Abstract
Key pecking of pigeons was reinforced with grain on a progressive ratio schedule during the presence of either of two key colors. Under one color, all responses were shocked; under the other color, responses were not shocked. A single response on a second key switched the key color and reset the progressive ratio, provided that the first step of the progressive ratio had been completed. A preference developed for the longer ratios of the progressive ratio under the non-shock key color. When the severity of punishment suppressed responding under the key color correlated with shock, the subjects continued to switch to the shock condition after a moderate degree of responding under the non-shock condition. Severe punishment also resulted in frequent, ineffective responses on the switching key and extended pausing under the key color associated with shock.