The sources of national obligations and flexibility in European Union laws, from 1958 to 2019

National obligatory commitments are the most defining and least studied features of international law and, specifically, European Union law. We introduce two complementary measures - the obligation ratio and the flexibility ratio - which capture, respectively, the extent to which EU laws impose obligations on national administrations and grant them some flexibility in implementation. Using a computational linguistic method, we extract these measures from nearly the entire legislative production from 1958 to 2019 (more than 9,000 regulations and directives). In addition to confirming some existing findings, we show how the involvement of the European Parliament in law-making, bicameral conflict, Euroscepticism in party positions and the public, which is plausibly associated with the risk of noncompliance, as well as actual noncompliance and policy complexity, are the most important drivers of the use of obligatory and flexible provisions.