What 50 years of research tell us about pausing under ratio schedules of reinforcement.

2007 marked the 50th anniversary of the publication of Charles Ferster and B. F. Skinner's magnum opus, Schedules of Reinforcement (1957), which reported the results of experiments carried out under contracts to the Office of Naval Research with Harvard University between 1949 and 1955. Although Skinner had previously discussed periodic reconditioning (what was later to become the fixed-interval [FI] schedule) and fixed-ratio (FR) reinforcement (he hadn't yet used the term reinforcement schedule) in The Behavior of Organisms (1938), it wasn't until the publication of Schedules of Reinforcement that he (and Ferster, working as a research fellow under Skinner's direction) distinguished several simple and complex schedules based on nonhuman (pigeons and rats) performances. In addition, they investigated the effects of many different types of those schedules along with a variety of other variables (e.g., drugs, deprivation level, ablation of brain tissue, added counters, etc.) that were described in terms of rate of response and depicted on cumulative records. Their research documented the power of schedules of reinforcement to control behavior and established the study of schedules as a focus within the experimental analysis of behavior. Ferster and Skinner's research painted a detailed picture of the performances of the subjects as seen on a response-by-response basis. However, their wide-ranging effort did not attempt to provide systematic information about the effects of parametric variations across conditions and subjects. It remained for subsequent researchers to fill in the gaps. Fifty years and innumerable experiments later, it is common for textbooks on learning to illustrate FR performances with cumulative records and to describe the resulting patterns as “break and run” (see Figure 1). As explained by Lattal (1991), “following a period of nonresponding (a break, more precisely the postreinforcement pause) after food delivery, there is a relatively quick transition to a high steady rate of responding (a run) that is maintained until the next food presentation when the pattern repeats” (p. 95). FR performances are often contrasted with those under variable-ratio (VR) schedules, in which the size of the ratios varies within the schedule. Under VR schedules, performances are “characterized by high response rates with little systematic pausing either after reinforcement or at other times” (p. 95).1 Figure 1 Cumulative records showing performances under FR 90 and VR 50 schedules of reinforcement (Ferster & Skinner, 1957). Reproduced from Figure 2 (Lattal, 1991) and reprinted with permission of Elsevier Science Publishers. Among the myriad response patterns observed under various schedules of reinforcement, pausing under ratio schedules (especially FR schedules) has attracted special attention. Unlike the pause that follows reinforcement on interval schedules, the pause that follows reinforcement on ratio schedules reduces overall reinforcement rates. Because reinforcement rates under ratio schedules depend strictly on response rates, optimal performance would be for subjects to resume responding immediately. Yet they pause, and the resultant loss of reinforcement persists over extended exposure to the schedule without diminution. The critical question, then, is: Why would an animal pause when that very action delays reinforcement and reduces overall reinforcement rate? One possibility is that the animal is fatigued after working so hard. For example, pauses are generally shorter under FR 10 than under FR 100. Because the FR 100 involves more work, it is possible that subjects rest longer before resuming work. Another possibility is that food consumption creates satiation, which weakens the motivational operation of food deprivation. However, we will see that neither of these seemingly reasonable explanations survive simple tests and that the picture is much more complicated. Ratio pausing within the laboratory is also of special interest because it resembles the human problem of procrastination: In both cases, an action is put off even though the resulting delay may be disadvantageous. Social commentators have noted that procrastination is a major contributor to behavioral inefficiency in schools, industry, and our daily lives in general (Steel, 2007). Identification of the variables that control the ratio pause in the laboratory may help to reveal techniques for the modification of procrastination. Our primary purpose in this article is to identify the variables that govern pausing under ratio schedules. Research on pausing under ratio schedules has come a long way since Ferster and Skinner's (1957) pioneering contributions. They set us on our way, but in the ensuing 50 years increasingly sophisticated questions and experimental designs have revealed a more complex picture. A fundamental issue in research on ratio schedules, then, concerns how pausing should be summarized and described. In the present article we review the research on pausing under ratio schedules of reinforcement in an attempt to glean an understanding of why pausing under such schedules occurs. We first take a look at research on pausing with simple FR schedules, then consider research with complex (multiple and mixed) schedules with FR components, and, finally, look at pausing under VR schedules. We then discuss the real-world implications of ratio pausing, in particular, the bearing of laboratory research on the pervasive social problem of human procrastination. Finally, we offer a set of conclusions that might shed light on the theoretical mechanisms that govern pausing under ratio schedules.