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Schedules of reinforcement at 50: a retrospective appreciation.


A half century has come and gone since the publication of Schedules of Reinforcement (Ferster & Skinner, 1957), a landmark work in experimental psychology, and inarguably Skinner's most substantive empirical contribution to a science of behavior. The book introduced psychologists to an innovative methodology for studying the moment-to-moment dynamics of behavior-environment interactions, presented volumes of real-time data exhibiting an orderliness nearly unprecedented in psychology, and established the research agenda of the science Skinner termed the experimental analysis of behavior. Although he drew sharp and abundant criticism for his forays into social theorizing, Skinner's experimental work was frequently recognized, even by those not sharing his epistemology, as a rigorous and important development in behavioral science, as attested to by numerous awards, including the American Psychological Association's (APA's) Distinguished Scientific Contribution Award (APA, 1958) and the National Medal of Science (APA, 1969).

Skinner has frequently been acknowledged as among psychology's most eminent figures (Haggbloom et al., 2002). Unfortunately, much of his reputation, among both psychologists and laypersons, stems largely from his writings on freedom, dignity, autonomous man, the limitations of cognitive theorizing, and social engineering. Although this work received considerable attention and provoked much dialogue both within and outside of psychology (see, e.g., Wheeler, 1973), a focus on Skinner's social views may serve to shortchange an appreciation of the important experimental work for which he initially became known. Consequently, it may be fitting to revisit Schedules of Reinforcement (SOR) and to consider its ramifications for a science of behavior more than 50 years since the publication of this influential work. The purpose of the present article is to discuss the psychological climate in which SOR was conceived, the many research programs within behavioral science that benefitted from both the methodology and the conceptualization provoked by SOR, and contemporary basic and applied research utilizing schedules to illuminate many dimensions of human and nonhuman behavior, and to consider some ways in which instructors of psychology might update and extend their coverage of schedules of reinforcement in the classroom.

History and Development of Schedules Research

As is common in science, work on schedules of reinforcement was prompted by serendipitous and practical matters having little to do with formal empirical or theoretical questions. In a very personal account of his development as a scientist, Skinner (1956) discussed how the idea of delivering reinforcers intermittently was prompted by the exigencies of laboratory supply and demand. Because the food pellets used in the inaugural work on operant behavior were fashioned by Skinner just prior to experimentation, their rapid depletion during the course of experimentation was nearly axiomatic:
  Eight rats eating a hundred pellets each day could easily keep up
  with production. One pleasant Saturday afternoon I surveyed my
  supply of dry pellets, and, appealing to certain elemental theorems
  in arithmetic, deduced that unless I spent the rest of that
  afternoon and evening at the pill machine, the supply would be
  exhausted by ten-thirty Monday morning. ... It led me to apply our
  second principle of unformalized scientific method and to ask myself
  why every press of the lever had to be reinforced.
  (Skinner, 1956, p. 226)

This insight would lead to a fruitful program of research, carried out by Ferster and Skinner, over the course of nearly 5 years, in which the patterning and rate of response of nonhuman animal subjects were examined under several fundamental schedules. The research itself, programmatic and unabashedly inductive, was carried out in a style that would scarcely be recognizable to today's behavioral researchers. In many cases, experiments were designed and conducted in a single day, often to answer a specific question provoked by surveying cumulative records resulting from the previous day's research. The research did not follow the prescriptions of the hypothetico-deductive method, because the data from any given experiment pointed to the logical subsequent experimental manipulations:
  There was no overriding preconception that ruled where research
  should or should not go. All that new facts needed for admission to
  scientific respectability was that they meet minimal operational
  requirements. New concepts had to be publicly replicable to be
  verified and accepted. (Green, 2002, p. 379)

Somewhat of an anomaly among monographs, Schedules of Reinforcement offered the reader an unusually large ratio of graphed data to actual text. This configuration, although unfamiliar to many behavioral scientists, was no mistake. That the book is dominated not by verbiage but by volumes of cumulative records, portraying response characteristics across a wide array of schedule types and parameters, is testament to the spirit of the natural science of behavior that was unfolding in the Harvard pigeon lab. Ferster described the tenor of the schedules research with enthusiasm:
  Much of the early research in the various areas of schedules of
  reinforcement was playful, in the sense that the behavior of the
  experimenter was maintained and shaped by the interaction with the
  phenomena that were emerging in the experiment.
  (Ferster, 1978, p. 349)

Because both the experimental methods and the data being generated broke new ground, there was the potential for nearly continuous discovery, as each distinct schedule produced noticeable, and sometimes dramatic, effects on the patterning and rate of behavior. The classic fixed-interval "scallop" evidenced by a gradual increase in response rate near the end of the interval, the postreinforcement pauses characteristic of both fixed ratio and interval schedules, and the high response rates generated by ratio schedules compared to interval schedules were distinct reminders that not all behavior-consequence relationships are created equal. Moreover, these dynamic changes in behavior were not hidden in statistical models or summarized descriptive measures; they were made evident by the cumulative recorder, an apparatus nearly perfectly adapted to the research program itself. For Skinner, the method and corresponding apparatus had more than proved themselves on pragmatic grounds:
  The resolving power of the microscope has been increased manyfold,
  and we can see fundamental processes of behavior in sharper and
  sharper detail. In choosing rate of responding as a basic datum and
  in recording conveniently in a cumulative curve, we make important
  temporal aspects of behavior visible. (Skinner, 1956, p. 229)

