Delays to reinforcement are ubiquitous in the natural environment, where those delays often vary. Still, most research examining delayed reinforcement has focused only on fixed delays. Little research has examined the effects of variable delays. Nonresetting fixed and mixed delays to reinforcement were imposed on the responding of 4 pigeons previously maintained on a multiple variable-interval (VI) VI schedule of immediate reinforcement. Mixed delays consisted of two alternating delay values, the mean of which equaled the value of the fixed delay. A progressive delay procedure was used, in which delay durations increased across successive sessions. Conditions included using unsignaled (Mixed-Change) and signaled delays (Mixed-Signaled) in which both mixed delay values changed across sessions. A third condition (Mixed-Constant) involved using unsignaled delays in which one of the mixed delay values remained constant while the other progressively increased across sessions. Mixed delay components maintained higher rates of responding in some conditions. Response rates decreased more and changes in interresponse times were greater with unsignaled than signaled delays. With unsignaled delays, changes in responding were a function of the average obtained delay within each component of the multiple schedule, regardless of whether the delays were fixed or mixed. Obtained delays tended to be shorter in the mixed delay component, resulting in higher rates of responding
