Cumulative Record. Chapter 24: The Flight from the Laboratory. Quote 10

When we have studied the performances generated by various contingencies of reinforcement in a single arbitrary response, we can move on to two or more concurrent responses. Instead of one lever to be pressed by a rat or one key to be pecked by a pigeon, our experimental space now frequently contains two or three levers or keys, each with its own set of reinforcing contingencies. In the present experiment, we are to account for the fact that psychologists have stopped pressing the experimental lever and have turned to other available manipulanda. (364-365)