  These "molecular" changes in probability of responding are most
  immediately relevant to our own daily lives ... There is also much
  to be said for an experiment that lasts half an hour--and for a
  quick and comprehensive look at the fine grain of the behavior in
  the cumulative curve. (Skinner, 1976, p. 218)

There was clearly a great deal of excitement in the air among the small cadre of researchers, many of them colleagues or students of Skinner's, who pioneered research on schedules of reinforcement. The enthusiasm was not universally shared, however, among experimental psychologists, and the novelty of both the research methodology and data generated in operant laboratories proved a sizable obstacle to dissemination of the research. Many journal editors, themselves practitioners and advocates of the large- group, null-hypothesis testing designs of Ronald Fisher, were reluctant to publish studies whose principal data were from individual organisms, neither aggregated across participants nor analyzed via acceptable tests of statistical inference:
  Operant conditioners were finding it hard to publish their papers.
  The editors of the standard journals, accustomed to a different kind
  of research, wanted detailed descriptions of apparatus and
  procedures which were not needed by informed readers. They were
  uneasy about the small number of subjects and about cumulative
  records. (The cumulative record, was, in fact, attacked as a "subtle
  curve-smoothing technique," which concealed differences between
  cases and should be abandoned in favor of "objective statistics.")
  (Skinner, 1987, p. 448)

In response to the unfavorable climate offered by the traditional experimental journals, those involved in the early schedules research took it upon themselves to establish a separate journal friendly to the innovative methods and data of operant research. Ferster, SOR's first author, spearheaded an effort that led, in 1958, to the establishment of the Journal of the Experimental Analysis of Behavior (JEAB), the primary outlet for operant research for the past half-century. Not surprisingly, JEAB was dominated by research on schedules of reinforcement, although perusal of the journal's earliest volumes reveals that there was much more on the minds of scientists submitting their work to the journal (Nevin, 2008). Indeed, the journal's first volume contained studies utilizing an impressive array of species (rats, pigeons, rhesus monkeys, chimpanzees, and human adults and children), response classes (lever pressing, key pecking, wheel running, grooming, and stuttering), and consequences (food, water, shock, tones, tokens, money, and colored lights correlated with reinforcement).

Among the most noteworthy aspects of this pioneering work was the usefulness of operant methodology to many scientists from disciplines seldom associated with behavior analysis. The early volumes of JEAB read like a Who's Who of scientists whose names would eventually be synonymous with such disciplines as behavioral medicine (Brady, Porter, Conrad, & Mason, 1958), behavior pharmacology (Dews, 1958, 1970; Dews & Morse, 1958), animal psychophysics (Blough, 1958, 1959, 1971), and animal cognition (Premack & Bahwell, 1959; Premack & Schaeffer, 1963; Shettleworth & Nevin, 1965; Terrace, 1963, 1969). For many of these researchers, the free-operant methodology introduced by Skinner allowed for an unprecedented degree of experimental control, and this paid especially high dividends in work with nonhuman animals. The response rates and patterns emitted by animals under common schedules of reinforcement exhibited remarkable stability, even over prolonged experimental sessions. This stability, or steady-state, behavior thus became a reliable baseline for evaluating the effects of other variables of interest, including drugs, acoustic and visual properties of antecedent (discriminative) stimuli, and characteristics of reinforcers, such as magnitude, delay, and quality. In particular, the programming of concurrent schedules not only allowed for these kinds of comparisons but also set the stage for the experimental analysis of choice behavior, a topic that dominated the experimental analysis of behavior for decades. The well-known matching law, first articulated by Herrnstein (1961, 1970), may represent the single most important empirical contribution of behavior analysis to a science of behavior.

Naturally, schedules of reinforcement served frequently as the principal independent variable, and this was a logical consequence of the work done previously by Ferster and Skinner (1957). What might surprise many psychologists, however, is the frequent attention given, in both SOR and the pages of JEAB, to a much larger collection of behavioral contingencies than depicted in introductory textbooks (continuous reinforcement [CRF], fixed-ratio [FR], variable-ratio [VR], fixed-interval [FI], and variable-interval [VI]). Indeed, Ferster and Skinner devoted a substantial portion of SOR to the study of a variety of both simple and complex schedules, as it became apparent early on that consequences could be delivered contingent on behavior along a sizable continuum of dimensions. In addition to studying response patterns across various ratio and interval parameters, it was possible to arrange compound schedules, in which two or more separate schedules were operating either successively or concurrently. Sometimes schedule alterations were signaled by discriminative stimuli, a procedure that allowed for the analysis of stimulus control, conditioned reinforcement, and the development of behavioral chains. In addition, Ferster and Skinner explored the possibility of producing very low or very high rates of behavior by arranging reinforcement contingent only on specified interresponse times (irt). These differential reinforcement schedules would become of theoretical interest because of their implications for how response classes are defined (Catania, 1970; Malott & Cumming, 1964), and several variations of differential reinforcement have become standard alternatives to punishment in applied behavior analysis (Cooper, Heron, & Heward, 2007).

