The present paper concerns itself with the various aspects of the mother-infant interaction. While amount and variety of stimulation are recognized as important parameters influencing development, the argument is made that a contingency relationship resulting in a generalized expectancy on the part of the infant is, by far, more important. Briefly, if the mother responds consistently and with short latencies, she helps develop within the infant an expectation that his action can be effective in influencing his environment. This expectation, as a generalized expectancy, provides the motivation for additional environmental interaction and cognitive development. Data from a study of mother-infant interaction and tests of cognitive development at three months support this hypothesis.