Differential psychology studies the ways in which individuals differ in their behavior and the processes that underlie it. This is a discipline that develops classifications (taxonomies) of psychological individual differences. This is distinguished from other aspects of psychology in that although psychology is ostensibly a study of individuals, modern psychologists often study groups, or attempt to discover general psychological processes that apply to all individuals. This particular area of psychology was first named and still retains the name of “differential psychology” by William Stern in his book (1900).
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While prominent psychologists, including Stern, have been widely credited for the concept of individual differences, historical records show that it was Charles Darwin (1859) who first spurred the scientific interest in the study of individual differences. His interest was further pursued by his half-cousin Francis Galton in his attempt to quantify individual differences among people.
For example, in evaluating the effectiveness of a new therapy, the mean performance of the therapy in one treatment group might be compared to the mean effectiveness of a placebo (or a well-known therapy) in a second, control group. In this context, differences between individuals in their reaction to the experimental and control manipulations are actually treated as errors rather than as interesting phenomena to study. This approach is applied because psychological research depends upon statistical controls that are only defined upon groups of people.