In sum, the work begun by Ferster and Skinner (1957), and embraced by an enthusiastic group of scientists, established a strong precedent for researchers interested in the systematic study of behavior maintained by its consequences. The free-operant method would eventually find widespread application, not only in the laboratories of many behavioral and natural scientists but, increasingly, in applied endeavors as well. Featured prominently in much of this research, however, is the primacy of schedules of reinforcement. The pervasiveness of schedule effects was suggested by Zeiler (1977):
  It is impossible to study behavior either in or outside the
  laboratory without encountering a schedule of reinforcement:
  Whenever behavior is maintained by a reinforcing stimulus, some
  schedule is in effect and is exerting its characteristic influences.
  Only when there is a clear understanding of how schedules operate
  will it be possible to understand the effects of reinforcing stimuli
  on behavior. (p. 229)

Naturally, not all who have made use of schedule methodology endorse the claim that behavior is principally controlled by schedules of reinforcement. Several prominent researchers in behavior analysis have suggested that schedules are, in fact, artificially imposed contingencies that neither mimic real world functional relationships nor cast explanatory light on the role of the environment in the organization of behavior. Shimp (1976, 1984), for instance, has long argued that the molar behavior patterns observed under conventional reinforcement schedules are merely the by-product of moment- to-moment adjustments of behavior to more localized contingencies. There is, Shimp suggested, no reason to view molar response rates as fundamental units of behavior when more molecular analyses reveal similar order and offer a more nuanced view of the temporal organization of behavior. Similarly, Schwartz has claimed that the contingencies of reinforcement manipulated by both laboratory and applied behavior analysts have no counterpart in nature and do not reflect fundamental properties of behavior. He has suggested that research has merely shown that reinforcement may influence behavior when other causal factors are eliminated or suppressed, and this often occurs only under contrived and artificial conditions (Schwartz, 1986; Schwartz & Lacey, 1982). Finally, there is a substantive research literature on human operant behavior that questions the generality of nonhuman animal schedule performance. Differences that emerge in response patterning between humans and nonhumans, especially on temporal schedules, have provoked numerous dialogues on the interaction between verbal behavior and schedule performance, contingency versus rule-governed behavior, and the sensitivity of behavior to scheduled contingencies (e.g., Catania, Shimoff, & Matthews, 1989; Hayes, Brownstein, Zettle, Rosenfarb, & Korn, 1986; Hayes & Ju, 1998; Matthews, Shimoff, Catania, & Sagvolden, 1977; Schlinger & Blakely, 1987). Thus, the applicability of reinforcement schedules in describing and accounting for human behavior remains a contentious issue for many.

Prominent Areas of Research on Schedules of Reinforcement

Today's behavior analysts, whether working in laboratories or applied settings, continue to pursue the implications of Ferster and Skinner's early work, and in doing so, they have extended considerably the scope of behavioral phenomena to which reinforcement schedules seem clearly relevant. As suggested by Zeiler (1977), the importance of the schedules research lay in its generic implications for behavior-consequence relationships, rather than in the detailed analysis of pigeons pecking keys for grain. In a sense, Ferster and Skinner (1957) had offered both a powerful methodology for studying behavior and a promissory note for those willing to mine further its implications for a science of behavior, especially with respect to human affairs.

Choice Behavior

As mentioned earlier, an especially important development in schedules research was the use of concurrent schedules as a vehicle for studying behavioral choice. The programming of two separate schedules of reinforcement (often two variable-interval schedules), operating simultaneously, made it possible to examine how organisms allocate their behavior to several available options. Among the most salient and reliable discoveries was that relative response rates (or allocation of time to a schedule) closely track relative reinforcement rates--the so-called matching law (Baum, 1973; Herrnstein, 1961, 1970). Once behavior stabilized on concurrent schedules, response rates and patterns served as reliable and powerful benchmarks for evaluating qualitative and quantitative differences in reinforcement. Thus, the use of concurrent schedules became invaluable as a tool for exploring other issues in behavioral allocation.

The study of choice behavior became a highly sophisticated business within the operant laboratory, and significant effort has been expended on identifying the local and extended characteristics of responding on concurrent schedules. This research has led to both significant quantification, as seen in the growth of mathematical models of operant behavior (Killeen, 1994; Mazur, 2001; Shull, 1991), and substantial theoretical discussion of whether matching behavior is best conceptualized at the molar or molecular level (Baum, 2004; Hineline, 2001). But such developments would remain of interest to a fairly small group of laboratory scientists were it not for the general applicability of the choice paradigm itself. In fact, the relevance of the concurrent schedules preparation is its conspicuous analogy to any circumstance in which an organism faces a choice between two or more behavioral options. The matching law, initially demonstrated by pigeons pecking keys for grain reinforcement, is now recognized as a remarkably robust behavioral phenomenon, characterizing the choice behavior of a large number of species, emitting various topographically diverse responses, for an array of reinforcers (Anderson, Velkey, & Woolverton, 2002; deVilliers, 1977; Houston, 1986; Pierce & Epling, 1983; Spiga, Maxwell, Meisch, & Grabowski, 2005).

