The research team, led by Katie Hoemann of the University of Kansas and including Jeffrey Girard, Marissa Ogren, Lauren Leotti, Lisa Feldman Barrett, Lisa Oakes, and Vanessa LoBue, took advantage of an existing longitudinal corpus of home videos known as the SEEDLingS data set, shared by Elika Bergelson’s research group. Rather than staging laboratory interactions, the study examined naturalistic footage of infants going about ordinary life with the people closest to them. This ecological approach matters because laboratory studies of emotion perception often present infants with posed, prototypical faces, yet those faces may bear little resemblance to what babies actually encounter in daily life.

The technical core of the study involved coding the facial behavior visible in the recordings using facial movement analysis, a method that quantifies the visible contractions of facial muscles rather than relying on subjective labels of what an expression means. The team asked two distinct questions: how much of the face were infants actually exposed to, and how often did the visible configurations match the hypothesized prototypes for emotion categories such as happiness, anger, sadness, and fear. This dual approach allowed the researchers to characterize both the quantity and the emotional quality of infants’ facial input.

The first major finding concerned visibility. Infants were exposed to far more partial faces than full faces across the entire age range studied. In the flow of everyday interaction, caregivers turn away, lean in close, move out of frame, or orient their faces at oblique angles, meaning that a baby’s view of another person’s face is typically fragmentary rather than complete. Exposure to full faces, while low overall, did increase across the observed time periods, hinting that infants’ visual access to complete facial configurations gradually improves as they and their caregivers develop more structured patterns of face-to-face engagement.

The second finding was more striking. Facial configurations hypothesized to serve as expressive prototypes for emotion categories were extremely rare in the input. When babies did glimpse a full face, it almost never displayed the canonical smile, scowl, or grimace that emotion recognition tasks assume they should be learning. This scarcity strikes at a central assumption of classical theories of emotional development, which posit that infants acquire emotional meanings by repeatedly pairing specific facial configurations with specific internal states. If the configurations themselves are seldom visible, that learning mechanism cannot operate as traditionally imagined.

Equally revealing was what happened on the rare occasions when hypothesized prototypes did appear. They often occurred in unexpected contexts, suggesting that the expressions did not necessarily function as cues to their presumed emotional meanings. A smile might accompany frustration rather than joy; a furrowed brow might signal concentration rather than anger. This contextual mismatch reinforces a growing body of evidence that emotions are expressed heterogeneously in daily life, and that the same facial movement can serve many different functions depending on the situation, the person, and the ongoing activity.

Descriptive analyses of the facial movement data added a quantitative layer to the story. Infants’ facial input was dominated by a small number of mostly simple facial configurations, and these frequent configurations did not correspond fully or uniquely to the hypothesized category prototypes. In other words, the statistical structure of the visual environment does not carve itself neatly along the lines of happiness, sadness, anger, and fear. Instead, babies see a recurring set of modest facial movements, many of them partial and many of them ambiguous with respect to any single emotional category.

The implications ripple outward across developmental science. If infants must learn much more than one-to-one mappings between facial configurations and context or behavior in order to acquire expressive vocabularies for emotion, then the learning problem they face is far more complex than previously assumed. Rather than memorizing a small dictionary of expression-to-meaning pairs, babies may need to track how facial movements unfold dynamically, how they combine with voices, gestures, and physical settings, and how particular individuals in their environment use faces in idiosyncratic ways. Emotion understanding, on this view, is constructed from rich, variable, and context-dependent evidence rather than absorbed from a fixed set of signals.

The study also carries methodological weight for the field. Decades of infant emotion perception research have relied on posed, prototypical stimuli precisely because they are easy to control and categorize. The new findings raise the question of whether performance in such tasks reflects learning about real emotional communication or merely sensitivity to artificial faces that infants rarely, if ever, see outside the laboratory. Naturalistic corpora like the one used here, combined with automated facial movement coding, offer a path toward reconciling laboratory findings with the realities of the developing child’s environment.

For parents, the takeaway is quietly reassuring rather than alarming. Babies appear to thrive without being fed a steady diet of textbook expressions, and the messy, partial, and context-bound faces of everyday life seem to be exactly the input from which human emotional understanding is built. The research, funded in part by the National Science Foundation and the Eunice Kennedy Shriver National Institute of Child Health and Human Development, marks a significant step toward grounding theories of emotional development in the world as infants actually experience it, one fleeting and fragmentary glimpse of a caregiver’s face at a time.

