As any parent knows, babies aren't born with a fear of heights. In fact, infants can act frighteningly bold around the edge of a bed or a changing table.
But at around 9 months, babies become more wary of such drop-offs. New research suggests infants build an avoidance of heights once they get more experience crawling and navigating the world on their own.
In one of their experiments, a group of scientists from the University of California, Berkeley, and Doshisha University in Kyoto studied babies that had not yet begun to crawl. Over the course of 15 days, some of the infants were trained to use a motorized baby go-cart that they could control.
After this period, the researchers watched how the babies reacted when they were held over a glass-covered edge. The infants who had experience with the go-cart got skittish around this virtual cliff. Their heart rates sped up, while the heart rates of the babies without the driving lessons remained steady, the scientists found.
The researchers also tested how these babies reacted to a so-called moving room, an enclosure where the walls move backwards and make whoever is inside feel like he or she is moving forward. The babies who had learned how to use a go-cart were more upset by this illusion.
In another part of the experiment, the researchers tested babies who had already started to crawl. The ones who were most upset by the moving room were also more afraid to crawl over a glass-covered virtual edge, even as their mothers encouraged them from the other side, as video from the experiment shows.
This finding suggests that as infants gain locomotor experience (in this case, crawling or navigating a go-cart), they come to rely more on visual information to help them move though an environment. The results also indicate that a fear of heights is not likely a hard-wired developmental change, but rather a shift that depends on experience, the researchers say.
An avoidance of heights has an obvious advantage: It keeps infants from falling and getting injured. So why doesn't it kick in before babies start crawling?
"One major benefit of such a delay is that infants are more prone to explore their environment and the movement possibilities afforded by that environment when they are less concerned about the consequences of their actions," the researchers write in the journal Psychological Science.
This lack of fear helps them develop movement strategies and learn how to navigate different types of surfaces, the scientists say.
"Paradoxically, a tendency to explore risky situations may be one of the driving forces behind skill development," the researchers add.