One reason for the relative disregard of individual differences in psychology is that research on this subject appears atheoretical and usually addresses correlation rather than causation. In this chapter, we suggest that quantitative genetics provides the basis for a general theory of the etiology of individual differences of scope and power rarely seen in the behavioral sciences. After a brief overview of quantitative genetics, we describe a general theory of individual differences in terms of 10 propositions and then consider the theory in the context of current trends in the philosophy of science.

We will not concern ourselves with the philosophical intricacies of the word “theory.” The term obviously means different things to different psychologists, as illustrated by formal differences among the best-known theories in psychology, such as learning theories, personality theories, and Piagetian theory. Nonetheless, from the pragmatic view of a behavioral researcher, theories should clarify our thinking by describing, predicting, and explaining behavior. At the very least, theories should be descriptive, organizing and condensing existing facts in a reasonable, internally consistent manner. However, they should also make predictions concerning phenomena not yet investigated and allow clear tests of these predictions to be made. At their best, theories explain phenomena as well as describe and predict them.