The science of psychology studies people at three levels of focus captured by the well-known quotation: “Every man is in certain respects (a) like all other men, (b) like some other men, (c) like no other man" [1].
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.
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Individual differences is essential whenever we wish to explain by individual's differ in their behavior. In any study, significant variation exists between individuals: For instance in reaction time, preferences, values, health linked behaviors to name just a few. Individual differences in factors such as personality, intelligence, memory, or physical factors such as body size, sex, age, and other factors can be studied and used in understanding this large source of variance. Importantly, individuals can also differ not only in their current state, but in the magnitude or even direction of response to a given stimulus. Such phenomena, often explained in terms of inverted-U response curves, place differential psychology at an important location in such endeavours as personalised medicine, in which diagnoses are customised for an individual's response profile.
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. Earlier studies show us a higher risk with the factors of social and behavioral domains in young children with a single parent. However, the variety of single parent families regarding gender of the main parent has rarely been taken into reason when understanding the relation between family and child's negative outcomes [2].