Shared smiles may shape infants' emotional development
· Medical Xpressby Kyra Gurney, University of Miami
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A mother and her baby smiling at each other is more than 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: Biological Sciences, 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 babies 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 of 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 affected their heart rates, researchers attached electrodes to the babies' chests and stomachs while trained observers watched them interact with their mothers. The observers recorded, 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.
The researchers found increases in heart-rate variability when the babies and their mothers were positively engaged at the same time. 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."
Messinger was also involved in a separate study looking at infant smiles. This research, which was also recently published in Philosophical Transactions of the Royal Society B: Biological Sciences, examined how babies and adults influence each other's smiling.
The researchers observed 58 infants at 4 and 8 months of age while they interacted 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 more likely to start a smiling interaction on their own and more likely to reciprocate an adult's smile than they were at 4 months.
"That suggests that the dyad becomes more dynamically responsive to one another at older ages," said Messinger, who was a first author of the paper along with Eliala Alice Salvadori, who conducted this research for her dissertation at the University of Amsterdam; Cristina Colonnesi, an associate professor at the University of Amsterdam; and Alan B. Cobo-Lewis, now an associate professor at the University of Maine.
Babies at both ages were most likely to return their mother's smiles, compared with smiles from fathers and a stranger, 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."
Publication details
Drew H. Abney et al, Mutual positive engagement with mother is uniquely associated with increases in human infant heart rate variability, Philosophical Transactions of the Royal Society B: Biological Sciences (2026). DOI: 10.1098/rstb.2026.0004
Eliala Alice Salvadori et al, Smiling in time: the interactive development of infant smiling with mothers, fathers and strangers, Philosophical Transactions of the Royal Society B: Biological Sciences (2026). DOI: 10.1098/rstb.2025.0142
Journal information: Philosophical Transactions B
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PediatricsChildren's healthPsychology & Mental health Provided by University of Miami Who's behind this story?
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