Perhaps the most relevant and interesting examples of the matching law are its applications to human behavior. Initial laboratory studies in which (a) responses typically included button or lever presses and (b) reinforcers included points on a computer console or snacks (e.g., peanuts) often produced data very similar to those acquired in the animal laboratory; that is, participants tended to allocate behavior in a manner consistent with the matching law (Baum, 1975; Bradshaw & Szabadi, 1988; Buskist & Miller, 1981). Applications of the matching law to behavior in applied settings have largely confirmed findings from the more controlled environment of the laboratory. Matching, for instance, has described allocation of self-injury in an 11-year-old child (McDowell, 1982), and choice of sexually provocative slides by a convicted pedophile (Cliffe & Parry, 1980). More recently, the matching relation has been demonstrated in archival analyses of sports performance. Both college and professional basketball players have been shown to match the relative rate of 2-point and 3-point shots to their respective reinforcement rates, once the differences in reinforcer magnitude are adjusted (Romanowich, Bourret, & Vollmer, 2007; Vollmer & Bourret, 2000). Reed, Critchfield, and Martens (2006) demonstrated that professional football teams allocated running and passing plays in a manner predicted by the matching law as well. Other studies have shown that spontaneous comments during informal conversations were made toward confederates who reinforced responses with verbal feedback, and that the relative rates of comments occurred in a manner consistent with the generalized matching law (Borrero et al., 2007; Conger & Killeen, 1974).

The general choice paradigm has become an enormously generative platform for studying many aspects of behavior that involve choosing between alternative activities or reinforcers, and there would seem to be few socially significant behaviors that could not be meaningfully viewed as instances of choice activity. For instance, both classic research and contemporary research on self-control, which typically pits an immediate, small reinforcer against a delayed, large reinforcer, have shed considerable light on the relative importance of reinforcer quality, magnitude, and delay in determining individual choice behavior (Epstein, Leddy, Temple, & Faith, 2007; Logue, 1998; Mischel & Grusec, 1967; Mischel, Shoda, & Rodriguez, 1989). A sizable literature attests to the extent to which reinforcer value decreases monotonically as a function of time delay, and this finding has potential implications for an array of behaviors said to reflect impulsivity, or lack of self-control. Indeed, the phenomenon known as delay discounting has informed many recent analyses of behavior problems characterized by impulsivity, including smoking (Bickel, Odum, & Madden, 1999), substance abuse (Bickel & Marsch, 2001; Vuchinich & Tucker, 1996), and pathological gambling (Dixon, Marley, & Jacobs, 2003; Petry & Casarella, 1999).

Behavioral Pharmacology

Articles that appeared in the first volume of JEAB portended a significant role for schedules of reinforcement in the examination of the relationship between drugs and behavior. By 1958, the year of the journal's inaugural issue, schedule performance, including response rate and patterning, was well known for several species and across several parameters of many different schedule types. Consequently, predictable behavior patterns were the norm, and these served as very sensitive baselines against which to judge the effects of other independent variables, including chemicals. In reminiscing about the early laboratory work on behavioral pharmacology, some of the discipline's pioneers reflected on the advantages of schedules research for scientists interested in drug-behavior relationships:
  When Charlie (Ferster) showed me the pigeon laboratory in early
  1953, it was immediately apparent from the counters and cumulative
  records that behavioral phenomena were being studied in a way that
  was well suited for application to pharmacology. In spite of my
  reservations about pigeons as subjects for drugs, in a short time we
  had planned a joint project, on effects of pentobarbital on
  fixed-interval responding, and immediately started experiments ...
  A sort of natural selection based on results then determined what
  lines seemed interesting and would be continued. ...
  (Dews, 1987, p. 460)

The marriage of operant methodology, particularly schedules research, with pharmacology paid immediate dividends, and much of the agenda for the young science of behavioral pharmacology was established by the work being done by Peter Dews, Roger Kelleher, and William Morse. Among the more important observations coming out of this early research was the concept of rate dependence--that the effects of any particular drug were substantially modulated by the current rate of the behavior, and this was largely determined by the schedule maintaining the behavior:
  Under conditions where different food-maintained performances
  alternated throughout an experimental session, pentobarbital was
  shown to affect behavior occurring under a fixed-interval (FI)
  schedule differently from behavior controlled by a fixed ratio (FR)
  schedule. ... This simple but powerful series of experiments lifted
  the evaluation and interpretation of drug effects on behavior from
  that of speculation about ephemeral emotional states to quantitative
  and observable aspects of behavior that
  could be manipulated directly and evaluated experimentally.
  (Barrett, 2002, p. 471)

Although the general notion of rate dependency appeared early in behavioral pharmacology, its explanatory scope was eventually questioned, and the discipline turned ultimately to other behavior principles expected to interact with drug ingestion, including qualitative properties of both reinforcement and punishment (Branch, 1984). In addition, considerable research focused on the function of drugs as discriminative stimuli, as this work dovetailed well with traditional pharmacological efforts at identifying physiological mechanisms of drug action (Colpaert, 1978). More recently, attention has been directed to self-administration of drugs and the identification of drugs as reinforcing stimuli (Branch, 2006).

