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Three studies were run to test the fear hypothesis of immobility reactions in chickens. The first experiment, as a replication of earlier work, found significantly enhanced immobility on the part of animals given preinduction electric shock. The second experiment showed that duration of immobility varied reliably as a function of the amount of shock. Experiment 3 demonstrated that a similar enhancement could be produced by brief preinduction exposure to a loud noise.

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The habituation of tonic immobility in chickens was examined in six studies. It was shown that repeated elicitation of immobility, and not just handling, was responsible for reduced response durations after multiple exposures to manual restraint. Habituation was a function of the number of stimulus presentations and, in addition, proved surprisingly durable, with diminished reactions lasting at least 2.5 mo. Strain differences were found in the number of trials required to reach a criteria of habituation, and habituation proceeded faster when immobility termination was self-paced as opposed to experimenter induced. Also, massed trials produced robust sensitization effects rather than diminished responsiveness.