In the Visual Cliff Experiment, a baby pen is set up that has a high shelf on one side and a drop-off on the other. Both have a pattern so that the drop-off can be clearly seen. Plexiglass is placed over the side with the drop-off, so if an infant crawls onto it, the baby will be just fine. Still, the drop is unnerving.
The first experiment was set up in 1960, when E.J. Gibson and R.D. Walk wanted to find out if infants had any practical conception of physical reality. They placed the infants on the shelf, and had their mothers show a toy to them. The babies eagerly crawled toward the toy. They then had their mothers put them back on the shelf, and move around to the far side of the visual drop-off. Again, the mothers showed the babies the toy and encouraged them to come and get it.
The researchers found that, starting at around eight months, the babies were reluctant, or even refused, to crawl over what they saw as a cliff. The babies avoided the cliff even when they put their hand on the glass and felt its solidity. Most of the thirty-six babies tested just wouldn't go over. Younger babies could be tempted to try to wriggle themselves over the cliff, making researchers believe that they hadn't gotten the visual or spatial maturity to understand that anything was amiss yet.
(Incidentally, the researchers tried the same thing with baby goats, who avoided the cliff one hundred percent of the time even when the goats were only a few hours old. This sample may be biased, though, since it's unlikely that the researchers got the goat's mother's cooperation in the experiment.)
About a decade later, scientists had decided that they hadn't yet gotten tired of freaking out infants. Where they'd gone wrong before was giving the infant a behavioral say in whether it wanted to be on the ground or plunked down in mid-air, apparently about to fall to its death. They attached cardiac monitors to babies too young to get themselves around and placed them on either side of the cliff. Weirdly, they seemed less distressed over the drop than they did when they were over the shelf. Their hearts decelerated on the drop-off side, and they exhibited more distress when placed, apparently, on solid ground. So clearly the babies could understand that there was a difference, but weren't concerned by the visual doom the way older kids were.
In fact, the older kids got, the more they seemed reluctant to step out over a cliff. A later study showed that just-walking babies were even less likely to walk out over the plexiglass than crawling babies.
In the last few years there has been a shift in interest, from what babies perceive to how they make decisions. The latest version of the visual cliff involves more input from the baby's primary caregiver than before. Again very young children were placed on a visual cliff presented with their caregiver — generally their mother — and a toy, on the far side of the drop. The children touched the glass and looked up at their mom. Their mothers were asked either to give them a look of fear, or to reassure the babies and beckon them over. To no one's surprise, the babies whose mothers showed fear stayed away, while the babies whose mothers encouraged them to walk over the plexiglass came over. It's not just physics, but outside guidance that will help a child decide what to do.
Perhaps the next experiment will involve a baby, a puppy, and a kitten in a race to get ice cream on the far side of the cliff. Or two babies. Let's see if competition will get kids to take risks that physics and maternal guidance can't. And for the sake of cinematic tradition — let's put the ice cream in a grail.
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