Stress reactivity and self-recognition.

This study examined longitudinally whether early differences in reactivity to stress are related to self-recognition at 18 months of age. Adrenocortical and behavioral stress responses to routine inoculation at 2 and 4 months and at 6 and 18 months were obtained. Previous findings indicated a developmental shift in adrenocortical functioning by 6 months of age that included the emergence of stable individual differences in cortisol response from this point in development. Self-recognition was related to a greater cortisol response and less rapid quieting at 6 to 18 months, whereas 2- to 4-month cortisol and quieting responses did not differentiate self-recognizers and non-self-recognizers. These findings indicate a role for temperament differences in facilitating the emergence of self-recognition.