Do babies know if hidden objects are still there?

One of Jean Piaget's most famous observations is the phenomenon of "object permanence"—the idea that babies younger than eight months old have no conception of an object once it's hidden from view. It's easy to see how he came to this conclusion. Click on the picture of my daughter Nora at six months of age to see a video of her spectacularly failing the object permanence test. Once the object is hidden under a napkin, she seems to lose all interest in it.

But does she really not understand that the object is still there, or is she simply interested in other things? A team of researchers led by Yuyan Luo developed a different methodology to answer this question (Yuyan Luo, Renée Baillargeon, and Laura Breuckner, University of Illinois; and Yuko Munakata, University of Colorado, "Reasoning about a Hidden Object After a Delay: Evidence for Robust Representations in 5-Month-Old Infants," Cognition, 2003).

Their procedure involved dividing 5-month-old babies into two groups. The first group was shown a box which was placed behind a screen on a small stage. The experimenter showed the baby that there was no room in the six inches between the screen and the back wall of the stage to fit any objects other than the box. Next, with the screen up, the babies were shown a cylinder that was taller than the screen. The babies were distracted for a few minutes, then presented with the testing phase. In this phase, the cylinder was moved behind the screen—right where the baby had only ever known to be filled with a box. If the baby remembered that the box was there, Luo et al. reasoned, then she would be surprised to see the cylinder move through the space that should have been occupied by the box.

Of course, five-month-olds can't tell us if they're surprised, so typically "surprise" is measured by how long the baby looks at the object which is supposed to surprise them. Babies did indeed stare at the cylinder for up to a full 60 seconds after it stopped moving.

The second group was shown the identical procedure, except that the box behind the screen was only a half-inch deep, leaving plenty of room for the cylinder to pass unimpeded. While this group also stared at the cylinder for a long period of time, they tended to lose interest about 20 seconds quicker than the first group. It's difficult to come up with any explanation for this difference other than that the babies remember that the object behind the screen shouldn't block the path of the cylinder. The reason they stared at all is simply because they hadn't seen the cylinder move on its own before.

So apparently babies do remember objects, even several minutes after they are hidden. In the video, Nora may have remembered the rattle after it was placed under the napkin but simply lost interest now that it was no longer moving.