Subject of Research: Naturalistic analysis of infants' everyday exposure to facial expressions of emotion from 6 to 17 months

Article Title: Infants’ facial input from 6 to 17 months rarely includes hypothesized expressive emotion prototypes

Article References: Hoemann, K., Girard, J. M., Ogren, M., Leotti, L., Barrett, L. F., Oakes, L. M., & LoBue, V. (2026). Infants’ facial input from 6 to 17 months rarely includes hypothesized expressive emotion prototypes. Communications Psychology. https://doi.org/10.1038/s44271-026-00539-z

Image Credits: AI Generated

DOI: 10.1038/s44271-026-00539-z

Keywords: infant development, emotion perception, facial expressions, facial action coding, longitudinal study, naturalistic observation, caregiver interaction, Communications Psychology, emotional development, SEEDLingS corpus, developmental psychology, social cognition

The first of the two studies set out to answer a question that has long interested developmental scientists: what actually happens inside a baby when a joyful exchange with a parent unfolds? To find out, the researchers measured changes in the heart rates of nearly 1,000 four-month-old infants as they engaged in positive interactions with their mothers. Electrodes were attached to the babies’ chests and stomachs, while trained observers watched the mother-infant pairs and recorded, moment by moment, whether each member of the pair displayed positive, neutral, or negative engagement. This combination of continuous physiological recording and fine-grained behavioral observation allowed the team to link what was happening between the mother and baby with what was happening inside the baby’s body.

The physiological measure at the center of the study was respiratory sinus arrhythmia, a form of heart-rate variability that is coupled with breathing. Scientists understand this variability to be a biological indicator tied to emotion regulation and to a person’s adaptability to different environments. In adults and in children, higher heart-rate variability in this range is generally associated with a more flexible capacity to respond to social and emotional demands. By calculating this measure from the infants’ heart-rate data, the researchers could ask whether moments of shared positivity were accompanied by shifts in the biological systems that support emotional self-regulation.

The answer was striking. The researchers found increases in the babies’ heart-rate variability precisely when the infants and their mothers were positively engaged at the same time. Crucially, the same physiological changes did not appear when only the baby was smiling, or when only the mother was showing positive emotion. Both partners had to be on the same positive wavelength for the effect to emerge. 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, put it plainly: “They both have to be positive. When they’re both positive, particularly for longer periods of time, you see this increase in the baby’s heart rate.”

That mutual requirement may be the study’s most important insight. It suggests that emotional development is not simply a matter of a happy baby or an affectionate parent acting alone. Instead, the physiological states associated with emotion regulation appear to be recruited during genuine two-way exchanges, in which both the caregiver and the infant are sharing positive affect simultaneously. “Play is not just fun and games. It’s influencing the baby’s physiology,” said Messinger, who is also a professor of pediatrics. “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.” In other words, the interaction itself, not the mood of either partner in isolation, is what seems to matter.

The scale and design of the study lend weight to its conclusions. The research was part of a national study on drug exposure conducted at four sites across the country, including Miami, and involved several universities. When the team analyzed the heart-rate data, they controlled for prenatal drug exposure and social adversity, helping to ensure that the observed link between shared positivity and heart-rate variability was not simply an artifact of those factors. 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. With nearly 1,000 infants contributing data, the study represents one of the larger physiological investigations of early mother-infant interaction.

The second study, also published in Philosophical Transactions of the Royal Society B, approached infant smiling from a different angle by asking how babies and adults influence each other’s smiling over time. The researchers observed 58 infants at two ages, 4 months and 8 months, while the babies interacted with their mothers, their fathers, and a stranger. This design allowed the team to compare smiling dynamics across different relationship partners within the same infants, revealing patterns that would be invisible in studies focused on mothers alone.

The results showed a clear division of labor in early smiling exchanges. Adults initiated and reciprocated smiling interactions at higher rates than infants did, and seeing an adult smile significantly increased the rate at which babies began smiling themselves. Babies, on the other hand, were more likely to be the ones who ended shared smiling moments, a pattern the researchers interpret as a possible sign that infants are learning to moderate their own excitement. The team also found developmental change: 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 with 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 the research for her dissertation at the University of Amsterdam, and Cristina Colonnesi, an associate professor at the University of Amsterdam. Another first author, Alan B. Cobo-Lewis, completed his postdoctoral fellowship and undergraduate studies at the University of Miami and is now an associate professor at the University of Maine.

