Don't Give Up on Lab Experiments: Why the Field Still Needs the Lab

Recent enthusiasm for field experiments, and especially for natural field experiments (NFEs), in which subjects go about their daily activities unaware that any study is taking place, has sometimes been read as a verdict against the laboratory. I argue that such a verdict is wrong. Through the lens of a simple rational-choice model, I show that the four standard experimental designs (laboratory, artefactual field experiment (AFE), framed field experiment (FFE), and NFE) are comparative-static restrictions of one maximization problem, each identifying a parameter the others cannot. The model reveals that each design type has distinctive strengths and weaknesses across various dimensions of knowledge creation, including the enforcement of the conditions for causal identification, the faithfulness of the experimental environment to the theory being tested, the identification of economic primitives via theoretical structure, and the ethics of studying human subjects. On each of the dimensions, the four design types are complements rather than rivals. Nowhere is this complementarity more evident than between the two extremes. The lab enforces the conditions for causal identification that the NFE must inherit from the market; the NFE recovers the parameter that governs behaviour in the wild, free of the selection, scrutiny, and environmental distortions the lab cannot escape. A research programme using all four designs together demonstrates something no single design can produce. The framework further accommodates the recent rise of online and survey experiments as natural extensions. Our discipline's recent drift away from laboratory evidence is leaving an important structural gap that natural field experiments, however well conceived, cannot fill.