Reading 10.4 – Form, function, and operational definitions

Chapter 10 – Radical Behaviorism

Reading 10.4 – Form, function, and operational definitions

Let’s just lay it out there: There are only two fundamental approaches to defining a behavior (response class). This limitation stems from the nature of behavior as a physical phenomenon. The definition of behavior in Strategies and tactics of behavioral research and practice (Johnston, Pennypacker, & Green, 2020) goes as follows:

Behavior is an organism’s interaction with its environment that involves functional
relations between biological features of the organism and the environment (p. 38).

Defining behavior in terms of an interaction between the organism and its environment allows for the result to include discernable movements or not. In either case, these results may have effects on the environment, which may in turn have effects on behavior, and so on. Our science has established the intricate details of these reciprocal effects, which we describe in terms of functional relations.

Topographical and functional definitions

Any particular behavior may be defined in terms of its form or its function. A behavior that involves movement may be defined on the basis of the form or topography of its responses in three-dimensional space. Alternatively, the same behavior may be defined in terms of the functional relations these responses have with the environment. Each type of definition ignores the criteria for the other, although a definition may be put together that uses both kinds of criteria. Chapter 2 in S&T covers the core issues in defining behavior, and Chapter 4 goes into detail on the topic of selecting and defining response classes.

The characteristics of behavior as a physical phenomenon allow only these two fundamental types of definitions. Although you can overlay either type with variations and refinements of all sorts, they always reduce to either a topographical or a functional core. Let this sink a bit in before reading further.

Operational definitions 1

At this point, some readers are probably wondering about operational definitions. Where do they come in? You may be surprised at the answer.

References to operational definitions are routine these days in ABA literature and textbooks (e.g., Cooper, Heron, & Heward, 2020). The general idea is to come up with a definition of a target behavior that will guide measurement decisions. A summary label for the behavior rarely provides sufficient specificity, so the researcher or clinician describes the features observers are supposed to use to decide which of all the things an individual does should be recorded as an instance of the target behavior and, by exclusion, which should not. For example, an intervention may target “tantrumming,” but observers need to know exactly what about this kind of behavior should be recorded or not.

What seems to justify calling the resulting definition “operational” is that it specifies the observing criteria that are supposed to be used in a particular project or clinical intervention. The same summary label for the behavior (e.g., tantrumming) might be used in a variety of research projects or interventions, but it is “operationalized” in each case by a relatively detailed definition. An operational definition specifies at least part of the observing and recording operations involved in measuring a target behavior. This reasoning makes sense, of course. There are usually good reasons for why a detailed definition operationalizing a summary behavioral label will need to vary somewhat from one case to another.

But when is a definition of a target behavior not operational? Can you have one definition of a particular behavior that is operational and another definition of the same behavior that is not operational? Answering this question requires a clear understanding of what it is we are defining. We are not defining the label we are using to identify the behavior, but the actual behavior itself. Given its features, what about it do we want to record? Any behavior will have far more characteristics than we are interested in, and a definition specifies which ones we want to capture and which ones we want to ignore. Our choices may influence how we decide to label it, but we cannot escape making these decisions.

So, it is impossible to define a behavior for purposes of measurement without requiring decisions about the features that are or are not of interest. In other words, all definitions are operational. We cannot usefully talk about a particular behavior without some clarity regarding the key features that describe our interests. If we’re talking about tantrumming behavior emitted by Mary, we need to know about the key topographical and functional elements that warrant that label in her case. We always have to talk in operational terms because behavior is complex and varied, and summary labels just don’t give us the information we need for practical (effective) discussion. This argument means that there is no reason to specify definitions of behavior as being operational in nature. They are all operational, and the adjective is therefore unnecessary and useless.

Operational definitions 2

The sense of “operational definition” discussed above had another life before the interests of applied behavior analysis came along and appropriated the phrase, and it still does. There was a time when talking about operational definitions involved a discussion of mentalism, operationism, and an important paper by Skinner published in 1945. As explained in Box 10.2 on operationism, and discussed in more detail by Jay Moore (1980, 2008), the principle of operationism came from a physicist named P. W. Bridgeman. He argued that “we mean by any concept nothing more than a set of operations: the concept is synonymous with the corresponding set of operations” (Bridgeman, 1927, 5). (Also see Boxes 4.3 and 4.4 in S&T, which make these same points.)

Psychology found this approach useful because it offered a way to deal with mental events without worrying too much about their existence. One thing led to another, and what evolved was the view that language is a symbolic activity in which private subjective meanings exist in a mental domain. That is, words have meaning in a private mental lexicon.

Not surprisingly, Skinner had serious problems with this approach to operationism, and he delivered a paper explaining his contrary views at a conference on operationism (Skinner, 1945). He proposed that the meaning of a word lies not in a mental dictionary, but in an analysis of the environmental factors that determine the emission of the word as a verbal response. In other words, he did not treat verbal behavior any differently from other behavior. The contrast with psychology’s approach to operationism was stark.

Skinner’s 1945 paper is famous, at least among behavior analysts, because his argument was immediately accepted as an important element in the conceptual foundation of the field. As a result, the phrase “operational definition” was widely understood in the context of Skinner’s paper as referring to tactic for understanding everyday terms through a search for their sources of control as verbal responses. In other words, the only occasion for behavior analysts to talk about operational definitions was when the task was understanding the “meaning” of term referencing personal qualities that were at least unclear and often fictional. For behavior analysts, the phrase referred to operationism as a topic or, perhaps, to the tactic Skinner gave us to understanding the “meaning” of verbal behavior in a way that avoided mentalism.

We are still young

There was a time when this is all that “operational definitions” meant. Why the past tense? Because many of today’s ABA practitioners refer to all definitions of target behaviors as operational definitions because that is what their textbooks told them to do. It doesn’t take too long before everyone’s doing it, and many terminological issues have crept into the field’s technical vocabulary this way. Terminological slippage is hardly unique to behavior analysis, of course. The struggle to clean up technical terminology is an ongoing battle in all science-based disciplines. A field’s terminology reflects its understanding of its subject matter and, thus, its maturity. In some ways, we are still young.

Although ABA marks its beginnings in the 1960s, the vast majority of today’s practitioners were trained relatively recently. The BACB certification program only began a few years ago, and not many of today’s tens of thousands certificants were practicing before its inception. The BACB credentialing program established contingencies that not only encouraged training programs to blossom and attracted students to them, but also created a growing market for textbooks. For most of today’s practitioners, these texts define ABA, never mind what has gone before.

Unfortunately, this means that many ABA practitioners not only mistakenly view operational definitions as a particular type of definition, but are also likely to be unfamiliar with Skinner’s earlier treatment of operationism, which remains viable in discussions of the meaning of words as verbal responses. This reading explains the first problem and at least identifies the second. Together, they represent the larger problem our field faces in resolving a variety of other terminological issues.


Bridgman, P. (1927). The logic of modern physics. New York: MacMillan.

Cooper, J., Heron, T.E., & Heward, W.L. (2020). Applied behavior analysis (3rd ed.) Upper Saddle River, NJ: Pearson.

Johnston, J. M., Pennypacker, H. S., & Green, G. (2009). Strategies and tactics of behavioral research and practice (34th edition). NY: Routledge.

Moore, J. (1980). On the principle of operationism in the science of behavior. Behaviorism, 3, 120-138.

Moore, J. (2008). Conceptual foundations of radical behaviorism. Cornwall-on-Hudson, NY: Sloan Publishing.

Skinner, B. F. (1945). The operational analysis of psychological terms. Psychological Review, 52, 270-277.