How smart are babies? A new U.S. study revealed they could tell whether people are friends or strangers by their co-laughter.

Co-laughter or simultaneous laughter shared between individuals can tell plenty of information about people's social relationships. For example, adult listeners can know if they have an affinity or not.

The question is, can babies do the same? To answer it, the researchers from New York University and UCLA conducted two experiments involving infants.

The Test Of Sound

For the study, now published in Scientific Reports, the researchers worked with 24 full-term and healthy babies who came from the maternity wards of New York's local hospitals. The team excluded 10 more infants due to factors such as pre-existing medical conditions and fussiness.

In the first experiment, the babies looked at a black-and-white checkboard. They also listened to Bach's concerto before and after the trials to familiarize them to auditory cues of the procedure.

The researchers then exposed the babies to sounds of co-laughter coming from American English-speaking undergraduates. They either signed up individually and paired with strangers or came with friends. The conversations lasted for about 13 minutes, from which the team drew co-laughter segments.

The data revealed that the babies listened longer on the co-laughter between friends than between strangers. It indicated that not only could they differentiate laughter, but they also preferred to hear one coming from affiliated individuals.

Reading The Visuals

For the second experiment, the researchers wanted to find out if they could match the co-laughter with the right social relationship.

The team then played video clips showing two women having two types of interaction. In one scene, they were facing each other as if they were friends. In the other, they were on each other's backs. The frames froze at a scene where both faced the babies with a neutral expression. The infants then heard alternating sounds of co-laughter.

The babies' reactions coincided with the team's hypothesis — that is, if they could identify the mismatch, they would end up staring longer.

The two experiments revealed two essential points about babies. One, the ability to know the nature of relationships can begin even during infancy. Second, understanding the differences will be more important as they grow older.

"Infants' sensitivity to different kinds of laughter might be one of the early emerging tools they use to understand and navigate the complex social world," said Athena Vouloumanos, one of the co-authors and an associate professor in NYU.

Babies Continue To Surprise Adults

This isn't the first study that showcased what babies can do. A research published in Science revealed they are capable of logical reasoning even before they can learn how to speak.

Infants can also be sensitive and responsive to touch. Stroking them gently before a medical procedure can work just as well as a topical anesthetic.