Even in contemporary research on behavioral pharmacology, schedules of reinforcement play an integral role, particularly when drugs are used as rein-forcers for operant responses. Indeed, one method for evaluating the reinforcing strength of a drug is to vary its "cost," and this can be done in operant studies through the use of a progressive ratio schedule. In a progressive ratio schedule, response requirements are increased with the delivery of each rein-forcer (Hodos, 1961). In such a study, a participant may receive injection of a drug initially based on a single response, but the required ratio of responses to reinforcers increments with each reinforcer delivery. Under such schedules, a single drug injection may eventually require hundreds or thousands of responses. The point at which responding terminates is known as the "break point," and this behavioral procedure allows for a categorization of drugs in a quantitative manner that somewhat parallels the abuse potential for different drugs (Young & Herling, 1986). The use of progressive ratio schedules in evaluating drug preference and/or choice in nonhumans and humans alike has become a standard procedure in behavioral pharmacological research (Hoff-meister, 1979; Katz, 1990; Loh & Roberts, 1990; Spear & Katz, 1991; Stoops, Lile, Robbins, Martin, Rush, & Kelly, 2007; Thompson, 1972).

Behavioral Economics

In operant experiments with both nonhumans and humans, participants respond to produce consequences according to specific reinforcement schedules. The "reinforcers," of course, differ from study to study and include such diverse consequences as food, tokens, points on a computer monitor, money, social praise or attention, music, toys, and so on. It is not terribly difficult, however, to see how these consequences could be viewed as "commodities," with participants functioning as "consumers" in standard operant experiments. In fact, the analogy between behavioral experiments and the phenomena of economics has been recognized by scientists for several decades (Lea, 1978). For those interested in the study of microeconomic behavior, the operant laboratory afforded a controlled environment and established experimental methods well suited to this purpose. In addition to viewing participants as consumers and reinforcers as commodities, operant analyses of economic behavior conceptualize reinforcement schedules as representing constraints on choice. Thus, different schedules of reinforcement can be seen as alterations in the availability, or price, of a commodity, and response allocation can be observed as a function of these changes in availability.

The use of reinforcement schedules, then, allows for an experimental investigation of such economic principles as elasticity--the change in consumption of a good as a function of its price. The consumption of some reinforcers, for example, may be affected more than others by increases in price, usually represented by increases in ratio schedule parameters. Similarly, by making different commodities available concurrently and according to the same price (schedule of reinforcement), the relative substitutability of the commodities can be ascertained. In essence, researchers involved in behavioral economics view the operant laboratory as a very useful environment for pursuing the classic demand curves of economic theory, but with the added benefit of precise manipulation of independent variables and objective measurement of dependent variables (Allison, 1983; Hursh, 1984; Hursh & Silberberg, 2008). Fortunately, the benefits of this cross-fertilization have been realized on both sides. Research in behavioral economics has had important, though unforeseen, consequences for the experimental analysis of behavior as well. Studies of the elasticity and substitutability of reinforcers conventionally used in the operant laboratory (e.g., food, water) demonstrated that demand curves for these reinforcers depend substantially on how they are made available. An especially important variable, though historically ignored by behavior analysts, was whether the reinforcer used in experimentation was available only within the context of experimental sessions, or available in supplementary amounts outside the experimental space, as is common with nonhuman animal research. This distinction between "open" (supplemental food made available outside of experimental sessions) and "closed" (no supplemental food available outside experimental sessions) economies proved immensely important to understanding response allocation in studies of choice, and was a direct result of viewing operant experiments as investigations in microeconomics (Hursh, 1978, 1984). In general, the synthesis of microeconomic theory with the experimental methodology of behavior analysis has moved the latter field toward a more systematic assessment of reinforcer value, as well as a greater appreciation of the delicate nature of reinforcer value, given the variables of price and alternative sources of reinforcement (Hursh & Silberberg, 2008).

The Pervasive Nature of Schedules of Reinforcement in Human Behavior

Any psychology professor teaching about schedules of reinforcement has the somewhat dubious challenge of describing to students what experiments on key-pecking pigeons might have to do with the lives of human beings as we move about our very busy environments. Ordinarily, this assignment entails a description of the scientific logic behind using simple, well-controlled anologue strategies to examine fundamental principles before proceeding with larger scale generalizations. A few students "get" that there are important functional equivalencies between birds pecking keys and humans pressing buttons on an ATM. For those who are not so convinced, descriptions of behavior under schedule control must exceed some higher threshold of ecological validity to be meaningful. These descriptions inevitably must go beyond the conventional description of basic contingencies (e.g., CRF, FR, VR, FI, VI) and their putative relevance to piecework, checking the mail, and gambling. Fortunately, this task is not especially daunting. The ubiquity of schedule effects on behavior in the natural environment can be readily appreciated once one acknowledges that all behavior operates on the environment. Of course, the consequences of behavior are not always as dramatic or programmed as the delivery of a food pellet or a winning slot machine configuration. Subtle behavior-environment relationships, common in the "real world" we all inhabit, offer both numerous and authentic examples. Turning the ignition key starts the engine of a vehicle, and turning one's head in the direction of a novel sound brings a potentially important stimulus into one's visual field. Entering one's correct password on a keyboard allows access to an electronic environment of one's choice. In none of these scenarios would we be likely to speak of the consequences described as "feeling good" or as "rewards" in the vernacular sense, but their function in maintaining these common repertoires can hardly be ignored.

