A mother and her baby smiling at each other is more than just a heartwarming exchange—this type of positive interaction may help babies develop emotion regulation skills and the physiological foundations of emotional well-being.
That's according to a new study that measured changes in the heart rates of nearly 1,000 4-month-old babies as they enjoyed positive interactions with their mothers. The study, published in Philosophical Transactions of the Royal Society B , found that emotional development may be shaped through positive exchanges between babies and their caregivers—such as smiling and laughing—because these interactions facilitate physiological changes in the baby that are important for emotion regulation.
"Play is not just fun and games. It's influencing the baby's physiology," said Daniel Messinger , a professor in the Department of Psychology at the University of Miami College of Arts and Sciences and a senior author on the paper. "There's something about positive sociality that's associated with this internal baby state. The baby's physiology is linked to what's going on in the interaction."
To see how the infants' social interactions impacted their heart rates, researchers attached electrodes to the babies' chests and stomachs while trained observers watched them interact with their mothers, recording moment by moment whether each member of the pair showed positive, neutral, or negative engagement. Using the heart rate data, the researchers then calculated respiratory sinus arrhythmia—heart-rate variability associated with breathing—which is understood to be a biological indicator tied to emotion regulation and adaptability to different environments.
Daniel Messinger
The researchers found increases in heart-rate variability when the babies and their mothers were positively engaged at the same time. Importantly, they did not observe the same physiological changes when only the baby was smiling or when only the mother was showing positive emotion.
"They both have to be positive," explained Messinger, who is also a professor of pediatrics. "When they're both positive, particularly for longer periods of time, you see this increase in the baby's heart rate."
The research was part of a national study on drug exposure at four sites across the country, including Miami, that involved several universities. The researchers controlled for prenatal drug exposure and social adversity when they analyzed the data. Among the members of the research team was Samantha G. Mitsven , a University of Miami alumna who worked on the study while completing her Ph.D. in developmental and child psychology.
Messinger was also involved in a separate study looking at infant smiles. This study, which was also recently published in Philosophical Transactions of the Royal Society B , examined how babies and adults influence each other's smiling.
The researchers observed 58 infants at 4 and 8 months of age while they were interacting with their mothers, their fathers, and a stranger. They found that adults initiated and reciprocated smiling interactions at higher rates than infants, and that seeing an adult smile significantly increased the rate at which babies began smiling. Babies, on the other hand, were more likely to end the shared smiling moments, which may indicate that they were learning to moderate their excitement.
The researchers also discovered that at 8 months, the babies were both more likely to start a smiling interaction on their own and more likely to reciprocate an adult's smile, compared to when they were 4 months old.
"That suggests that the dyad becomes more dynamically responsive to one another at older ages," said Messinger, who was a first author on the paper along with Eliala Alice Salvadori , who conducted this research for her dissertation at the University of Amsterdam, and Cristina Colonnesi , an associate professor at the University of Amsterdam. One of the other first authors, Alan B. Cobo-Lewis , is a University alumnus. He completed his postdoctoral fellowship and undergraduate studies at the University of Miami and is now an associate professor at the University of Maine.
Compared to the smiling interactions with fathers and an unknown adult, babies at both ages were most likely to return their mother's smiles, the researchers found.
"This was, importantly, the first paper that involved not only moms and dads, but also strangers," Messinger said. "The strangers initiate and sustain smiles more than moms, but infants are more likely to smile back at the mom. So, the relationship really counts there."
