A 9-month-old infant placed on a wooden platform spies a brightly colored ball and crawls awkwardly but relentlessly toward the toy. He reaches the platform's edge and vainly thrusts an arm in the air. The ball lies beyond his reach on another platform, across a sheer drop of more than 2 feet. Unperturbed, he keeps right on going, as if to crawl through thin air.
Never fear, an attending adult plucks the reckless baby from harm's way. Yet the child's not always so rash. The same infant stays put if the adult seats him with his legs dangling over the platform's edge. There, the baby sits with ease and gazes at the tempting toy. Given a second chance to crawl, however, the infant will again take the plunge.
Consider this a lesson in the surprisingly close affinity of learning to early postural milestones—sitting, crawling, and walking. As they master each posture, infants must learn new ways to control their bodies and to factor depth information into their actions,