Mountain goats are born understanding where they should and shouldn’t climb, but baby humans need practice puttering around before they can make sound judgments. Now New York University developmental psychologist Karen Adolph has found that for each new phase of motor development, infants have to relearn how to keep themselves safe.

Adolph tested how infants judge risk by setting 12- and 18-month-old infants at the top of an adjustable wooden “cliff” and having their mothers beckon them over the edge. (Lab staff guarded the babies closely and caught any who actually tumbled off.)

Babies who had been crawling for months generally did not go over drop-offs that were too big for them, nor did babies who had been walking for a while. But many babies who had
just started walking marched straight over drop-offs beyond their capabilities—even the highest, most obvious three-foot plunge.

What that means, Adolph explains, is that crawling infants do not learn to be afraid of heights. Instead they learn what their crawling bodies can do, and when their style of locomotion changes, they need practice to recalibrate how they perceive their abilities.

Adults adjust to changing motor limitations every day: they may shift their body weight to ease up on a sore leg or take smaller, more deliberate steps when there is ice on the ground. Adolph says we learn that adaptability as infants by experimenting with physical limits and making mistakes.

For parents, of course, there’s another lesson in the research: unless your kids are mountain goats, keep a watchful eye on them when they start to walk.