Also, real-world consequences are often of an ethereal nature in the everyday environment, seldom delivered in a manner well-described by any of the conventionally studied reinforcement schedules. Social reinforcement, for instance, can be especially fickle, and consequently quite difficult to simulate in the operant chamber. Parents do not always pick up crying babies, married couples respond to each other's verbal statements with fitful consistency, and teachers attend too infrequently to students who are academically on task. Moreover, these "natural contingencies" are of a reciprocal nature, with each actor serving both antecedent and consequential functions in an ongoing, dynamic interplay. This reciprocity was well appreciated by Skinner (1956), who often credited his nonhuman animal subjects with maintaining his interest in the learning process: "The organism whose behavior is most extensively modified and most completely controlled in research of the sort I have described is the experimenter himself ... The subjects we study reinforce us much more effectively than we reinforce them" (p. 232).

More recently, complex interpersonal contingencies have been pursued by a number of researchers, most notably child development researchers interested in the interplay between infants and caregivers. In much of this work, the focus is on the child's detection of social contingencies. Numerous studies have demonstrated that infants respond very differently to social stimulation (e.g., caregiver facial expressions, verbalizations, physical contact) delivered contingent on infant behavior as compared with responses to the same stimulation delivered noncontingently (Fagen, Morrongiello, Rovee-Collier, & Gekoski, 1984; Millar & Weir, 1992; Tarabulsy, Tessier, & Kappas, 1996). Caregiver behavior that is unpredictable or unrelated to infant behavior is seen as a violation of expected social behavior and is correlated with negative affective reactions in infants (Fagen & Ohr, 1985; Lewis, Allesandri, & Sullivan, 1990). This contingency detection research suggests that human infants are highly prepared to identify social reinforcement contingencies, particularly those connected to their own attempts to engage caregivers. Tarabulsy etal. (1996) have argued that disruptions to, or asynchrony in, infant-caregiver interactions, known to correlate with insecure attachment (Isabella & Belsky, 1991), are fundamentally manifestations of social contingency violations.

Similar to contingency-detection research in developmental psychology, reinforcement schedules have also been used to evaluate causal reasoning in adult participants. Reed (1999, 2001, 2003) has demonstrated that participants render postexperiment judgments of causality as a function of the contingency described by the schedule. In these experiments, the individuals' responses on a computer keyboard resulted in brief illumination of an outlined triangle on the monitor according to programmed schedules. Participants moved through several schedules sequentially, and experiments pitted schedules that ordinarily produce high response rates (ratio and differential reinforcement of high rate; drh) against those usually associated with lower response rates (interval and differential reinforcement of low rate; drl). Although reinforcement rates were held constant across schedules by way of a yoking procedure, ratio and drh schedules still produced greater response rates as well as higher causality judgments. Participants viewed their behavior as being more responsible for the illumination of the triangle when emitting higher rather than lower response rates. These results were interpreted as evidence that the frequent bursts of responding that produce reinforcement in ratio schedules create a greater sense of behavior-consequence contingency than the pauses, or longer interresponse times (irt) common to interval schedules (Reed, 2001). This research, utilizing conventional operant methodology, is important because it brings a functional analysis to bear on response topographies seldom considered within the purview of behavior analysis.

Of course, being creatures of habit, scientists conducting contemporary research on schedules tend to remain closely tethered to the traditional dimensions of count and time (ratio and interval schedules) developed by Ferster and Skinner (1957). There are, however, many other ways in which behavior-environment relationships can be configured. Williams and Johnston (1992), for instance, described a number of reinforcement schedules in which both the response and the consequence could assume either discrete (count-based) or continuous (duration) properties, and observed response rates to vary as a function of the specific contingency. Broadening the properties of reinforcement schedules in this way helps bring research on schedules closer to the dynamic behavior-environment interchanges characteristic of human behavior in the natural environment. As an example, in a conjugate reinforcement schedule, the duration or intensity of a reinforcing stimulus is proportional to the duration or intensity of the response. Conjugate reinforcement schedules were utilized early in the development of applied behavior analysis, particularly in the pioneering work of Ogden Lindsley, who discovered that arranging a continuous relationship between behavior and its consequences allowed for remarkably sensitive assessment of attention in patients recovering from surgery (Lindsley, 1957; Lindsley, Hobika, & Etsten, 1961). The procedure was also used by advertising researchers to evaluate both attention and preference for television programming, interest in commercials, and preference for music type (monophonic vs. stereophonic) and volume. In most of this research, participants pushed buttons or stepped on pedals, and stimulation was presented as long as the response occurred, but terminated or decreased in duration with cessation of responding (Morgan & Lindsley, 1966; Nathan & Wallace, 1965; Winters & Wallace, 1970). By requiring the participants to maintain stimulation through a continuous response, the method allowed for a direct assessment of interest or attention, rather than unreliable measures such as direction of gaze or verbal reports.

