Income Alters Choice Between Drug and an Alternative Nondrug Reinforcer in Monkeys

The effects of income (duration of access: 20, 60, or 180 min) on self-administration of orally delivered phencyclidine (PCP) and saccharin (sacc) or water were examined in 6 rhesus monkeys (Macaca mulatta) that were trained to respond for these reinforcers under concurrent fixed-ratio (FR) schedules. The FR requirement for PCP was varied (4, 8, 16, 32, 64, and 128), whereas the FR value for water and sacc remained at 32. Income level had a greater effect on sacc than on PCP deliveries. As income decreased from 180 to 20 min, sacc deliveries decreased by approximately 90%, whereas PCP deliveries decreased by approximately 54%. At FRs 16 and 32, preference for PCP vs. sacc was reversed at the high and low income conditions. When the PCP FR was increased, deliveries decreased and this effect was enhanced from a 20% to a 70% reduction when sacc (vs. water) was concurrently available. Income level and the availability of nondrug alternative reinforcers independently altered the demand for drug.