We have been identifying transitional ages in infant rats for the acquisition of partial reinforcement effects and other paradoxical effects of reinforcement as a means, ultimately, of relating these indicants of emotional development to neuroanatomical and neurophysiological development. Our work also involves the study of habituation and Pavlovian conditioning at various ages in infancy. The guiding principle of this work is a theory encompassing the effects of primary, conditioned, and counterconditioned frustration, and a general theory of persistence of which frustration theory can be regarded as a special case. Insofar as the transitional ages for emotional (frustrative) effects can be identified, they can be applied to investigations of behavioral teratology.