A curious phenomenon has long puzzled developmental psychologists: Why do babies seem incredibly smart in some experiments yet utterly clueless when tested with other tasks designed to measure the same knowledge? Hide a toy under a blanket, and babies under about seven months do not reach for it, suggesting that they have not yet grasped object permanence, the idea that objects continue to exist when out of sight. Experiments involving merely looking at objects, however, show that babies as young as two and a half months are aware of the concept.

The standard explanation for such discrepancies is that babies have the concept early on, possibly from birth, but that for many months their underdeveloped motor skills keep them from lifting the blanket. Still, this account may be insufficient, because even babies taught to pull a blanket will not necessarily do it to uncover a hidden object.

So psychologist Yuko Munakata of the University of Colorado at Boulder has proposed a new key to the puzzle: that knowledge develops over time. "It's not like babies have this aha! insight one day--it's a gradual understanding," she explains. In a paper to be published in Psychological Science, Munakata and Jeanne L. Shinskey of the University of South Carolina offer some evidence that infants "gradually develop stronger representations of objects through experience" and that some tasks require stronger mental representations than others do.

In testing 24 babies, the researchers first made each seven-month-old thoroughly familiar with a simple clay toy. They then measured how often the baby reached for this toy compared with a new toy. As predicted by many previous studies, babies overwhelmingly preferred the novel to the familiar, 88 versus 39 percent of the time. But that was true only when the toys were in plain view. When the researchers turned off the light (and watched through an infrared camera), they found a complete reversal of infants' usual novelty preference. In the dark, the babies reached for the area where they last saw the new toy only 20 percent of the time, compared with 32 percent for the familiar one.

The researchers conclude that becoming familiar with a toy strengthened the babies' mental representations of it enough to search for it. Conversely, the babies did not reach for the cool new toy because they had not been around it long enough to recall it while in darkness.

That is just one interpretation of the data, according to Rutgers University psychologist Carolyn Rovee-Collier. It is possible that babies reach for familiar objects in the dark because that is a different game. She also wonders whether the pattern of reaching for familiar objects in the dark would hold for different degrees of familiarity, because novelty may be a special case rather than simply a lower level of familiarity.

Rachel Keen, a psychologist at the University of Massachusetts Amherst, proposes another possibility: babies are not afraid of the dark and in fact may find darkness pleasantly intriguing. She posits that "a familiar object in the novel environment of darkness is somewhat novel," noting that an intermediate level of novelty has proved more appealing than either total novelty or complete familiarity. Munakata says she is conducting another experiment that will test that explanation. But even that may not settle the question. As Keen puts it, "It's difficult to make inferences about what goes on in babies' minds, even though we do it all the time."