CARDIAC-FUNCTION AND BEHAVIORAL REACTIVITY DURING INFANCY

Longitudinal cardiac data from the end of the fetal period to 21 month s of age were examined for change and stability over age and relations to the temperamental characteristics of high and low reactivity at 4 months of age and fear to the unfamiliar in the second year. Heart per iod and power in the cardiac spectra changed dramatically over the fir st 2 years, and individual differences were not preserved until 9-14 m onths of age. Sleep heart period at 2 weeks of age and low frequency p ower at 2 months of age were better predictors of the temperamental ca tegories than later measures of the same variables, suggesting that ca rdiac function early in life may be an especially sensitive index of t emperamental qualities.