Perhaps the most socially revealing finding came from the comparison across partners. At both ages, babies were most likely to return their mother’s smiles, compared with smiling interactions involving their fathers or an unknown adult. The adults who were strangers actually initiated and sustained smiles more often than the mothers did, yet the infants responded preferentially to their mothers. “This was, importantly, the first paper that involved not only moms and dads, but also strangers,” Messinger noted. “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.” The finding underscores that infant smiling is not a reflexive response to any friendly face, but a selectively tuned behavior shaped by the specific relationship between the baby and the person in front of them.

Together, the two studies sketch a picture of early emotional development as a genuinely interactive process. The heart-rate study shows that mutual positive engagement, sustained over time, is associated with increases in a physiological indicator linked to emotion regulation, while the smiling study shows that these exchanges become more fluid, reciprocal, and relationship-specific as infants grow from 4 to 8 months of age. For parents, the practical message is encouraging rather than demanding: ordinary moments of shared laughter and smiling are not idle pleasures but may be part of the biological work of early development. For scientists, the results open a window onto how social experience and physiology intertwine in the first months of life, suggesting that the foundations of emotional well-being are built not in solitude, but in the back-and-forth of a shared smile.

Subject of Research: The role of shared positive caregiver-infant interactions in infants' physiological and emotional development

Article Title: Shared smiles may shape infants’ emotional development

Article References: Shared smiles may shape infants’ emotional development. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: infant development, emotion regulation, heart-rate variability, respiratory sinus arrhythmia, mother-infant interaction, smiling, caregivers, developmental psychology, physiology, social interaction, University of Miami, Philosophical Transactions of the Royal Society B

The research, published in Physiological Reports, focused on high-frequency heart rate variability, or HF-HRV, a measure derived from electrocardiogram recordings that reflects beat-to-beat fluctuations in heart rhythm within a specific frequency band. HF-HRV is closely associated with respiratory sinus arrhythmia, the natural speeding and slowing of the heart with breathing, which in turn is linked to parasympathetic, or vagal, nervous system activity. Because this branch of the autonomic nervous system is thought to support calmness, social engagement, and emotional regulation, HF-HRV has become a popular window onto the physiological states of caregivers and children during interaction.

Previous studies of physiological synchrony have typically averaged heart rate variability over fixed time windows and compared those averages between partners. Such approaches can detect gross similarity but may miss a crucial feature: the possibility that one person’s physiological rhythm precedes or follows the other’s by a meaningful interval. Eye contact, vocal exchanges, and physical touch between parents and children can change within seconds, and the physiological consequences of those moments may ripple forward or backward in time. The Japanese team, led by Aoi Sakurai, set out to build and test a method that explicitly accounts for these temporal shifts.

The study recruited 15 mother-infant dyads with healthy infants aged five to seven months, recruited between December 2022 and March 2023 through local childcare newspapers and social media in Sendai City. Each pair was scheduled for two recording sessions roughly two weeks apart. Mothers were asked to feed and change their infants beforehand to ensure calm, and then both wore small wireless ECG sensors, positioned at the upper sternum for mothers and near the clavicular region for infants, while spending five minutes in unrestricted free play on a yoga mat, talking, touching, and playing as they naturally would.

Turning raw infant ECG into usable data proved to be the study’s greatest technical hurdle. Of the 30 recorded sessions, 14 were excluded because of device failures, session interruptions, or electrode detachment and movement artifacts, leaving 16 analyzable sessions from 10 unique dyads. The researchers processed the surviving signals with a custom MATLAB pipeline that combined automated QRS detection with careful visual inspection and manual correction, flagging suspicious beats as missing values rather than interpolating them. Artifact rates in the retained data were remarkably low, at roughly 0.29 percent for mothers and 0.02 percent for infants, a testament to the labor-intensive cleaning process.

The core of the analysis was a lag-sensitive cross-correlation technique. The team computed HF-HRV power in 30-second windows advanced in half-second steps, using frequency bands of 0.15 to 0.40 hertz for mothers and 0.24 to 1.04 hertz for infants, reflecting developmental differences in breathing rate. For each maternal reference window, the infant’s signal was shifted across a search range spanning minus 30 to plus 30 seconds, and the correlation between the two traces was calculated at every offset. When a positive correlation coefficient of 0.8 or higher emerged as a local peak, that moment was marked as a synchronization point, classified as Lag-plus if the infant’s pattern aligned after a positive offset, Lag-minus if the mother’s pattern aligned after a negative offset, or Lag-zero for exact alignment.

