Do babies know a hidden toy still exists?

Probably, and earlier than Piaget thought, but scientists still argue about how to read the evidence. Object permanence is the understanding that whether an object can be sensed has no effect on whether it continues to exist. If a toy slides behind a cushion, does it stop existing for the baby, or does the baby just not look for it?

Piaget believed it was one of an infant's most important accomplishments, since without it objects would have no separate, permanent existence. He studied it by watching what happened when a favorite toy was covered with a blanket or removed from sight. Peek-a-boo plays on the same gap: children who have yet to fully develop object permanence respond to the sudden hiding and revealing of a face. A classic sign of the same stage is the A-not-B error: after finding a hidden object at location A, a child fails to retrieve it at location B.

Renée Baillargeon doubted that reaching tells the whole story. She argued that Piaget's finding of failure until 8 to 12 months was rooted in a lack of motor ability, not a lack of understanding. So her studies measured how long infants looked at impossible versus possible events. Babies looked longer at events that defied physical laws applied to hidden objects, which she took to mean they do understand object permanence. She and Julie DeVos reported it in infants as young as 3.5 months old.

Not everyone agrees. Schoner and Thelen argued that longer looks may merely mean infants notice a difference between displays, such as more movement or different colors, rather than surprise at an impossible event. There is not yet scientific consensus on when understanding of object permanence emerges.