Faculty Bibliography

Although boys outshine girls in a range of motor skills, there are no reported gender differences in motor performance during infancy. This study examined gender bias in mothers' expectations about their infants' motor development. Mothers of 11-month-old infants estimated their babies' crawling ability, crawling attempts, and motor decisions in a novel locomotor task-crawling down steep and shallow slopes. Mothers of girls underestimated their performance and mothers of boys overestimated their performance. Mothers' gender bias had no basis in fact. When we tested the infants in the same slope task moments after mothers' provided their ratings, girls and boys showed identical levels of motor performance.

Nine-month-old infants were tested at the precipice of safe and risky gaps in the surface of support. Their reaching and avoidance responses were compared in two postures, an experienced sitting posture and a less familiar crawling posture. The babies avoided reaching over risky gaps in the sitting posture but fell into risky gaps while attempting to reach in the crawling posture. This dissociation between developmental changes in posture suggests that (a) each postural milestone represents a different, modularly organized control system and (b) infants' adaptive avoidance responses are based on information about their postural stability relative to the gap size. Moreover, the results belie previous accounts suggesting that avoidance of a disparity in depth of the ground surface depends on general knowledge such as fear of heights, associations between depth information and falling, or knowledge that the body cannot be supported in empty space.

Infants acquire independent mobility amidst a flux of body growth. Changes in body dimensions and variations in the ground change the physical constraints on keeping balance. The study examined whether toddlers can adapt to changes in their body dimensions and variations in the terrain by loading them with lead weights and observing how they navigated safe and risky slopes. Experiment 1 verified the reliability of a new psychophysical procedure for testing infants' responses in 2 experimental conditions. In Experiment 2, this procedure was used to compare infants' responses on slopes in feather-weight and lead-weight conditions. The lead weights impaired infants' ability to walk down slopes. Babies adapted to altered body dimensions by treating the same degree of slope as safe in the feather-weight condition but as risky in the lead-weight condition. Exploratory activity on the starting platform predicted adaptive responses on risky slopes.

This article presents a developmental account of changes in the visual guidance of locomotion. In contrast to the impressive efficiency of adult locomotion, locomotor activity is not under prospective control at the onset of human mobility. Infants require extensive crawling and walking experience before responding adaptively to variations in the terrain. At the same time that they are learning to navigate in increasingly varied environments, their bodies and skills are rapidly changing. Learning generalizes from safe, flat ground to novel surfaces but it does not transfer to new methods of locomotion. We account for these patterns of generality and specificity of learning by focusing on the role of exploratory behavior in detecting threats to balance control.

The effects of infants' age, body dimensions, and experience on the development of crawling was examined by observing 28 infants longitudinally, from children's first attempts at crawling until they began walking. Although most infants displayed multiple crawling postures en route to walking, development did not adhere to a strict progression of obligatory, discrete stages. In particular, 15 infants crawled on their bellies prior to crawling on hands and knees, but the other 13 infants skipped the belly-crawling period and proceeded directly to crawling on hands and knees. Duration of experience with earlier forms of crawling predicted the speed and efficiency of later, quite different forms of crawling. Most important, infants who had formerly belly crawled were more proficient crawling on hands and knees than infants who had skipped the belly-crawling period. Transfer could not be explained by differences in infants' age or body dimensions alone. Rather, experience using earlier crawling patterns may have exerted beneficial effects on hands-and-knees crawling by shoring up underlying constituents common to all forms of crawling postures.