'Twinkle twinkle, little star"—we can all sing the old nursery classic. But the song sounds different when we sing it to an infant, and that change in the voice may help strengthen emotional ties between parent and child.

Sandra Trehub, Ph.D., of the University of Toronto, recorded mothers and fathers singing to their babies and to an empty room. Though the parents were told to make the second rendition as similar to the first as possible, others listening to the tapes could easily distinguish the two. "The parents tried to duplicate their performance to their infants, and they weren't able to do it," Trehub says. It seems that most parents, especially mothers, have a special "lullaby voice" that they use in singing to their babies: high in pitch and slow in speed. The voice is partly a product of the emotion parents feel for their child—emotion that provokes involuntary changes in facial expression and body posture, which in turn affect the sound of the song. To evoke such a response, the baby must actually be present: "The expressiveness of this voice can't be faked," says Trehub.

The babies themselves were able to perceive and respond to such expressiveness. In fact, says Trehub, "infants were so proficient in their perceptual skills that I found myself wondering—what are these skills for?" Babies, she speculates, are monitoring their environments for signs of security and caring. The words of lullabies may mean little to infants, but their emotional message comes through loud and clear.