Visual Cliff: In developmental psychology, a research apparatus designed to look like a cliff, but which actually has a clear plexiglass platform so that an infant could safely crawl from one end to the other. No matter which side of the visual cliff they are placed on (i.e., "safe level ground" or "dangling over open air"), non-crawling infants do not show increased physiological response (fear). This is because they have not yet developed depth perception and so cannot be tricked by the research optical illusion. Depth perception develops as the crawling ability develops. Crawling infants placed on the "safe, high" side of the visual cliff are highly reluctant to crawl over the "edge" and off the "cliff." Crawling infants placed onto the plexiglass on the "open air" side show heightened physiological response. However, infants can be coaxed over the edge of the cliff if their mothers encourage them to crawl towards them. A smiling, gesturing mother can entice an infant to disregard all the perceptual information that is telling him that if he crawls one more foot he will tumble from a great height. (Conversely, the children of mothers who display a fearful expression will stay put on the "safe" side, even if an experimenter is trying to coax them over the edge by waving an interesting toy.) This simple research apparatus was originally designed to test the development of depth perception, but has since been used to demonstrate the power of the mother-infant attachment bond. If positive attachment can provoke such a "leap of faith," then what about the converse -- just how bad is it when children grow up without a strong attachment bond? What happens to children who never feel loved, never feel secure, never feel that there is someone in the world who will put his interest above her own? The state prisons and rehab facilities are full of them.