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The Experimental Analysis of Behavior

The 1957 American Scientist article, reproduced in full

B. F. Skinner

Not so long ago the expression “a science of behavior” would have been regarded as a contradiction in terms. Living organisms were distinguished by the fact that they were spontaneous and unpredictable. If you saw something move without being obviously pushed or pulled, you could be pretty sure it was alive. This was so much the case that mechanical imitations of living things—singing birds which flapped their wings, figures on a clock tolling a bell—had an awful fascination which, in the age of electronic brains and automation, we cannot recapture or fully understand. One hundred and fifty years of science and invention have robbed living creatures of this high distinction.

Science has not done this by creating truly spontaneous or capricious systems. It has simply discovered and used subtle forces which, acting upon a mechanism, give it the direction and apparent spontaneity which make it seem alive. Similar forces were meanwhile being discovered in the case of the living organism itself. By the middle of the seventeenth century it was known that muscle, excised from a living organism and out of reach of any “will,” would contract if pinched or pricked or otherwise stimulated, and during the nineteenth century larger segments of the organism were submitted to a similar analysis. The discovery of the reflex, apart from its neurological implications, was essentially the discovery of stimuli—of forces acting upon an organism which accounted for part of its behavior.

For a long time the analysis of behavior took the form of the discovery and collection of reflex mechanisms. Early in the present century, the Dutch physiologist Rudolph Magnus [1], after an exhaustive study of the reflexes involved in the maintenance of posture, put the matter this way: when a cat hears a mouse, turns toward the source of the sound, sees the mouse, runs toward it, and pounces, its posture at every stage, even to the selection of the foot which is to take the first step, is determined by reflexes which can be demonstrated one by one under experimental conditions. All the cat has to do is to decide whether or not to pursue the mouse; everything else is prepared for it by its postural and locomotor reflexes.

To pursue or not to pursue is a question, however, which has never been fully answered on the model of the reflex, even with the help of Pavlov’s principle of conditioning. Reflexes—conditioned or otherwise—are primarily concerned with the internal economy of the organism and with maintaining various sorts of equilibrium. The behavior through which the individual deals with the surrounding environment and gets from it the things it needs for its existence and for the propagation of the species cannot be forced into the simple all-or-nothing formula of stimulus and response. Some well-defined patterns of behavior, especially in birds, fish, and invertebrates are controlled by “releasers” which suggest reflex stimuli [2], but even here the probability of occurrence of such behavior varies over a much wider range, and the conditions of which that probability is a function are much more complex and subtle. And when we come to that vast repertoire of “operant” behavior which is shaped up by the environment in the lifetime of the individual, the reflex pattern will not suffice at all.

In studying such behavior we must make certain preliminary decisions. We begin by choosing an organism—one which we hope will be representative but which is first merely convenient. We must also choose a bit of behavior—not for any intrinsic or dramatic interest it may have, but because it is easily observed, affects the environment in such a way that it can be easily recorded, and for reasons to be noted subsequently, may be repeated many times without fatigue. Thirdly, we must select or construct an experimental space which can be well controlled.

2012-01SkinnerF1.jpgClick to Enlarge ImageThese requirements are satisfied by the situation shown in Figure 1. A partially sound-shielded aluminum box is divided into two compartments. In the near compartment a pigeon, standing on a screen floor, is seen in the act of pecking a translucent plastic plate behind a circular opening in the partition. The plate is part of a delicate electric key; when it is pecked, a circuit is closed to operate recording and controlling equipment. Colored lights can be projected on the back of the disk as stimuli. The box is ventilated, and illuminated by a dim ceiling light.

We are interested in the probability that in such a controlled space the organism we select will engage in the behavior we thus record. At first blush, such an interest may seem trivial. We shall see, however, that the conditions which alter the probability, and the processes which unfold as that probability changes, are quite complex. Moreover, they have an immediate, important bearing on the behavior of other organisms under other circumstances, including the organism called man in the everyday world of human affairs.

Probability of responding is a difficult datum. We may avoid controversial issues by turning at once to a practical measure, the frequency with which a response is emitted. The experimental situation shown in Figure 1 was designed to permit this frequency to vary over a wide range. In the experiments to be described here, stable rates are recorded which differ by a factor of about 600. In other experiments, rates have differed by as much as 2000:1. Rate of responding is most conveniently recorded in a cumulative curve. A pen moves across a paper tape, stepping a short uniform distance with each response. Appropriate paper speeds and unit steps are chosen so that the rates to be studied give convenient slopes.

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