Infancy

Although physical aggression emerges in infancy, concurrent with motor development, its possible connection remains poorly understood. Previous research has documented age-related changes in aggression but has not examined how fundamental motor transitions relate to these changes. This investigation examined associations between two key motor milestones (crawling and walking) and aggressive behavior patterns in early childhood. Mothers (N = 377) of infants aged 4-15 months reported on their children's locomotor abilities, aggressive behaviors (directed and exploratory), defiance, and temper loss. Results showed distinct developmental trajectories for different forms of aggression. The prevalence of directed aggression (e.g., hitting, kicking) increased linearly with age, and exploratory aggression (e.g., biting, hair-pulling) followed a curvilinear pattern, plateauing around 9 months of age before declining. Frequency patterns also differed. Exploratory aggression frequency declined linearly with age and directed aggression frequency decreased initially before trending upward around 11 months. The transition to independent walking, but not crawling, was associated with increased frequency of both types of aggression. These effects were mediated through increases in both defiance and temper loss; the indirect effect through temper loss was significantly stronger for directed than exploratory aggression in single-mediator models. These findings suggest that the transition to walking may serve as a reorganizing force in early aggressive behavior in infancy by influencing emerging behavioral opposition and negative emotional expression. Results contribute to a small but growing literature on the developmental systems that underlie aggression in infancy.

Musical engagement is a universal trait of human behavior and a foundational form of embodied communication. While this engagement is deeply rooted in bodily experiences, much of the existing research has focused on perceptual and neural mechanisms, with comparatively little attention paid to how the body's physical affordances, including the differing biomechanical properties of distinct effectors, shape early rhythmic engagement with music. The current study aims to characterize how rhythmic motoric responses to music vary in the context of an infant's developing physical skills and attributes. Pre-ambulatory infants were placed in an unweighting paradigm to vary levels of postural support as duration and rate of rhythmic movement were measured across core and distal effectors. Results indicated that greater postural support resulted in increased durations and quicker rates of rhythmic movement and enabled qualities of bounce and rebound within infants' movement repertoire, an effect driven primarily by core effectors. Rate of rhythmic movement was found to be organized primarily by effector type, with core movements consistently slower than distal movements across all conditions. Distinct trends between core and distal effectors were found, with vertically displaced core movements consistently slower in rate and increased in duration with age, while distal movements were faster in rate and showed no significant change in duration with age. These patterns align with principles of biomechanics and mass distribution, reflecting a hierarchical organization of rhythmic movement across the body that parallels patterns observed in adults. Findings suggest that postural support facilitates the emergence of more fully embodied musical engagement, offering insight into the sensorimotor foundations of early rhythmic movement.

Evidence of disturbances in communication skills among children born during the pandemic remains limited to outcomes within the first year of life and rarely accounts for key predictors. This study investigated the longitudinal association between COVID-19 stressors and communication skills during the first 1000 days of life, and examined whether parental depressive symptoms, behaviors, and cognitions mediated this association. 2664 French-speaking children (48.2% boys) born in Québec (Canada) between October 2020 and September 2021, were included in this observational prospective cohort study. Child communication skills were measured at 29 months using the Ages & Stages Questionnaires. COVID stressors (financial insecurity, social isolation, overall stress) were reported by both mothers and fathers at 5 months postpartum. Depressive symptoms were measured at 5 months postpartum using the Center for Epidemiologic Studies Depression Scale Revised (CESD-10). Parenting behaviors and cognitions (self-efficacy, perceived parental impact on the child, hostile-reactive behaviors, overprotection) were reported at 17 months postpartum using the Parental Cognitions and Conduct Toward the Infant Scale. COVID stressors were associated with increased parental depressive symptoms. Depressive symptoms significantly predicted maternal and paternal lack of perceived impact on child development and paternal overprotection, which, in turn, was negatively associated with child communication skills. Findings indicate significant indirect associations between COVID-19 stressors and child communication skills. This study suggests potential clinical implications by highlighting modifiable parental cognitions and behaviors as possible targets for intervention.

Turn-taking, the coordinated exchange of communicative signals between infants and caregivers, is vital for child communicative development. While it is considered a human universal, most research has focused on mother-infant dyads. Given fathers' increasing caregiving involvement, understanding their contribution to early communication is essential. Thus, we have examined early infant-father turn-taking and the role of fathers' mentalizing abilities, which may support turn prediction, and paternal involvement, as it may shape the frequency and quality of exchanges. We observed 39 father-infant dyads in natural interactions at 7 weeks and 6 months postpartum. Vocalizations were micro-coded and turn-taking was defined as a vocal response by one partner within 3000 ms of the other's vocalization. Mentalizing was measured via the Reflective Functioning Scale of the Parent Development Interview, while involvement was indexed by weeks of paternity leave. Results showed little change in turn-taking over time, except for a slight decline in infant turns and increase in paternal overlaps. Mixed effect models revealed significant between-dyad variability; higher mentalizing was associated with fewer paternal overlaps, and greater involvement was linked to shorter overlap durations. Findings suggest that infants are active turn-takers from early on and paternal mentalizing may support attuned coordination. Despite a relatively homogenous sample, results underscore fathers' importance in early communicative development.

