A group of 193 children, classified as high or low reactive to stimulation at 4 months and observed again at 14 and 21 months, were observed at 4% years of age for behavioral signs of inhibited or uninhibited behavior. Children who had been high reactive were less spontaneous and less sociable than those who had been classified as low reactive, but only a small proportion of children maintained a consistently inhibited or uninhibited phenotype at all ages.

A group of 193 children, classified as high or low reactive to stimulation at 4 months and observed again at 14 and 21 months, were observed at 4% years of age for behavioral signs of inhibited or uninhibited behavior. Children who had been high reactive were less spontaneous and less sociable than those who had been classified as low reactive, but only a small proportion of children maintained a consistently inhibited or uninhibited phenotype at all ages.

Previously, we proposed a theoretical framework that classified infants into qualitative categories of reactivity, rather than on a continuous dimension. The present research used an objective statistical procedure (maximum covariance analysis, or MAXCOV) to determine if a qualitative latent structure, consistent with our theoretical conjectures, would be found to underlie quantitative indices of reactivity to stimuli in a sample of 599 four-month-old infants. Results of the MAXCOV analysis showed clear evidence of a latent discontinuity underlying the behavioral measures of infant reactivity. The base rate of the latent class (or taxon) was estimated at 10%. Infants within the putative high-reactivity taxon, compared with infants not in the taxon, were elevated on measures of behavioral inhibition at 4.5 years. These results provide objective empirical support for a central tenet in our theoretical model by supporting the taxonicity of infant reactivity.

Four-month-old infants from Boston, Dublin, and Beijing were administered the same battery of visual, auditory, and olfactory stimuli to evaluate differences in level of reactivity. The Chinese infants were significantly less active, irritable, and vocal than the Boston and Dublin samples, with American infants showing the highest level of reactivity. The data suggest the possibility of temperamental differences between Caucasian and Asian infants in reactivity to stimulation.

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