The results were striking. Highly correlated intervals accounted for a mean of 74.5 percent of the total analyzed interaction time, a figure well above the variable or lower synchrony levels often reported in previous mother-infant studies. More intriguing still was the distribution of lags: within the highly correlated time, Lag-minus intervals made up about 54.4 percent and Lag-plus intervals about 44.6 percent, while exact zero-lag alignment accounted for a mere 1.1 percent. In other words, under this analytical definition, mother-infant heart rhythm synchrony was almost always offset in time, and the direction of the offset varied both across sessions and within them, with some dyads showing a bias toward one direction over limited periods.

Because high correlations between two slowly fluctuating biological signals can arise by chance from autocorrelation and shared measurement conditions, the team also compared their observations against a null model. For each session, they generated 100 surrogate datasets by randomly permuting the order of 30-second segments within the maternal and infant time series, destroying the original correspondence while preserving local structure. Observed synchrony exceeded the session-specific surrogate distribution in 13 of the 16 sessions, with a mean observed-minus-surrogate difference of about 0.205, and the positive difference held across all 10 dyads in a sensitivity analysis. This suggests that the alignment was more than coincidental temporal structure, though the authors are careful to frame the surrogate test as exploratory, since it relied on a single null model and a modest number of iterations.

The theoretical backdrop is Polyvagal Theory, which positions the vagal system as central to physiological co-regulation in close relationships. The findings are consistent with the idea that caregivers and infants mutually regulate one another’s autonomic states, and they echo earlier reports of heart rhythm synchrony between pregnant women and their fetuses, between marital partners, and even between dogs and their owners. Yet the authors caution against overinterpretation. The lag categories should not be read as evidence that one partner behaviorally leads or regulates the other, and the study measured HF-HRV without recording respiration, so the results reflect temporal physiological coordination rather than respiration-controlled estimates of vagal activity.

The limitations are candidly acknowledged. The sample was small and limited to mothers and five- to seven-month-old infants, with no fathers, no baseline or still-face conditions, and no non-partner dyad pairings to rule out shared environment or common movement. No synchronized behavioral coding was performed, so the physiological alignment cannot yet be tied to specific events like eye contact, vocalization, or touch. The predefined plus-or-minus 30-second lag range may itself have shaped the findings. Still, the study demonstrates the feasibility of a lag-sensitive approach that could, if replicated in larger and more diverse samples, eventually help researchers probe whether physiological synchrony differs in populations facing psychosocial risk, such as mothers with postpartum depression, offering an objective, minimally invasive complement to the behavioral observations that have long dominated attachment research.

Subject of Research: Lag-sensitive analysis of mother-infant heart rate variability synchrony during free interaction

Article Title: Lag‐sensitive cross‐correlation analysis of mother–infant HF‐HRV synchrony during free interaction: An exploratory feasibility study

Article References: Sakurai, A., Koide, K., Kasahara, Y., Nakamura, Y., & Yoshizawa, T. (2026). Lag‐sensitive cross‐correlation analysis of mother–infant HF ‐ HRV synchrony during free interaction: An exploratory feasibility study. Physiological Reports, 14(18), Article e71116. https://doi.org/10.14814/phy2.71116

Image Credits: AI Generated

DOI: 10.14814/phy2.71116

Keywords: heart rate variability, mother-infant synchrony, HF-HRV, cross-correlation, autonomic nervous system, respiratory sinus arrhythmia, polyvagal theory, attachment, physiological co-regulation, surrogate data, free play, infant development

Restraint, as the researchers define it, is any practice that limits an infant’s ability to move or change posture freely. That includes caregiver restraint, such as being held or carried, and device restraint, such as sitting in a high chair, stroller, swing, or car seat. The definition deliberately avoids a fixed list of equipment. Instead, it focuses on the infant’s real-time opportunities for movement, which means an infant strapped into a high chair counts as restrained, while an infant placed in a large crib where they can crawl and reposition freely does not. This broad framing matters because restraint is not simply a safety measure or a convenience for caregivers. It fundamentally shapes what infants can do, see, hear, and touch. Previous work has linked restraint to reduced leg movement and diminished spinal and hip muscle activity, and to fewer opportunities for object interaction and reduced exposure to adult speech. At the same time, restraint can be beneficial: supported sitting frees a baby’s hands for exploring objects, and being carried places infants higher off the ground, giving them richer views of faces and distant surroundings.

Measuring this everyday phenomenon has always been the bottleneck. Parent surveys, whether retrospective questionnaires or ecological momentary assessments delivered by phone several times a day, depend on memory or on sparse sampling, and neither can produce a continuous record of the day. Video recording solves the continuity problem in principle, but in practice it is cumbersome. Stationary cameras lose sight of infants behind furniture, handheld cameras can alter behavior, and the annotation process is so labor-intensive that prior studies typically captured only half an hour to an hour of footage, often biased toward periods when infants were awake and active. The periods most relevant to restraint research, such as a baby seated in front of a screen or parked in a seat during mealtime, were precisely the ones most likely to be skipped. What was needed was a method that could run all day, unobtrusively, without asking anything of the caregiver.

