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Subjects worked on a problem, engaged in an intervening activity, and then resumed work on the problem. Different intervening activities represented various mechanisms that might produce incubation (e.g., set breaking, facilitation by analogy, review of the problem's elements). These various treatment groups were compared to a control group that worked on the problem continuously. None of them showed evidence of incubation, despite previously reported incubation with the same problem. Since these systematically negative findings are consistent with those of several recent studies, the status of incubation as an objectively demonstrated phenomenon is questioned and directions for further research are suggested.
Olton et al. (Wed,) studied this question.