The stability of infant temperament and autonomic patterning (heart period and cardiac vagal tone) was examined longitudinally when infants were 5, 10, and 18 months of age. Behavioral measures of reactivity and regulation to frustration tasks, and maternal perceptions of infant temperament were obtained at each age along with baseline measures of cardiac activity. No stability was found from 5 to 10 months while some stability of behavior and autonomic patterning was identified from 10 to 18 months, with the exception of negative reactivity. High levels of cardiac vagal tone (V) were associated with negative reactivity at 18 months. When examining groups based on degrees of reactivity and regulation, we found infants who responded negatively to frustration but who also displayed more regulatory behavior to have higher v. 0 1996 John Wiley & Sons, Inc.
Theories of temperament exist which propose that individual differences are constitutionally based and relatively stable over time. The study of temperament in infancy has been exceptionally important toward identifying these characteristics as infants' responses to stimuli are believed to be primarily biologically driven. But such studies are not without their difficulties. Establishing the stability of behaviors across the 1st year of life as well as identifying their constitutional basis is often hampered by the rapid growth and development that typifies the infancy period. Moreover, behavior becomes more complex over time. For example, the stability of a crying response may be obscured by developmental changes in crying behavior which occur through maturation of the nervous system (Emde, Gaensbauer, & Harmon, 1976) and the responsivity of the environment (Bell & Ainsworth, 1972; Fish, Stifter, & Belsky, 1991). Rapid physiological changes also make it difficult to detect the relationship between behavior