The solution the team built rests on inertial measurement units, miniature sensors that record linear acceleration from accelerometers and angular velocity from gyroscopes. In this study, four lightweight MC10 Biostamp sensors were secured in a customized infant garment at the right thigh, left thigh, right ankle, and left ankle. Each sensor sampled motion at 62.5 hertz, a rate fine enough to capture the subtle signatures of infant movement, and the battery lasted more than a full day. The dataset drew on 146 home-visit sessions from 66 infants, split into a younger cohort of 30 babies aged 4 to 7 months and an older cohort of 36 babies aged 11 to 14 months. During each visit, a GoPro camera recorded the first 1.5 hours of the session, and trained coders labeled every stretch of that footage as restrained, unrestrained, or missing when the infant left the frame. Two independent coders achieved 98.79 percent agreement, providing a trustworthy ground truth against which the machine learning model could be judged.

The modeling pipeline is a careful exercise in feature engineering. Raw motion data were aggregated within sliding windows, and within each window the researchers computed a rich set of statistics for every signal from every sensor: means, medians, minima, maxima, percentiles, skewness, kurtosis, standard deviations, and sums, along with cross-sensor and cross-orientation correlations and pairwise differences. In total, 436 features described each window. A random forest classifier, trained with leave-one-session-out cross-validation, then mapped those features onto the binary restrained or unrestrained label. Crucially, the team varied the window length, testing windows of 4, 16, and 30 seconds, because the choice of temporal scale involves a genuine trade-off. Short windows offer fine temporal resolution but may lack enough context to distinguish, say, a baby kicking freely on the floor from a baby kicking while strapped into a seat. Longer windows capture more context but risk blurring abrupt transitions.

All three window lengths performed well, with accuracies between 87 and 89 percent and kappa values between 0.70 and 0.73, but the 30-second window showed the most favorable pattern. The model carried a slight bias toward overestimating unrestrained time, likely because unrestrained periods were more common in the training data, and the longer window shrank that bias: the difference between predicted and human-annotated restraint prevalence fell from 4.70 percent with 4-second windows to 1.70 percent with 30-second windows. The authors offer an intuitive explanation for why longer windows help. Infants may look motionally similar whether restrained or not while they sit still, but the telltale differences emerge when they move: a caregiver’s lap or a seat absorbs and damps motion in ways that a bare floor does not. Those damping signatures appear only in movement episodes, and a 30-second window is far more likely to contain one. Longer windows also generalize better across the bewildering variety of restraint forms, from stationary seats to swings to being carried around the house. The practical lesson for the field is that window length should be matched to the natural duration of the behavior being detected, and restrained bouts, which typically last minutes rather than seconds, call for longer windows than the 2-to-4-second windows used in earlier body-posture classifiers.

When the final 30-second model was applied to full-day recordings, it produced estimates that converged with prior survey-based findings. Younger infants aged 4 to 7 months spent an average of 55.38 percent of their awake time restrained, while older infants aged 11 to 14 months spent 33.42 percent, a statistically significant age-related decline that mirrors earlier reports. Individual variability was striking, with some infants restrained far more than others even within the same age group. The average awake wearing time in the 133 analyzable full-day sessions was about 6.18 hours, after excluding naps and periods when the garment was removed, which caregivers logged on paper forms. The authors acknowledge that reliance on parent logs is a limitation, since caregivers may forget or misrecord these intervals, and they suggest future work could cross-validate wearing time using automated sleep detection from audio recordings or sensor-based movement detection.

The study is candid about the boundaries of its model. Machine learning classifiers are constrained by their training data, so strong performance on this dataset does not guarantee comparable accuracy on new populations, new sensor brands, or unfamiliar forms of restraint. The authors point to culturally distinctive practices such as the Tajik gahvora cradle, Tseltal slings, and the use of sandbags or heavy blankets elsewhere, all of which could produce hip and ankle movement patterns absent from the current training set. Applying the model in such contexts will require fresh ground-truth coding and possible re-tuning. There is also the deeper issue that detecting when restraint happens does not reveal what the experience is like. An infant in a high chair during animated face-to-face feeding lives a very different moment from an infant in the same chair while a caregiver washes dishes, and interpreting restraint’s developmental consequences will require pairing sensor data with audio, proximity tracking, and other contextual streams.

