The aim of this paper is to explain how and why individual differences emerge despite accounting for biological andsocio-cultural differences, why people behave differently in the same context, and how behavior becomes stable and consistent. We review the experimental work on variability and stereotypy. In animal research, in contrast to expectations, there is interindividual variability in behavior under extreme environmental control. In addition, intraindividual consistency (stereotypy) is detected in animals whose behavior is not fully adjusted to the contingencies. The differences in what is learned (the kind of contingency relations) among laboratory animals can be explained by: a) the differences between effective contingencies and programmed contingencies, and b) the relationship between exploration and rate of reinforcement. In experimental studies in humans, learning differences in identical environments depend, further to the above, onwhat was previously learned by the individual (experience and education) and the thoroughness and internal consistency of task instructions. From these concepts, we propose a psychological theory of personality that explains: (a) how we learn different relationships from the same experience; (b) how behavioral individual differences emerge (variability); and (c)why each individual’s behavior becomes stable and consistent.