In much of the thinking concerning mentally retarded persons, it appears to be assumed that retarded behavior is simply a consequence of low intelligence or inadequate cognitive functioning. This assumption is made despite the commonplace view in psychology that any behavior cannot simply be a function of formal cognitive functioning. In addition to cognitive determinants of behavior, achievements and motivational factors also play a crucial role (Seitz, Abelson, Levine, & Zigler, 1975; Zigler & Butterfield, 1968). These authors have argued that performance on an IQ test or any “cognitive task” reflects a combination of three factors: (1) formal cognitive processes such as memory, reasoning, and abstracting ability; (2) achievement factors involving specific knowledge of the problem to be solved; and (3) a wide range of personality and motivational factors. In this chapter we discuss personality determinants of retarded behavior. The demonstration of the importance of personality and motivational factors in retarded behavior has been the focus of much of our work. The fact that our work is focused on such factors does not mean that we believe that the cause of retardation lies in motivational or personality differences: The cognitive functioning of retarded individuals has a profound and pervasive influence on their behavior. The crucial questions are: Just how great is this influence, and how does it vary across tasks with which retarded persons are confronted?