Even with those caveats, the implications are considerable. A validated, automated system for quantifying restraint opens questions that were previously unanswerable: what immediately precedes a caregiver’s decision to pick up or put down a baby, how restraint bouts are distributed across the day, whether prolonged individual bouts matter differently from the same total time fragmented into short ones, and how speech input, object interaction, and visual experience differ between restrained and unrestrained periods. The authors note that Tajik and US infants can show comparable overall restraint time yet different motor outcomes, suggesting that the temporal texture of restraint, not just its total, may be developmentally decisive. The team also envisions extending the approach to locomotion, steps, and falls, and to the underexplored outdoor environments where strollers, carriers, and car seats dominate. Sensor data, video annotations, and analysis code are publicly shared, lowering the barrier for other labs to build on the work. What began as a simple parental intuition, that babies spend much of their day constrained, can now be measured with second-by-second precision, and that measurement may reshape how developmental science understands the architecture of a baby’s everyday learning environment.

Subject of Research: Machine learning detection of infants' daily restraint from wearable inertial sensors

Article Title: Quantifying infants’ everyday restrained experiences in the home using wearable inertial sensors

Article References: Quantifying infants’ everyday restrained experiences in the home using wearable inertial sensors. (n.d.). https://doi.org/10.3758/s13428-026-03185-9

Image Credits: AI Generated

DOI: 10.3758/s13428-026-03185-9

Keywords: infant development, wearable sensors, machine learning, physical restraint, motor development, inertial measurement units, home observation, random forest, caregiving, Behavior Research Methods, developmental psychology, motion classification

The editorial team behind the issue argues that the field has spent decades focusing heavily on emotion recognition: how accurately observers can identify what someone else is feeling from a face, a voice or a posture. The new collection deliberately shifts the lens inward, toward the person doing the expressing. Emotional expressions, the editors propose, should be understood in terms of their origins and development, their continuity across animal species, and the practical roles they play in behaviour and social life. In this framing, emotions become a visible inner world, a living link between brain, body and environment, rather than a private mental state that occasionally leaks to the surface.

Among the most striking contributions is a study co-conducted by Cristina Colonnesi of the University of Amsterdam, which examines how infants in their first year of life actively construct positive moments with the people around them. The researchers observed that eight-month-old babies, markedly more than four-month-olds, initiate episodes of shared positive emotion themselves. A typical sequence involves a baby smiling and then drawing an adult into mutual laughter, creating a loop of shared enjoyment that the infant, not the caregiver, set in motion. Rather than primarily reacting to another person’s smile, older babies increasingly deploy their own positive expressions as social tools, using them to start and sustain warm, enjoyable interactions.

That developmental shift may look subtle, but its implications are considerable. The ability to initiate, rather than merely respond to, shared positive emotion suggests growing flexibility in how infants use their emotional behaviour in social contexts. It hints that the foundations of humour, joint attention and even the social bonding that characterises later friendship may be laid in the first year of life, not through language or instruction, but through the infant’s own expressive behaviour. Shared laughter between a baby and an adult is, in this view, not a charming by-product of caregiving but a developmental achievement produced by the infant’s emerging expressive control.

A second line of work, presented by Milica Nikolić and colleagues, turns to self-conscious emotions: shame, guilt, shyness and pride. These emotions are distinctive because they arise when children begin to view themselves through the eyes of others, and the research shows that they are already visible in a child’s facial expressions, body posture and gaze during the first few years of life. Crucially, they prove to be far more than internal feelings. The studies reviewed in the issue indicate that these emotions have characteristic outward signatures and, more importantly, predictable behavioural consequences that begin shaping a child’s social world very early.

The consequences differ sharply depending on the emotion. When children feel guilty about something that has gone wrong, they are subsequently more likely to try to make amends, whether by helping others, sharing resources or repairing damage they have caused. Shame produces the opposite pattern: children who feel ashamed are more likely to withdraw, avoid eye contact and steer clear of the situations that triggered the feeling. Nikolić and her colleagues are careful to note that the evidence base is still limited, but they argue that these self-conscious emotions likely play an early role in shaping social behaviour and reputation, functioning as internal regulators that push children toward repair in one case and retreat in the other.

The third major contribution, from Disa Sauter and her team, dismantles one of the most persistent assumptions in popular psychology: that feeling good means smiling. In a study covering twenty-two different positive emotions, grounded in participants’ own personal experiences, the researchers mapped which facial muscles and movements are activated for each emotion. The results reveal that many positive feelings carry their own distinct facial profiles that involve no smile at all. Interest and determination are prime examples; in the case of determination, the defining feature is a raised chin rather than any movement of the mouth. Other positive emotions, such as amusement and elation, do involve distinct smiles, characterised by upturned corners of the mouth and often accompanied by closed eyes and strongly raised cheeks.

