The Constructs of Inhibition and Lack of Inhibition to Unfamiliarity
Jerome Kagan Nancy Snidman Harvard University
J. Steven Reznick Yale University
Most commentators on human nature have noted that the variation in the reaction to unfamiliarity and possible risk is a major characteristic that differentiates adults. Even though it is rare to find a large number of people who are consistently shy, cautious, and timid, or, on the other hand, outgoing, relaxed, and bold in all contexts, there is a small group, perhaps 10%, who consistently bring one or the other of these behavioral styles to untried situations that have the potential to create uncertainty. Jung's category of introversion-extraversion represents the most complete elaboration of this idea, and empirical indices of these and related constructs are among the most stable and the most heritable in contemporary psychological research ( Conley, 1985; Loehlin, 1982; Plomin, 1986). The actualization of these two qualities in childhood has been described by Thomas and Chess ( 1977), who generated great interest among psychologists in the study of temperament.
The hypothesis that children differ in the ease with which inhibition of action and/or withdrawal are evoked by unfamiliarity and challenge is implicit in Sheldon's tripartite typology, as well as Schneirla's ( 1959, 1965) suggestion that approach or withdrawal from events is a fundamental differentiating feature in animal species. Moreover, the empirical record affirms this idea for many animal groups, including mice, rats, cats, dogs, wolves, pigs, and monkeys. Blizard ( 1981) found these strain differences among rats, Scott and Fuller ( 1974) discovered that timidity was one of the two most differentiating characteristics in five breeds of dogs, and Suomi ( 1984) as well as Stevenson-Hinde and her colleagues ( 1980) reported that variation in timid behavior was a stable quality in macaque monkeys reared in a laboratory from infancy to puberty.
In childhood, a temperamental quality defined by initial tendency to with-