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To investigate whether 2 chimpanzees had expectations
regarding the outcome of their responses on a computerized task,
food reward that typically was given for correct responses was
withheld on some correctly completed trials. There were two types
of these probe trials: those which the chimpanzees performed
correctly on their own, and those during which the chimpanzees
needed the experimenter's assistance to complete the trial
correctly. For both chimpanzees, reward procurement behaviors
directed toward the experimenter occurred significantly more often
on correctly completed probe trials than on incorrectly completed
trials. This indicated increased expectation of food reward on
correct trials as compared to incorrect trials. For 1 of the 2
chimpanzees, reward procurement behaviors were significantly
more likely to occur on probe trials on which the chimpanzee
received no assistance from the experimenter than on trials in
which the experimenter assisted the chimpanzee. This behavioral
difference was not predicated on reinforcement history, as all
correctly completed nonprobe trials were rewarded whether or not
assistance was provided by the experimenter. These data indicate
that this chimpanzee may have a rudimentary sense of "equity"
regarding what outcome should accompany the successful
completion of trials that is dependent on the level of assistance
provided by an experimenter during the trial.