The technical significance of this work lies in its demonstration that positive facial behaviour is a heterogeneous family of expressions rather than a single gradient of smile intensity. Feeling good, the researchers show, manifests in a variety of facial configurations, including spontaneous ones, and the classic smile is only one member of that family. For fields ranging from affective computing to clinical screening, which often rely on smile detection as a proxy for positive affect, the findings suggest that such proxies may systematically miss positive states such as interest or determination, and may misread them when they appear without any mouth movement at all.

A fourth study, authored by postdoc Christopher Riddell, Milica Nikolić and colleagues, extends the comparative dimension of the issue by examining mimicry of the play face, an exaggerated, playful expression resembling a smile that typically appears during rough-and-tumble play. The researchers compared infants, adults and orangutans. Both human infants and adults were more likely to produce a play face themselves after observing one in another person than after seeing a neutral expression, and their own expressions mirrored what they saw even without any direct interaction with the person being mirrored. Orangutans, by contrast, rarely showed this behaviour. For them, play faces arise primarily during direct social play, whereas in humans contagious smiling can break free of the play context altogether and spread through observation alone.

Taken together, the nineteen articles of the special issue, published under the title Mechanisms, development, phylogeny and functions of emotional expressions, make a coherent case that emotional expressions are central to development from the very start, shaping both individual behaviour and the formation and regulation of social relationships, from a baby’s first self-initiated smile to the complex body language of shame and pride. In their editorial, Nikolić, Kret, Nummenmaa and Sauter call on the research community to study emotions more within real-life contexts, attending to how expressions emerge, develop and function for both the expresser and the people around them. If the collection is right, the smile that lights up a living room, the guilty glance after a broken vase and the raised chin of a determined child are not merely decorations on an inner life. They are the mechanisms through which that inner life reaches out and builds the social world.

Subject of Research: The development, evolution and social functions of emotional expressions in humans and other animals

Article Title: From a baby's smile to a monkey's grin: how emotions shape our behaviour

Article References: From a baby's smile to a monkey's grin: how emotions shape our behaviour. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: emotional expressions, infant development, smiling, self-conscious emotions, shame, guilt, facial expressions, play face, orangutans, mimicry, social behaviour, developmental psychology

The centerpiece of the work is a tightly controlled feeding experiment. Participants consumed an identical standardized carbohydrate meal on separate occasions, once alone and once in the company of a person to whom they felt close. Continuous glucose monitoring revealed that the same food produced a different metabolic story depending on the social context. When participants ate in companionship, their post-meal glucose excursions were smaller, meaning the spike in blood sugar after eating was blunted, and their glucose levels returned to baseline more quickly. In physiological terms, the body needed to deploy less of its regulatory machinery to keep a potentially destabilizing nutrient load under control. Because repeated large glucose swings are implicated in the long arc of metabolic disease, the finding has immediate relevance for one of the most pressing public health challenges of the era.

Crucially, the researchers did not treat this single result as an isolated curiosity. Across a series of preregistered experiments, they tested whether the same proximity effect would appear in other homeostatic domains. It did. When adults were exposed to cold, those who were near a close social partner maintained their temperature regulation more effectively than those tested alone. The pattern also emerged in the earliest stages of life: infants, whose survival depends entirely on the bodies of their caregivers, showed more efficient stress regulation when held or kept in close contact. The consistency of the effect across glucose control, thermoregulation, and stress responsivity is what elevates the work from an interesting observation to a candidate general principle, one that cuts across organ systems that are usually studied in isolation from one another.

Two features of the findings are particularly striking from a technical standpoint. First, the benefits of companionship appeared even without physical touch. Mere proximity to a loved individual was sufficient, suggesting that the relevant signal is not tactile contact or shared heat but something subtler, likely a learned association in which the presence of a specific person predicts safety and reduced physiological demand. Second, the researchers found that the effect was not explained by reductions in subjective stress. This is a critical distinction, because a simpler interpretation of the data would be that companions merely feel calmer, and that calmness secondarily improves metabolic readings. The evidence points instead to a direct regulatory benefit, as if the body recalibrates its operating costs downward when it detects a reliable ally nearby, independent of how stressed the person reports feeling.

