Selective adaptation is made possible by ways in which governments, elites, and other interpretive communities express their own normative preferences in the course of interpretation and application of practice rules. Selective adaptation depends on a number of factors, including perception, complementarity, and legitimacy. Perception influences understanding about foreign and local norms and practices. Originally a principle of nuclear physics, complementarity describes a circumstance by which apparently contradictory phenomena can be combined in ways that preserve essential characteristics of each component and yet allow for them to operate together in a mutually reinforcing and effective manner.