More recently, conjugate reinforcement schedules have been used to study memory in infants. Rovee-Collier developed a procedure in which infants in cribs developed a kicking response when their foot, attached via a string to a mobile suspended over the crib, caused movement in the mobile. Because both the intensity and duration of mobile movement is proportional to the intensity and duration of leg movement, the contingency represents a conjugate reinforcement schedule. The procedure has become a standard method for evaluating not only contingency awareness in infants but also memory for the contingency over time, and the effects of contextual stimuli on durability of memory. Using the method and distinct contextual cues, Rovee-Collier and colleagues have established impressive long-term memory for infants as young as 3 months of age (Butler & Rovee-Collier, 1989; Rovee-Collier, Griesler, & Earley, 1985; Rovee-Collier & Shyi, 1992; Rovee-Collier & Sullivan, 1980).

The conjugate reinforcement schedule may very well represent one of the most natural and common contingencies affecting humans and nonhumans alike. The movement of a car at a fixed speed requires a constant pressure on the accelerator (assuming one is not using cruise control); an angler controls the retrieval speed of a lure by the rate at which line is reeled in; and the rate at which a manuscript scrolls on a computer monitor is a direct function of speed of operation of the mouse's roller. There are, undoubtedly, many more examples of behavior-environment interplay in which contingent stimulation tracks response intensity or duration, especially amid the many machine-human interfaces characterizing the modern world. Ferster and Skinner (1957) were understandably constrained in their ability to study many of these configurations, due to the limited technology at their disposal, much of it fashioned by Skinner himself. But today's researchers, beneficiaries of more dynamic and powerful electronic programming equipment, face fewer barriers in arranging reinforcement schedules along multiple dimensions. Such research may serve as an important reminder, especially to those outside of behavior analysis, of the generality and applicability of schedules of reinforcement to naturally occurring human behavior.

Schedules of Reinforcement in the "Decade of Behavior"

The American Psychological Association has declared 2000-2010 the "Decade of Behavior." As we near the end of this decade, reflecting on the legacy of a landmark work like Schedules of Reinforcement (Ferster & Skinner, 1957) seems appropriate, especially if one considers the somewhat subservient role behavior has come to play in contemporary psychological science. Baumeister, Vohs, and Funder (2007), having surveyed contemporary social and personality psychology, reached a sobering conclusion: In a field at one time celebrated for its methodological ingenuity, classic "field" experiments, and direct assessment of social influence effects, self-report measures have become the predominant dependent variable in contemporary research. There are, of course, numerous advantages to utilizing self-reports as the primary mechanism for collecting psychological data, among them convenience and potential ethical considerations, as pointed out by Baumeister and colleagues (Baumeister et al., 2007). The behavior of filling out a scale, however, is not isomorphic to the behavior to which the scale's items are said to refer, and the limitations in drawing viable inferences from self-report data are well known (Nisbett & Wilson, 1977). In fact, behavior analysts have contributed an important piece to this scientific dialogue in the form of analyses of the conditions under which verbal and nonverbal behavior exhibit correspondence (Baer, Osnes, & Stokes, 1983; Critchfield, 1993; Matthews, Shimoff, & Catania, 1987; Wulfert, Dougher, & Greenway, 1991), though this research literature is not likely to be familiar to those outside the field.

Behavior analysts may be comforted by the observation that other behavioral scientists have increasingly come to lament a science dependent on self-reports for its primary data. For instance, a recent series of articles explored both the scientific and clinical repercussions of psychology's reliance on arbitrary metrics (Blanton & Jaccard, 2006; Embretson, 2006; Greenwald, Nosek, & Sriram, 2006; Kazdin, 2006). An arbitrary metric is a measure of a construct, such as depression or marital satisfaction, whose quantitative dimensions cannot be interpreted with confidence as reflecting fundamental properties of the underlying construct. Because such constructs are ordinarily conceptualized as multidimensional, no discrete numerical index could be taken as an independent and comprehensive measure of the construct itself. Nor can such measures be usefully assessed as indicators of change in the construct in response to treatment regimens or independent variable manipulations. Such measures, however, are the bread and butter of assessment practice throughout much of the behavioral sciences, especially in clinical and applied settings. As Baumeister et al. (2007) noted, employing arbitrary metrics has become standard practice in empirical research in social psychology, a discipline once devoted to direct observation of ecologically valid behaviors.