Prof. Atzil framed the discovery as a challenge to one of the deepest assumptions in biomedical science. We used to think of the body as an isolated system that regulates itself, she noted, but the findings show that human physiology is inherently social. In her account, being close to someone you love lowers the physical energy the body spends on self-regulation, producing a direct metabolic dividend for togetherness. This framing carries an evolutionary implication that the researchers make explicit: if bodies genuinely run better in company, then social bonding may not be merely a strategy for protection, cooperation, or reproduction layered on top of individual physiology. It may be woven into the very economics of homeostasis, giving organisms a concrete energetic reason to form and maintain attachments in the first place. Human bonds, on this view, are partly metabolic instruments.

The mechanistic story that emerges is one of energetic accounting. Keeping internal conditions stable, a process biologists call homeostasis, is not free. Every correction the body makes, secreting insulin to clear glucose from the bloodstream, shivering or redirecting circulation to defend core temperature, releasing and then dampening stress hormones, consumes energy and imposes wear on tissues. If the nervous system can verify, through continuous monitoring of the social environment, that a protective partner is present, the allostatic load, the cumulative cost of staying ready for threat, can be lowered. The parasympathetic branch of the autonomic nervous system, which favors restorative states, can gain greater influence, and predictive circuits in the brain can downshift their threat forecasts. The result is a body that achieves the same regulatory targets with less expenditure, which is precisely the signature the experiments detected.

The developmental dimension of the work deserves particular emphasis. Infants cannot regulate their own temperature, glucose, or stress responses competently on their own; for most of human evolutionary history, an infant separated from caregivers faced rapid physiological collapse. The finding that infant stress regulation is more efficient in close proximity is therefore unsurprising in outline, but it gains new meaning when read alongside the adult data. It suggests that the social modulation of physiology does not disappear with maturity, as many researchers assumed, but persists throughout life as a quiet background process. The adult eating lunch with a friend and the newborn cradled against a parent may be running variations on the same ancient regulatory program, one in which the presence of a bonded other is counted as an external resource that the body can draw upon to lighten its internal workload.

The public health implications arrive at a moment when dysregulated blood sugar has become a global concern. Rates of type 2 diabetes and prediabetes continue to climb across income levels and continents, and standard advice centers on diet composition, portion control, physical activity, and medication. This study does not overturn any of that guidance, and the researchers are careful to state that social closeness is not a substitute for medical care, nutrition, or exercise. But it does introduce a variable that almost no clinical guideline currently addresses: the identity of the people sitting at the table. If identical meals produce meaningfully different glycemic responses depending on companionship, then epidemiological models of metabolic health that ignore relational context may be systematically missing part of the picture. Blood sugar regulation, as Atzil puts it, is not just about what we eat but who we eat with, and relationships may hold an active biological role in physical wellbeing.

The research design strengthens the credibility of these claims in ways worth noting for readers weighing how much to trust a single headline. The experiments were preregistered, meaning the researchers committed to their hypotheses and analysis plans before collecting data, a safeguard against the selective reporting that has troubled social science in the past. The meal challenge was standardized, removing the obvious confound that shared meals might simply involve different foods. The replication of the effect across distinct physiological domains, and across age groups from infancy to adulthood, makes it unlikely that the result is an artifact of one particular task or population. At the same time, open questions remain. The precise neural and hormonal pathways that translate the perception of a companion into altered insulin dynamics or thermal efficiency have not yet been fully mapped, and future work will need to identify whether oxytocinergic signaling, vagal tone, or other mechanisms carry the signal.

What the study ultimately offers is a revised map of what a human body is. For more than a century, physiology has been practiced largely as the science of the sealed individual, a self-contained machine measured in isolation within laboratory walls. The concept of Social Physiology proposes instead that the boundary of the regulating system extends beyond the skin, into the network of trusted others whose presence the nervous system tracks at all times. Human connection, long celebrated in poetry and studied in psychology as a source of meaning and happiness, now appears in the laboratory as something harder-edged: a measurable reduction in the biological cost of staying alive. The people beside us, the researchers conclude, constitute a powerful and overlooked factor in health, one that operates not through inspiration or encouragement but through the quiet, continuous recalibration of glucose curves, thermal defenses, and stress responses. Relationships, it turns out, do not merely support our emotional lives. They help run the machinery of the body itself.

Subject of Research: The effect of social proximity on human glucose, temperature, and stress regulation

Article Title: Eating with a loved one lowers your blood sugar

Article References: Eating with a loved one lowers your blood sugar. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: social physiology, blood glucose, homeostasis, stress regulation, thermoregulation, metabolic health, Science Advances, Hebrew University of Jerusalem, social bonding, preregistered experiments, infant development, public health