Do you ever wonder what's going through your baby's mind when she smiles at you? Researchers at the University of California, San Diego, think they know. Their study, published in PLOS ONE, suggests that babies who are smiling are attempting to make whomever they're interacting with smile back—all while the infants exert as little effort as possible. "I used to wonder if my daughter was trying to communicate with me when she was an infant and smiled," the lead researcher tells the San Diego Union-Tribune, and this study shows "it might not have just been wishful thinking on my part. Babies are very goal-oriented." As the study explains, "By the time infants reach 4 months of age both mothers and infants time their smiles in a purposeful, goal-oriented manner. In our study, mothers consistently attempted to maximize the time spent in mutual smiling, while infants tried to maximize mother-only smile time." In fact, a press release notes, researchers found that babies use sophisticated timing in order to get what they want.
Researchers first looked at a previous study that observed face-to-face interactions of 13 moms and their babies when the infants were aged 4 to 17 weeks. By using an algorithm to "reverse engineer," they were able to determine the babies' goals based on their behavior. But if babies want their mothers to smile as much as possible while they smile as little as possible (though, as one researcher cautions, the study did not determine whether the babies were conscious of what they were doing), then shouldn't infants refrain from smiling if their mother is already smiling? That's where the sophisticated timing comes in: Infants used "maximally efficient ... wait times" between smiles, because, if they stop smiling entirely, their mother is likely to also stop smiling at some point. Researchers then programmed a robot with a realistic, toddler-like face to behave in the same fashion as the babies in the study, and had that robot interact with students. The same thing happened: The robot minimized its own smiles all while maximizing the smiles it received in return. (American babies don't laugh as much as Dutch babies do.)