A closer look
Before we knew how babies perceived depth, Eleanor Gibson threw some heavy glass over a massive drop-off and watched to see if they'd happily crawl to their doom. The 'Visual Cliff' experiment involved placing a checkered pattern directly under one side of the glass, and several feet below on the other. Even when their mothers enthusiastically cheered them on, the vast majority of crawling infants flat-out refused to cross the deep side. It brilliantly proved that humans aren't entirely blank slates; we come factory-equipped with a healthy, instinctual terror of falling off cliffs.
