Updating Watson & Marks (1971): How Has Our Understanding of the Mechanisms of Extinction Learning Evolved and Where Is Our Field Going Next?

•Many theoretical and empirical developments have occurred since Watson & Marks (1971).•The match between the underlying fear and exposure content seems to be important.•Role of fear activation in outcome is mixed and needs more investigation.•Within-session extinction is a limited predictor of outcome.•Between-session extinction has more, but still mixed, support as mechanism in extinction.

J. P. Watson and I. M. Marks published a seminal article in Behavior Therapy entitled “Relevant and Irrelevant Fear in Flooding-A Crossover Study of Phobic Patients” in 1971 that paved the way for important theoretical developments and empirical studies that examined the mechanisms underlying extinction learning. Indeed, in the 44 years since their article was published, our knowledge about how exposure therapy works has increased considerably. In this review, we explore the progress our field has made in understanding extinction learning and how Watson and Marks' important work has influenced this progress. We provide a brief summary of the design and major findings of the Watson and Marks (1971) study, followed by a brief description of several theoretical conceptualizations of fear extinction that were developed following the article's publication. We also review empirical studies that illustrate the “state of the science” with regard to the following key issues that were explored in Watson and Marks' paper: (a) the effect of specificity of exposure stimuli content in exposure therapy on outcome; (b) fear activation as a mechanism of exposure; and (c) the associations between within- and between-session extinction learning and treatment outcome. The major findings of these three issues over the past 4 decades are summarized and discussed.