--- - |2 Fear and positive emotionality were considered in a growth modeling context. Mothers, primarily Caucasian (91.9%) and of middle socioeconomic status, participated in play interactions with infants at 4 months (N = 148). Infant fear and positive affectivity were evaluated at 6, 8, 10, and 12 months of age. A linear trajectory was superior in explaining growth for parent report and observation‐based indicators of positive affectivity and parent report of fearfulness; a piecewise model explained the nonlinear growth of observation‐based fear. Responsiveness in mother–infant interactions emerged as a significant predictor of the fear trajectory, with higher sensitivity predicting lower levels of observed fear. Reciprocity, tempo, emotional tone, and intensity of mother–infant interactions also made significant contributions to temperament development; however, analyses addressing these were exploratory. - Child Development, Volume 89, Issue 5, Page 1519-1534, September/October 2018.