Learning Concepts and Categories: Is Spacing the “Enemy of Induction”?

Inductive learning—that is, learning a new concept or category by observing exemplars—happens constantly, for example, when a baby learns a new word or a doctor classifies x-rays. What influence does the spacing of exemplars have on induction? Compared with massing, spacing enhances long-term recall, but we expected spacing to hamper induction by making the commonalities that define a concept or category less apparent. We asked participants to study multiple paintings by different artists, with a given artist’s paintings presented consecutively (massed) or interleaved with other artists’ paintings (spaced). We then tested induction by asking participants to indicate which studied artist (Experiments 1a and 1b) or whether any studied artist (Experiment 2) painted each of a series of new paintings. Surprisingly, induction profited from spacing, even though massing apparently created a sense of fluent learning: Participants rated massing as more effective than spacing, even after their own test performance had demonstrated the opposite. The spacing effect refers to the nearly ubiquitous finding that items studied once and revisited after a delay are recalled better in the long term than are items studied repeatedly with no intervening delay (e.g., Cepeda, Pashler, Vul, Wixted, & Rohrer, 2006; Dempster, 1996; Glenberg, 1979; Hintzman, 1974; Melton, 1970). The positive effects of spacing on long-term recall are large and robust, and have been demonstrated in a variety of domains, such as conditioning (even in animals as simple as Aplysia; see Carew, Pinsker, & Kandel, 1972), verbal learning (e.g., Bahrick, Bahrick, Bahrick, & Bahrick, 1993; Ebbinghaus, 1885/1964), motor learning (e.g., Shea & Morgan, 1979), and learning of educational materials (e.g., Bjork, 1979; Dempster, 1988). In many everyday and educational contexts, however, what is important to learn and remember transcends specific episodes, instances, and examples. Instead, it is most important to learn the principles, patterns, and concepts that can be abstracted from related episodes or examples. In short, educators often want to optimize the induction of concepts and patterns, and there are reasons to think that such induction may be enhanced by massing, rather than by spacing. As stated by E.Z. Rothkopf (personal communication, September 1977), ‘‘spacing is the friend of recall, but the enemy of induction. ’’ SPACING AS THE ENEMY OF INDUCTION There is a compelling logic behind Rothkopf’s assertion. Massing allows one to notice the similarities between successive episodes or exemplars, whereas spacing makes doing so more difficult. Thus, for example, spacing presentations of individual paintings by a given artist will make it more difficult to notice any characteristics that define the artist’s style because spacing increases the chances that those characteristics will be forgotten between successive presentations. MASSING AS THE FRIEND OF INDUCTION The logic behind Rothkopf’s assertion is so compelling that, to our knowledge, it has never been tested. Perhaps the most direct evidence that massing facilitates induction comes from a study by Kurtz and Hovland (1956), who asked participants to study simple drawings that varied on four dimensions: size, shape, position, and coloring. There were four categories of drawings that participants learned to identify; for example, a ‘‘Kem’’ was defined as a drawing containing a circle positioned near the top of the display. Each category was presented eight times, with the individual items either interleaved with items from the other Address correspondence to Nate Kornell, Department of Psychology, 1285 Franz Hall, UCLA, Los Angeles, CA 90095, e-mail: nkornell@ ucla.edu. PSYCHOLOGICAL SCIENCE Volume 19—Number 6 585 Copyright r 2008 Association for Psychological Science categories (i.e., spaced) or massed together. No item was ever repeated exactly. On a memory test following the study phase, participants’ performance was better for drawings in the massed condition than for drawings in the spaced condition. Gagné (1950) obtained a similar result using four categories of nonsense-figure/nonsense-syllable pairs: Error rates were reduced when the highly similar category members were grouped together, instead of being interleaved. Less direct evidence comes from experiments that compared exact and nonexact repetitions (i.e., verbatim repetitions vs. paraphrased or gist repetitions). In such experiments, the spacing effect appears to diminish or disappear altogether for nonverbatim repetitions (e.g., Appleton-Knapp, Bjork, & Wickens, 2005; Dellarosa & Bourne, 1985; Glover & Corkill, 1987). Similarly, Melton (1970) demonstrated that spacing effects do not occur when participants fail to recognize that a repeated item is a repetition. Given such findings, and given that inductive learning involves exposure to a variety of different exemplars and does not involve exact repetition, it seems possible that spacing effects will disappear or turn into massing effects in tasks requiring induction. Finally, research in the domain of motor learning also provides indirect evidence that inductive learning may profit from massing, rather than spacing. Learning a motor skill, such as a tennis serve, involves induction in the sense that exposure to one’s own proprioceptive feedback is an important component of learning—and such repetitions are, by necessity, not exact, especially for novices learning complex skills. Although spaced practice is effective in many areas of motor learning (e.g., Shea & Morgan, 1979), massed practice can be more effective for learning complex motor skills (Wulf & Shea, 2002). GOALS OF THE PRESENT RESEARCH Thus, there are both logical and empirical reasons to expect massing, not spacing, to facilitate induction. One goal of the experiments reported here was to investigate the size of the massing effect—that is, the advantage of massed study over spaced study—in an inductive-learning context relevant to educational practice. Another goal was to investigate participants’ subjective assessments of massed versus spaced study in the context of induction. Prior research has demonstrated that people often rate massing as more effective than spacing, even in contexts in which spacing is actually superior (Baddeley & Longman, 1978; Simon & Bjork, 2001; Zechmeister & Shaughnessy, 1980). Such massing illusions may derive from the fact that metacognitive judgments are often grounded in feelings of fluency (e.g., see Benjamin, Bjork, & Schwartz, 1998). Presenting the same item twice consecutively makes processing the second presentation seem highly fluent, providing a (misleading) impression of learning, whereas spacing decreases the fluency of processing the second presentation. In other words, massing provides a sense of ease, which learners assume will translate to good memory on a later test, whereas spacing is often a ‘‘desirable difficulty’’ (Bjork, 1994) in the sense that it enhances long-term retention. If massing produces a sense of fluency of induction, participants may prefer massing to spacing in the induction task we used in these experiments. EXPERIMENTS 1A AND 1B In this experiment, participants were asked to learn the styles of 12 different artists by viewing six different paintings by each artist. In Experiment 1a, spacing was manipulated within participants: Paintings by each of 6 of the artists were presented massed, and paintings by each of the other 6 artists were presented spaced. In Experiment 1b, spacing was manipulated between participants: For a given participant, the paintings were presented either all massed by painter or all interleaved (spaced). After the learning phase, participants were shown new paintings by the same 12 artists and asked to select, from a list of all the artists’ names, the artist who had painted each new painting. After the test, participants in Experiment 1a were asked what presentation condition, massing or spacing, they felt had been more effective for learning a given artist’s style.