Unpacking black boxes: Mechanisms and theory building in evaluation

There is growing interest in the concept of ‘‘mechanism’’ across many areas of the social sciences. In the field of program and policy evaluation, a number of scholars have also emphasized the importance of causal mechanisms for explaining how and why programs work. However, there appears to be some ambiguity about the meaning and uses of mechanism-based thinking in both the social science and evaluation literature. In this article we attempt to clarify what is meant by mechanisms in the context of program evaluation by identifying three main characteristics of mechanisms and outlining a possible typology of mechanisms. A number of theoretical and practical implications for evaluators are also discussed, along with some precautions to consider when investigating mechanisms that might plausibly account for program outcomes.