We investigated the effort associated with incidental- and intentional-memory encoding of repeated human faces (Experiment 1) and car fronts (Experiment 2). Some stimuli were repeated, and the lag (number of intervening items) was varied. Encoding effort was inferred by secondary task costs associated with concurrently performed temporal productions, a sensitive index of effort. Recognition memory performance results were typical; however, temporal productions of the second presentation were about 80 ms shorter for massed (immediate) repetitions than for all other lags. These findings clarify models of the spacing effect. Abstract THE SPACING EFFECT: EVIDENCE FROM DURATION PRODUCTIONS AND RECOGNITION MEMORY Richard A. Block, Frank A. Bosco, & Travis S. Schanz Montana State University, Bozeman, MT Memory improves if two or more presentations of an event are separated by other events. This finding is called the spacing effect, or distributed practice effect. It is a reliable effect on memory with many practical consequences. A spacing effect has been found for a wide variety of materials, ages of subjects, and memory tests. Because it is so ubiquitous, it is not well understood. Researchers have proposed several models. The spacing effect may result from two or more processes, one involving massed (immediately repeated) presentations versus distributed presentations and the other involving the number of items intervening between the first and second presentation. We call the first effect the spacing effect and the latter effect the lag effect. We used a concurrent task to investigate the spacing effect. Subjects were asked to delimit a specific stimulus exposure duration. This method, called temporal production, is considered to be a sensitive measure of attentional demands. Subjects received brief training (with feedback) on producing 2-s durations. Then they self-paced the exposure of 100 stimuli (including some filler items), attempting to expose each for 2 s. Some stimuli were presented once (P1 only), and some were presented twice (P1 and P2) at lags of 0, 1, 5, or 13 intervening stimuli. Memory condition was also manipulated: Some subjects received intentional-memory instructions, and others received incidental-memory instructions. Finally, we tested recognition memory for presented stimuli, along with nonpresented stimuli. Introduction Experiment 1: Faces The stimuli were unfamiliar human faces. A total of 64 subjects was assigned to each memory condition (intentional vs. incidental). In contrast, the recognition data show both spacing and lag effects. As expected, memory was better in the intentional than in the incidental condition. Cepeda, N. J., Pashler, H., Vul, E., & Wixted, J. T. (in press). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin. Hintzman, D. L., Block, R. A., & Summers, J. J. (1973). Modality tags and memory for repetitions: Locus of the spacing effect. Journal of Verbal Learning & Verbal Behavior, 12, Hintzman, D. L., & Rogers, M. K. (1973). Spacing effects in picture memory. Memory & Cognition, 1, Zakay, D., Block, R. A., & Tsal, Y. (1999). Prospective duration estimation and performance. In D. Gopher & A. Koriat (Eds.), Attention and Performance XVII: Cognitive regulation of performance: Interaction of theory and application (pp ). Cambridge, MA: MIT Press. The temporal productions show a spacing effect, but no lag effect. In addition, they were longer in the intentional than in the incidental condition. The stimuli were frontal views of cars. As before, 64 subjects were assigned to each memory condition (intentional vs. incidental). Experiment 2: Cars Discussion The temporal productions show a spacing effect (in the intentional condition), but no lag effect. In addition, they were longer in the intentional than in the incidental condition. References In contrast, the recognition data show both spacing and lag effects. As in Experiment 1, memory was better in the intentional than in the incidental condition. In both experiments, recognition memory showed typical spacing and lag effects: Performance was a monotonically increasing, but decelerating function of lag between the two presentations (P1 and P2). However, temporal productions of P2 showed a different effect: They were about 80 ms shorter for massed repetitions than for all other lags. One exception was the incidental-memory condition in Experiment 2, which showed no spacing effect. Perhaps this reflects the relatively less automatic processing of cars versus faces. Compared to subjects in the incidental- memory condition, subjects in the intentional-memory condition made longer temporal productions and showed better recognition-memory performance. These findings clarify models of the spacing effect. The spacing effect and the lag effect result from separable processes. The spacing effect is explainable in terms of a habituation-recovery model or an attentional-deficiency model. However, the lag effect is apparently not a consequence of varying attentional demands, because the temporal productions did not vary as a function of lag (beyond that of an immediate repetition). An encoding-variability model of the lag effect is one viable model in the light of our data.