Montreal researchers say this type of reaction lends support to an adult's natural inclination to raise the pitch of his or her voice when speaking to infants.
The preference for repeated infant vowel sounds seems to kick in as early as four months, before infants are babbling "goo goo gaa gaa" themselves, say Prof. Linda Polka of McGill's School of Communication Disorders and her team in the journal Developmental Science.
They call it the "first documented evidence that infants prefer listening to infant vowel sounds over adult vowel sounds."
"An infant's speech is very high frequency, and it could be part of what they like is those very high frequencies," Polka said.
In a series of experiments, up to 22 infants aged three to six months sat on a caregiver's lap facing a TV screen with a checkerboard pattern.
By gazing at the screen, the babies controlled how long they heard a series of repeated "eee" vowel sounds from a synthesizer. The setup allowed scientists to control the pitch and emphasize various frequencies.
Vowels are the first speech-like sounds babies learn to make. In the study, infants listened longer to a vowel with at least one infant vocal property like high pitch.
On YouTube, baby Camille smiles when she hears an infant and moves her mouth more.
Others, like baby Ethan, tried to vocalize "ee" when he heard an infant sound played for CBC News at the lab. Still others simply grinned with delight.
Pitch perfect baby talk
The findings offer clues about how hearing infant speech at four or five months might motivate babies to focus their attention to vocalize.
"What's important to me is what role does that play in their development? How is it helping them? And what happens to a child who for some reason can't process that kind of signal because they have a hearing loss, for example. Or babies who aren't engaging or interested in their own voice or other baby voices. Is there something there that we should pay attention to that we could use to help those children?"
In the meantime, parents' intuitive knowledge to use a high, infant-like voice to speak to babies is "right on," Polka said.
Clearly, babies are very interested in their peers. Polka recalled how her three children are close in age and the younger ones were "totally riveted" with their older sibling's speech.
At a Wholeplay class in Toronto, Tracey Norman did a hokey-pokey dance with her three-month-old, Pearl.
Norman said Pearl enjoys when she imitates baby sounds back to her and recalled how engaged the infant was at her first mom and baby yoga class.
"She just listened to all the sounds and was so happy for the whole hour," Norman said. "I think it was all the baby sounds."
The Montreal team hopes to look at other vowel sounds next.