To many scientists, the most striking aspect of people is how similar they are. Geneticists report that the human genome overlaps enormously across individuals and groups. Anthropologists find that significant aspects of social life are experienced universally (Brown, 1991). To the layperson, however, the most striking aspect of people is how much they differ. The language we naturally use to talk about people is primarily a language of individual differences (Goldberg, 1981). Many cultures explain social behavior by reference to psychological attributes that differentiate people from one another (Fiske & Taylor, 1991). Individual differences are a staple of everyday social discourse.

Personality psychologists recognize universals. However, like laypersons, they revel in the differences. In this chapter, we review personality psychologists' various strategies for describing and explaining individual differences in personality functioning. We begin by exploring the intercorrelated patterns of affect, cognition, and behavior, which under the names of dispositions or traits have been a primary focus of personality psychology's research enterprise. We review efforts to identify a comprehensive structure of dispositions and the achievement of a consensus on five individual-difference factors. We then turn to questions of human temperament and the parallel efforts to identify the structure of temperamental qualities. Our final section explores intelligence and alternative models of human intellectual capabilities.

As we will see, many investigators in these areas use factor analysis to identify the structure of individual differences in psychological tendencies.