Negative Contrast and Partial Reinforcement Effects as a Function of Crowded Rearing Conditions in the Rat

Summary

Two experiments were designed with the use of conventional learning situations to test the hypothesis that crowding leads to a decrease in emotionality. In both experiments, weanling rats were housed either individually or in groups of nine and started instrumental training at the age of four months. Phase 1 consisted of a 2 × 2 factorial design combining two levels of housing and either two levels of reward magnitude (large vs. small), as in Experiment 1 (N = 36), or two levels of reinforcement schedule (50% vs. 100%), as in Experiment 2 (N = 36). In phase 2 of Experiment 1, all Ss received small reward but maintained their original housing conditions. A negative contrast effect was found regardless of housing conditions, although this effect tended to be of a lesser magnitude under crowded conditions. In Phase 2 of Experiment 2, all Ss underwent extinction. A significant partial reinforcement extinction effect was obtained in the individually but not group-housed rats. The results were discussed in terms of both Amsel's frustration hypothesis and Capaldi's generalization-decrement hypothesis.