That special tone of voice adults use with babies may be an important part of how infants learn language, a new study suggests.

Researchers have long suspected that "baby talk, "with its short sentences, slow pace and sing-song tone, helps infants start to distinguish words from other sounds. Direct evidence, however, has been lacking, and not particularly easy to get.

But in the new study, published in the journal Infancy, researchers found that the typical intonation of baby talk, with its swooping changes in pitch from word to word, seems to help babies begin to recognize where words begin and end.

Though it's not certain why this is, it's likely that a sing-song tone captures babies' attention better than the more monotone manner adults use with each other, according to lead author Erik D. Thiessen, a psychology professor at Carnegie Mellon University in Pittsburgh.

It seems very natural for adults to switch to baby talk when speaking to an infant, and it's probably no "coincidence" that the speech style has a practical use.

"The way that you want to talk to your baby turns out to be good for them," Thiessen said. "Parents' instincts are right."

For their study, Thiessen and his colleagues zeroed in on the effect of "infant-directed" intonation by having 40 infants, 6.5-to-7.5-months-old, listen to a set of nonsense sentences, spoken either in baby-talk or adult-like tones.

After the babies were familiarized with the sentences, the researchers tested whether speech intonation made a difference in the infants' ability to distinguish individual words.

They did this by first drawing the babies' attention to a flashing light, and then playing the sound of either a whole word or a partial word repeatedly until the baby looked away. If an infant gazed longer when a whole word was played versus a partial word, it was taken as a sign that they recognized the word as something distinct.

Thiessen's team found that babies who heard the sentences in baby-talk tones listened longer when a whole word was played than when a partial word was played. In contrast, infants who heard the sentences in adult intonations showed no such preference for whole words.

The two sets of sentences the babies heard differed only in tone, that is, the infant-directed sentences carried none of the other features of baby talk, such as short sentences and long pauses after words. This suggests that the sing-song quality of baby talk itself aids in language learning.

That does not mean, however, that the other characteristics of baby talk play no role in learning. Instead, Thiessen suggested that they probably all act together to facilitate language development.

SOURCE: Infancy, March 2005.