Stability and change in cortisol and behavioral response to stress during the first 18 months of life

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Abstract
Observation of cortisol and behavioral responses to routine inoculation was conducted at 18 months for infants in a longitudinal sample whose stress responses had been observed at 2, 4, and 6 months of age. At 18 months, infants showed an increase in cortisol level over base to the perturbation. The magnitude of this response did not differ from the 6-month response. Moreover, level of cortisol response at 18 months was related to level of cortisol response at 6 months, but not at 2 or 4 months of age. In light of previous findings for a decline in cortisol response between 2 and 6 months and for the emergence of consistent individual differences in cortisol response by 4 to 6 months, the present findings indicate that a developmental shift in adrenocortical functioning has occurred by 6 months of age. © 1995 John Wiley & Sons, Inc.