Step by Step, Day by Day: The First 3 Months of Infant Walking at Home.

Infants make learning look easy. They acquire a range of complex foundational skills-walking, talking, reaching, and manipulating objects-rapidly and efficiently. Nonetheless, learning new skills requires both time and practice. Although time and practice are related, the passage of time is not equivalent to the accumulation of practice. Here, we tested the relative contributions of elapsed time and accumulated practice to skill development by focusing on infant walking, a critical behavior for everyday activity. We followed 28 infants (18 boys, 10 girls, M walk onset age = 12.21 months, SD = 1.82) across the first 3 months of walking at home-the period when improvements in walking skill are most rapid and dramatic and the place where development really occurs. We calculated walking age (i.e., elapsed time since walk onset) from prospective tracking of walk onset, quantified walking practice (e.g., accumulated steps per hour) from observations of everyday activity, and measured walking skill (e.g., step length) with a pressure-sensitive gait mat. Longitudinal improvements in walking skill were consistent for all infants, and linear growth for the group reflected individual trajectories. In contrast, developmental change in walking practice was inconsistent for most infants: Spontaneous walking increased and decreased across monthly comparisons. Although walking age was the strongest predictor of improvements in walking skill, everyday practice explained unique variance beyond the effects of elapsed time. Likely, accumulating quantities of everyday walking (the natural input for learning) facilitates skill improvements by providing infants with critical experiences with perceptually guided locomotion in varied environments.