Similarity in developmental profile among related pairs of human infants.

hopper groups can be explained by a single underlying mechanism, that is, the development of a set to respond (industriousness) or of a set to not respond (laziness). Presumably the treadle group learned that reinforcement is contingent on some response and they entered the autoshaping situation with a set or expectation that a similar contingency still applied. The hopper birds responded less because they anticipated a continuation of the noncontingency that existed in their first stage of training. Our interpretation is entirely parallel to that offered by Maier et al. (2) for the learned helplessness phenomenon. If that interpretation is valid, then the significance of subjects' ability to control their environment is not restricted to commerce with aversive stimuli or to any single motivational or reward system. Thus, speculations involving presumed physiological or biochemical correlates of helplessness in terms of stress reactions (3) are likely