This article analyzes cross-national and longitudinal variations in the incorporation of nonhuman animals into country constitutions and legislation. We argue that incorporation follows from the scientific rationalization and human rights-based ontological elaboration of nonhuman animals in world society, carried by a growing number of intergovernmental agreements and international nongovernmental organizations (INGOs). To test our ideas, we use event-history analyses on original data from 195 countries for the period 1972–2020. The models of constitutional incorporation show mixed results, with positive effects from human rights and INGOs but negative effects from science and intergovernmental agreements. The models of legislative incorporation show consistent positive effects from world factors, even when controlling for a range of domestic factors. Legal incorporation suggests an extension of the boundaries of “society,” driven by the rising prominence of highly rationalized and elaborated models of nonhuman animals, replete with dignity, sentience, and even tentative forms of rights and personhood.