Reinforcing effects of caffeine in coffee and capsules.
Caffeine itself acts as a strong reinforcer even in people who already consume a lot of it.
01Research in Context
What this study did
Six adults who drank at least five cups of coffee a day came to the lab.
They could press one button for a 100 mg caffeine capsule or another for placebo.
In later sessions they chose between caffeinated and decaf coffee instead.
The team counted how often each option was picked and asked how much they liked it.
What they found
People picked caffeine over placebo almost every time, whether it came in a pill or a cup.
Liking scores rose and fell with the amount of caffeine they took in.
Even heavy coffee drinkers still worked for pure caffeine, showing tolerance had not killed the effect.
How this fits with other research
Bromley et al. (1998) saw the same dose-response curve with cocaine in monkeys.
Both studies show that stronger drug doses pull more responses, just like any other reinforcer.
BURNSTEIN et al. (1964) got rats to lick more water when the payoff was wheel-running.
That odd pairing proves the rule: any stimulus that increases behavior is a reinforcer, caffeine included.
Last et al. (1984) remind us that reinforcer value shifts with context and time.
So the caffeine effect may look smaller later in a long session, not because the drug changed, but because the context did.
Why it matters
You now have clear evidence that the active ingredient, not the coffee ritual, is driving the behavior. When you fade edible or drink reinforcers, keep the active component constant at first. Then thin the dose, not just the delivery method, to maintain strength while cutting volume.
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02At a glance
03Original abstract
In a residential research ward the reinforcing and subjective effects of caffeine were studied under double-blind conditions in volunteer subjects with histories of heavy coffee drinking. In Experiment 1, 6 subjects had 13 opportunities each day to self-administer either a caffeine (100 mg) or a placebo capsule for periods of 14 to 61 days. All subjects developed a clear preference for caffeine, with intake of caffeine becoming relatively stable after preference had been attained. Preference for caffeine was demonstrated whether or not preference testing was preceded by a period of 10 to 37 days of caffeine abstinence, suggesting that a recent history of heavy caffeine intake (tolerance/dependence) was not a necessary condition for caffeine to function as a reinforcer. In Experiment 2, 6 subjects had 10 opportunities each day to self-administer a cup of coffee or (on different days) a capsule, dependent upon completing a work requirement that progressively increased and then decreased over days. Each day, one of four conditions was studied: caffeinated coffee (100 mg/cup), decaffeinated coffee, caffeine capsules (100 mg/capsule), or placebo capsules. Caffeinated coffee maintained the most self-administration, significantly higher than decaffeinated coffee and placebo capsules but not different from caffeine capsules. Both decaffeinated coffee and caffeine capsules were significantly higher than placebo capsules but not different from each other. In both experiments, subject ratings of "linking" of coffee or capsules covaried with the self-administration measures. These experiments provide the clearest demonstrations to date of the reinforcing effects of caffeine in capsules and in coffee.
Journal of the experimental analysis of behavior, 1989 · doi:10.1901/jeab.1989.52-127