The growth within the past decade of so-called "discovery" approaches to teaching, particularly in the sciences, has been explosive. These approaches emphasize "inquiry," require a high degree of flexibility in thinking, and stress, more than did traditional curricula, divergent rather than convergent thought processes. This study originated in an attempt to identify some of the variables affecting rigidity in problem-solving behavior in hope that the exclusion or control of such variables might increase flexibility of thought. Of particular concern were the blinding effects of habit, specifically with what happens when, to quote Luchins, a habit "ceases to be a tool discriminately applied but becomes a procrustean bed to which the situation'must conform; when, in a word, instead of the individual mastering the habit, the habit masters the individual" (32).The problem of behavioral rigidity has been of perennial interest to psychologists. Whether the term Einstellung-effect, mental set, disposition, readiness, determining tendency, or other similar term is used, the concept of a predisposition to respond in a given way which can affect perception, reasoning, and memory becomes a very important one. Chown says, "Few major topics in contemporary psychology appear to offer more promise than rigidity, and the amount of work reported on this subject has been increasing year by year" (11).With their concepts of "fixation" and "style of life," Freud and Adier, respectively, described behavior consistently inappropriate or, at least, responsive only to a limited set of cues in a variety of situations (4, 23). These clinical approaches, however, failed to describe clearly the conditions eliciting the behavior except, perhaps, at a high level of generality.While the greatest impetus to research in rigidity in American psychology came with the publication of Luchins5 original work, numerous earlier investigators hinted at its existence (32). Chant, for example, found that many errors in problem solving were caused by an "interpretive" approach, in which old meanings and interpretations determined the response, rather than a fresh approach to the materials at hand (10).