By the time they are nine months old, babies can tell there's a difference between happy tunes like the jazz song Tiger Rag and sad ones like Beethoven's Symphony No. 7.

How do researchers know?

They monitored the length of time babies ages 3 months to 9 months watched a picture of a face while the music played. When the music switched from happy to sad or vice versa, the older infants showed more interest in the face. (The music was previously judged as happy or sad by preschoolers and adults.)

The authors - Ross Flom from Brigham Young University along with researchers from Iowa and Minnesota - stress they don't know whether the babies felt joy or sorrow while listening or whether they could discern the emotion in the music.

So why does it matter?

For one, it shows young infants can categorize the songs "in a manner like adults," said Flom.

It also reiterates that infants are sensitive to emotion, whether it's through facial expressions, voice or song, he said.

"It underscores the old adage that the communicative message is not in the words per se, rather it is in the intonation or affective qualities of our speech," he said.

The study will be published in the Journal of Infant Behavior and Development.

- Heather May