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The writers are indebted to Mr. Norman Sears who obtained the reaction time scores of these subjects.

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In his volume on 'Conditioned Reflexes' (7), Pavlov reported that the experimental animals studied in his laboratory could be classified into four general types. These divisions were based on the differences among the animals in their reactions to the conditioning procedure. He later (8) reduced these categories to three types, excitatory, inhibitory, and central. Novikova (as reviewed by Razran ( 9)) from a study of positive and negative conditioned grasping responses found that the children comprising his experimental group were of four types, excitable or inhibitable, labile or inert, in their formation of positive and negative conditioned responses. In a more recent study of conditioned salivation in human adults Razran (10) identified three types of subjects, positive, negative, and indifferent. These studies suggest that individuals, human or animal, fall into distinct groups when compared on the basis of their relative reactivity to a conditioning influence.Experimental findings in psychology have in most cases been unfriendly to theories of psychological types. Studies of honesty, motor skill, introversion, and other traits have produced two common conclusions, first, individual differences in psychological traits are not distributed in bi-modal or multi-modal form but in conformity to the so-called normal probability curve, and second, psychological traits are not general but are strictly specific to each experimental situation. Despite Pavlov's findings there are strong indications that the first of these conclusions applies to ease of conditioning. Several recent studies have failed to find the conditioning * This investigation was carried out in the

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