In the wake of the legal realist repudiation of traditional formalism, there existed an absence of a satisfying conceptual account of appellate court lawmaking in America. This void was eventually filled, almost by default, by a notion of appellate court lawmaking that is a kind of idealized version of legislative lawmaking whereby appellate courts choose to make one plausible version or another of legal doctrine the authoritative law based on their balancing of societal costs and benefits deemed to be associated with each possibility. This approach evaluates each competing version of common law in terms of the degree to which each best advances the underlying social purposes sought to be advanced by the doctrinal area in question. This newly embraced approach is often called instrumentalism.

This chapter provides a detailed description and analysis of a famous instance of modern instrumentalist analysis by a high-profile appellate court. The case is Tarasoff v. Regents of the University of California, decided by the California Supreme Court in 1976, and announcing for the first time in the United States the existence of a formal tort law duty for a therapist to warn the intended victim of violent behavior by a patient.