The question that "how and why people differ" is the subject matter of the study of_____________.

individual differences

Different traits can exist in varying degrees in an individual. In this sense, each one of us is unique as s/he exemplifies a typical combination of various traits. The question which you may like to pose is how and why people differ. This, in fact, is the subject matter of the study of individual differences. For psychologists, individual differences refer to distinctiveness and variations among people’s characteristics and behaviour patterns.