not experience these stressors. However, as we noted, a large number of inhibited children had older siblings. The presence of an older sibling who unexpectedly seizes a toy or yells at the younger child might provide the chronic stress necessary to actualize the infant's diathesis. An infant not born with these temperamental characteristics would not be as stressed by the same social experiences. Finally, it is necessary to differentiate between shy, introverted adolescents who are endogenously prone to uncertainty because they belong to our temperamentally inhibited group, and shy adolescents who acquired their behavior profile during the school years as a result of experience and who are not temperamentally prone to uncertainty. The likelihood of reversibility in behavior is greater for this second group. Use of selected physiological indexes, including heart rate, cortisol, pupillary dilation, and vocal tension might permit investigators to differentiate between these two classes of children.
This research was supported in part by grants from the John D. and Catherine T. MacArthur Foundation and grant 40619 from the National Institute of Mental Health.
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