ABSTRACT Ratings of behaviors pertaining to inhibition were observed for 130 twins participating in a longitudinal study Ratings were available for four ages (12, 18, 24, and 30 months) and from three sources at each age direct observations obtained in a laboratory setting, direct observations obtained in conjunction with infant mental testing, and a temperament measure from a questionnaire completed by parents For the individual twins, the age-to-age correlations were in the moderate range (26 to 64) The situation-to-situation correlations were generally in the same range (17 to 64) When the twins were recombined into twin pairs, within-pair (intraclass) correlations indicated that monozygotic (MZ) twins were more concordant than dizygotic (DZ) twins for each of the behaviors at each of the ages Also, the MZ twins were more concordant for the direction and degree of behavioral change from age to age or from situation to situation These data provide additional evidence for the biological influence on behavioral inhibition, a characteristic that has been studied in temperament and personality research The results suggest that the trait of behavioral inhibition and a change in the trait are genetically conditioned in addiction, it is suggested that the concept of trait be expanded to include the person-centered biological regulation of change