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

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 had 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.