This state of affairs is particularly lamentable given the substantial advances made recently in scientific instrumentation. In fact, many of our more mundane daily routines are now readily monitored and measured by a host of small, powerful digital devices. This technology has often come from other disciplines, such as animal behavior, physical and occupational therapy, and the medical sciences, but its utility for behavioral science is easily acknowledged. Precision-based instrumentation is now available for the continuous measurement of a variety of behavioral dimensions. Increasingly powerful and portable digital cameras can capture the details of sequential interactions between participants, eye gaze and attention, facial expressions, and temporal relationships between behavior and environmental events, and laboratory models are often capable of integrating these data with corresponding physiological indices. In addition, accelerometers, telemetric devices, and actigraphs can be used to assess numerous behavior patterns, such as movement during sleep and gait and balance during ambulation, and large-scale movement, such as migration patterns. The enhanced mobility and durability of many such instruments make their exportation outside the laboratory and remote use more accessible than ever before. In sum, there would seem to be no legitimate reason a science of behavior would not make more use of a growing technology that renders direct measurement of behavior more accessible than ever before.

Of course, technological advances do not, in and of themselves, make for better science. As in other pursuits, it is the tool-to-task fit that seems most relevant. By today's standards, the operant equipment fashioned and used by Ferster and Skinner (1957) seems quite pedestrian. It was, however, more than adequate to the task of revealing the intricate and sensitive dance between an operant class and its environmental determinants. Mere demonstrations that consequences influenced behavior were not original; Thorndike (1911) had demonstrated as much. The advent of different kinds of behavior-consequence arrangements, though, allowed for a more sensitive evaluation of these parameters, although little would have come from this early experimentation if response patterns and rates had been uniform across different schedules. The fact that this did not happen lent immediate credibility to the research program begun by Ferster and Skinner and advanced by other operant researchers for the past half century.


The research that led to the publication of Schedules of Reinforcement (Ferster & Skinner, 1957) served to both justify and inspire an empirical science of behavior whose functional units consisted of behavior-environment relationships. The myriad ways in which behavior could be shown to operate on the environment offered a large tapestry for those wishing to explore the many shades and colors of the law of effect. In following this behavior-consequence interplay over long experimental sessions and at the level of individual organisms, the experimental analysis of behavior distanced itself from the large-group designs and statistical inference that had come to dominate psychological research. A perusal of Haggbloom et al.'s (2002) list of eminent figures in psychology reveals that Skinner is joined by Piaget (2nd on the list behind Skinner) and Pavlov (24th), both of whose fundamental contributions are well known even by nonpsychologists. What is perhaps unappreciated by behavioral scientists, however, is that the groundbreaking work of both Piaget and Pavlov resulted from data collected and analyzed at the individual level, with little regard for sample size and statistical inference (Morgan, 1998). That Skinner, Piaget, and Pavlov, clearly not kindred souls with respect to much of their scientific epistemology, found common ground in the study of the individual calls into question conventional methods of psychological science. Meehl (1978) may have expressed it most cogently 30 years ago:
  I suggest to you that Sir Ronald has befuddled us, mesmerized us,
  and led us down the primrose path. I believe that the almost
  universal reliance on merely refuting the null hypothesis as the
  standard method for corroborating substantive theories in the soft
  areas is a terrible mistake, is basically unsound, poor scientific
  strategy, and one of the worst things that ever happened in the
  history of psychology, (p. 817)

As mentioned previously, Schedules of Reinforcement was an unorthodox publication in part because the primary data presented were real-time cumulative records of response patterns, often emitted over prolonged experimental sessions. These displays were, in many respects, antithetical to the manner in which behavioral data were being treated within psychology by the 1950s. Consequently, some justification for this deviation in method may have seemed necessary, and Skinner (1958) offered the following rationale shortly after SOR's publication:
  Most of what we know about the effects of complex schedules of
 reinforcement has been learned in a series of discoveries no one of
  which could have been proved to the satisfaction of a student in
  Statistics A. ... The curves we get cannot be averaged or otherwise
  smoothed without destroying properties which we know to be of first
  importance. These points are hard to make. The seasoned experimenter
  can shrug off the protests of statisticians, but the young
  Psychologist should be prepared to feel guilty, or at least stripped
  of the prestige conferred upon him by statistical practices, in
  embarking upon research of this sort. ... Certain people--among them
  psychologists who should know better--have claimed to be able to say
  how the scientific mind works. They have set up normative rules of
  scientific conduct. The first step for anyone interested in studying
  reinforcement is to challenge that claim. Until a great deal more is
  known about thinking, scientific or otherwise, a sensible man will
  not abandon common sense. Ferster and I were impressed by the wisdom
  of this course of action when, in writing our book, we reconstructed
  our own scientific behavior. At one time we intended--though, alas,
  we changed our minds--to express the point in this dedication: "To
  the mathematicians, statisticians, and scientific methodologists with
  whose help this book would never have been written." (p. 99)

David L. Morgan

Spalding University

Correspondence concerning this article should be addressed to David L. Morgan, PhD, School of Professional Psychology, Spalding University, 845 South Third St., Louisville, KY 40203 (e-mail:

This article is dedicated to the memory of Peter Harzem, mentor and scholar, for his substantial empirical and conceptual contributions to a science of behavior.


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