This article presents a constraint-based model of novelty in which constraints contribute far more than performance criteria. Based on Reitman's classic analysis of the composition of a fugue, the model features paired constraints. One of each pair precludes or limits search for novelty among a specific set of existing responses; the other promotes or directs search among its opposites. The model shows how precluding specific aspects of Abstract Expressionism and promoting their opposites produced the novelty known as Pop Art. It then demonstrates how the same process-applied to an artist's own successive sets of responses-can maintain novelty. The evidence supports the argument that constraint selection is critical to generating and sustaining novelty.

Recent experimental research suggests 2 things. The first is that along with learning how to do something, people also learn how variably or differently to continue doing it. The second is that high variability is maintained by constraining, precluding a currently successful, often repetitive solution to a problem. In this view, Claude Monet's habitually high level of variability in painting was acquired during his childhood and early apprenticeship and was maintained throughout his adult career by a continuous series of task constraints imposed by the artist on his own work. For Monet, variability was rewarded and rewarding.

Potential contributors to sustained levels of variability in the topography of the rat's barpress were investigated in two experiments. Behavior was classified into discretely defined components, and changes in components and their sequential organization were analyzed. Experiment 1 showed that topographic variability in the rat is modulated by shifts in reinforcement schedules. Variability decreased between either dipper training or extinction and continuous reinforcement (CRF), and increased between CRF and extinction. Once the press was acquired, variability did not change if the schedule (CRF) did not change. Experiment 2 showed that, regardless of subsequent changes in topographic requirements, rats initially shaped to press under more stringent criteria sustained higher levels of variability during CRF, but not during extinction, than rats shaped with less stringent criteria. The results suggest that subjects learn not only what to do during reinforcement but also how differently or variably to do it.

The development of Richard Serra's sculpture is presented as a case study using a constraint-based model of novelty (Stokes, 2005, 2007). The model was developed from two problem-solving precedents: Reitman's (1965) idea that paired constraints direct and limit search in a problem space; and Simon's (1973), that search can only lead to novel solutions if the problem space is ill-structured. Ill-structured means that a problem space is incompletely specified or defined. The study shows how paired constraints restructure problem spaces in ways that make novelty possible and probable (Stokes, 2007). Possible means that novelty may or may not happen; probable, that paired constraints facilitate the happening.