The area of psychology concerned with measuring and comparing differences in individual and group behavior.
The earliest research in the field of differential psychology began in the late nineteenth century with Francis Galton's investigation of the effects of heredity on individual intelligence and his pioneering work in intelligence testing, which was further advanced by James McKeen Cattell and Alfred Binet. It was Binet who developed the first standardized intelligence test. Growth in related areas such as genetics and developmental psychology, as well as advances in psychological testing, all broadened the scope of the field considerably. While individual differences are often conceived of, at least popularly, in terms of categories ("gifted," "slow learner"), they are actually measurable on a continuum which, for most traits, follows the normal probability or "bell" curve first derived from the study of heights of soldiers. The majority of subjects cluster near the center with a gradual decrease toward the extremes.
Some areas of research focused on today by psychologists working in differential psychology are the effect of heredity and environment on behavioral differences and differences in intelligence among individuals and groups. Observations about group differences can be misused and turn into stereotypes when mean characteristics are indiscriminately ascribed to all individuals in a group, and when differences between groups are viewed as unchangeable and solely hereditary.
Eysenck, Michael W. Individual Differences: Normal and Abnormal. Hillsdale, NJ: L. Erlbaum Associates, 1994.