Dips in Development: Learning to Walk Temporarily Disrupts Infant Vocalization.

Infant development is a dynamic, cross-domain process. Infants acquire new skills, integrate them with well-established behaviors, and do so simultaneously. New skills can bolster development of existing behaviors or create moments of temporary instability for the developing system. How do multiple skills interact? To address this question, we mapped longitudinal change in infant vocalization against the backdrop of infants' evolving locomotor skills. We followed 22 infants across the 6 months surrounding the emergence of walking and examined changes in the production of vocalizations and in their coordination with locomotion during everyday activity at home. As a group, infants displayed positive linear growth in vocalization frequency, duration, and coordination with locomotion. However, a majority of individual infants showed evidence of a temporary decline in growth-a "dip"-in all three measures during the 2 months around walk onset. Moreover, although infants coordinated vocalizations with crawling (a well-established skill) across most of the observation period, above-chance coordination of vocalization with walking (a new skill) only emerged after 1 month of walking experience. Thus, learning to walk creates opportunities for infant communication, but is also a moment in development when progress slows, albeit temporarily.