturing? We investigate the relative influences of nature versus nurture by comparing expansionist lawmaking in the European Court of Justice (ECJ) and the Andean Tribunal of Justice (ATJ), the ECJ's jurisdictional cousin and the third most active international court. We argue that international judges are more likely to become expansionist lawmakers where they are supported by substate interlocutors and compliance constituencies, including government officials, advocacy networks, national judges, and administrative agencies. This comparison of two structurally identical international courts calls into question prevailing explanations of ECJ lawmaking, and it suggests that prevailing scholarship puts too much emphasis on the self-interested power-seeking of judges, the importance of institutional design features, and the preferences of governments to explain lawmaking by international courts.