The science of psychology studies people at three levels of focus captured by the well known quote: “Every man is in certain respects (a) like all other men, (b) like some other men, (c) like no other man" (Murray, H.A. & C. Kluckhohn, 1953).
Individual differences psychology focuses on this second level of study. It is also sometimes called Differential Psychology because researchers in this area study the ways in which individual people differ in their behavior. This is distinguished from other aspects of psychology in that although psychology is ostensibly a study of individuals, modern psychologists often study groups or biological underpinnings of cognition.
For example, in evaluating the effectiveness of a new therapy, the mean performance of the therapy in one 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 is because psychological research depends upon statistical controls that are only defined upon groups of people. Individual difference psychologists usually express their interest in individuals while studying groups by seeking dimensions shared by all individuals but upon which individuals differ.
Importance of individual differences
The study of individual differences is essential because important variation between individuals can be masked by averaging. For example, a researcher is interested in resting metabolic rate in humans. The researcher gathers a sample of men, women, and children, measures their metabolic rate and gets a single average. The researcher then tells the whole population that they should be eating 1,900 calories a day.
What's wrong with this study? The researcher has neglected individual differences in activity level, body size, sex, age, and other factors that influence metabolic rate. The average reported based on the results is masking multiple dimensions that should be used to determine daily caloric intake. Therefore, his or her conclusions are misleading if not outright false. This is an extreme example to make a point, but it illustrates the problems that can arise by averaging across groups.
Areas of study
Individual differences research typically includes personality, motivation, intelligence, ability, IQ, interests, values, self-concept, self-efficacy, and self-esteem (to name just a few). There are few remaining "differential psychology" programs in the United States, although research in this area is very active. Current researchers are found in a variety of applied and experimental programs, including educational psychology, industrial psychology, personality psychology, social psychology, and developmental psychology programs, in the neo-Piagetian theories of cognitive development in particular.
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