Smiling is not necessarily an emotional behavior for babies. According to a new study out of UC San Diego smiling is first and foremost a social and relational act destined to make the mother smile in return.

Babies often smile with a goal in mind: to make the person they are with smile in return. To prove this hypothesis, a team of computer scientists, roboticists and developmental psychologists from the University of California - San Diego programmed a toddler-like robot to behave like the babies they studied and had the robot interact with undergraduate students.

Their conclusions, published in the September 23 issue of PLOS ONE, reveal that the robot got the undergraduates to smile as much as possible while smiling as little as possible.

A mix of developmental psychology and robotics, this innovative study used the robot as a tool for scientific research. Scientific teams already use it to better understand nonverbal communication between children and adults, particularly in cases involving autism.

‘If you've ever interacted with babies, you suspect that they're up to something when they're smiling. They're not just smiling randomly,’ said Javier Movellan, a research scientist in the Machine Perception Laboratory at the University of California, San Diego, and one of the study's authors. ‘But proving this is difficult.’

This is why the team turned to optimal control theory, a tool often used in robotics. This method allowed the researchers to design and program a robot to perform a specific behavior based on specific goals. For the purpose of this study, the researchers used the method to reverse engineer what the babies' goals were based on their behavior.

The team used data from a previous study that observed the face-to-face interactions of 13 pairs of mothers and infants under the age of four months, including when and how often the mothers and babies smiled.

The researchers were surprised that the control theory data analysis found that 11 out of the 13 babies in the study showed clear signs of intentional smiling.

So the researchers developed a program that mimicked the babies' actions and transferred it onto Diego San, a toddler-like robot that Movellan's team had used for similar studies in the past.

Diego San interacted with 32 UC San Diego undergraduates individually during three-minute sessions, and when Diego San behaved like the babies in the study, the undergraduate students behaved like the babies' mothers: they smiled a lot even while the robot didn't have to smile that much.

‘What makes our study unique is that previous approaches to studying infant-parent interaction essentially describe patterns,’ said study co-author Dan Messinger from the University of Miami. ‘But we couldn't say what the mother or infant is trying to obtain in the interaction. Here we find that infants have their own goals in the interaction, even before four months of age.’