Affective reorganization in the infant, the mother, and the dyad: the role of upright locomotion and its timing.

Clinical theory is replete with rich observations that the onset of walking engenders an enhancement of emotional expression in the infant. Yet, no empirical research exists on this subject. Moreover, the importance of normal timing variations in the onset of developmental transitions has not been a topic of investigation in most developmental research. In this naturalistic home study, "earlier" and "later" walking groups were delineated on the basis of timing of entry into this developmental transition. Age-held constant analyses indicated that earlier walkers and their mothers generally showed a rise in positive exchanges as well as "testing of wills" across the transition to walking. These changes were less clear for the later walkers. Differences were observed also between the two groups even prior to walking. These findings suggest that future studies should take account of timing variations in the onset of a developmental transition.