There is some evidence suggesting that bilingual children differ from monolingual children in cognitive processing due to their exposure to a more complex environment. While several studies have supported this hypothesis, most research has focused on school-age children. The present study examined whether differences in inhibitory control between monolinguals and bilinguals are present early in life with the Early Childhood Inhibitory Touchscreen Task (ECITT). This task provides a reliable measure of inhibition control in infants, requiring them to shift their responses from a previously rewarded location to a new one. It minimizes language and memory demands while engaging infants through animations, making it an ideal measure for very young children. A sample of 60 infants (mean age 20.21 months) was tested, with language exposure measured on a continuum using the Language Exposure Assessment Tool (LEAT). Vocabulary was assessed using the McArthur-Bates Communicative Development Inventory (MCDI), and inhibitory control was assessed, as well as a range of executive functions, with the Early Executive Functions Questionnaire (EEFQ). The ECITT data replicate previous research, showing that overall performance on this task improves with age. However, inhibitory control skills did not vary as a function of the amount of second language exposure. The current results suggest that inhibitory control skills do not differ as a function of language exposure diversity in toddlers younger than 24 months.

This study reveals a new social interactional framework-joint agency-that permits us to better understand the early development of agency in infants being raised in culturally distinct socialization contexts. Sixteen caregiver-infant pairs (1; 0-2; 6 months) were observed in naturalistic settings within two Argentine populations-Indigenous Wichi and urban Eurodescendant. Guided in part by a Wichi category, husek ihi, the study adopts a dual analytic approach that balances cross-cultural comparability with sensitivity to interactional ecologies. The analysis proceeded in three sequential stages: (1) the development of an observational coding system to identify joint agency episodes (JAEs), (2) the characterization of these JAEs along social, semiotic, and material dimensions, and (3) the examination of the semiotic modalities through which joint agency unfolds in interaction. The evidence revealed that all infants engaged in JAEs, but that their organization differed as a function of cultural community. Among the Wichi, JAEs typically involved multiparty participation, non-verbally mediated, and engaged embodied modalities centering on available materials. In contrast, in the Eurodescendant community, JAEs were typically dyadic, verbally mediated, and centered on domestic toys. These findings offer a multimodal, culturally situated account of early agency, challenging models that treat agency primarily as the attribution of internal mental states.

Adversities in early life compromise children's nurturing care and impact their development. This study explored the biopsychosocial conditions associated with electroencephalography (EEG)-based brain activity in children living in rural Bangladesh, a setting with high rates of malnutrition and suboptimal psychosocial stimulation. In a substudy of the Benefits and Risks of Iron Supplementation in Children (BRISC) study in Bangladesh, we recorded resting EEG brain activity in a random subsample of 440 children at 11 months and 594 children at 20 months of age. EEG band power measures were derived for delta, theta, alpha, and beta power bands. Validated tools were used to assess maternal depression, household food insecurity, and psychosocial stimulation. Children's anthropometry, venous hemoglobin and ferritin concentrations, and inflammation were also measured. Linear regression analyses were conducted to identify the concurrent and lagged biopsychosocial conditions associated with EEG-based brain activity. Growth, nutritional status, psychosocial stimulation, and household food security were associated with concurrent and lagged brain activity. The direction of association differed by EEG frequency band, but the majority of biopsychosocial conditions were positively associated with activity in the higher frequency bands and negatively with lower frequency bands. Our findings indicate that brain activity in children can be influenced by a range of biopsychosocial adversities. Children's nutrition, quality and quantity of psychosocial stimulation, and food security influence their current and later brain activity in this resource-limited setting in South Asia. TRIAL REGISTRATION: ACTRN12617000660381.

This study examined the relationship among early attention skills in infants and measures of dyadic interaction during caregiver-infant play, including interpersonal synchrony and turn-taking behavior. Thirty-five caregivers and their one-year-old infants (mean age = 15.34 months) completed a 5-minute free play session and sessions were coded for communication behaviors (eye contact, vocalizations, and gestures), coordinated attention, and infant visual attention states (focused, unfocused, and casual attention). Coordinated attention with a caregiver was not related to caregiver-infant synchrony and turn-taking. Mean duration of casual attention (i.e., passive, exploratory attention toward an object or person), but not focused attention, was negatively associated with time spent in bouts of turn-taking. Results suggest that shorter bouts of casual attention during play may support longer periods of turn-taking with caregivers, an important social communication skill for infants as intentional communication continues to develop in the second year of life.

Early temperament, particularly regulation and reactivity, is critical to neurodevelopment and can be shaped by early biological and environmental factors. Prenatal substance exposure is one such influencing factor, often associated with lower regulation and heightened reactivity in infants. However, positive parenting may help mitigate these effects. This study examined how prenatal exposure to alcohol, tobacco, and marijuana influences infant temperament at 24 months, and whether maternal warmth at 12- and 24-months moderates or mediates these relationships. Participants were 404 mother-child dyads from a prospective, ethnically diverse urban cohort. Results showed prenatal alcohol and tobacco exposure was directly associated with poorer child regulation and reactivity, while marijuana was not independently linked to these outcomes. Maternal warmth at 12 and 24 months moderated some of the negative effects of prenatal tobacco and marijuana exposure, but not alcohol. These moderating effects varied by child outcome, timing, and frequency of substance use. No evidence was found for maternal warmth mediating any of the associations between prenatal substance use and child outcomes. Findings highlight the differential impact of substance type, timing, and frequency on child outcomes and underscore the importance of early parenting support as a resilience factor.

Object exploration presents infants and young children with valuable opportunities for learning across domains. What factors motivate and constrain exploratory behavior? In the present studies, we highlight the infant's contributions to object exploration by testing links between parent-reported temperament and observed exploratory behaviors. In two samples, Study 1: 24-month-olds (N = 35, 51% female; 88% White; 95% non-Hispanic) and Study 2: 6-month-olds (N = 96, 50% female; 82% White; 89% non-Hispanic), we assessed object exploration via coding of child behavior during two different lab-based free play tasks. Across both samples, infants rated higher in temperamental surgency engaged in a more rapid pace of object exploration (i.e., moving quickly between objects). Additionally, cluster analyses indicated that infants characterized by high activity level, high positive affect, and high negative affect spent more time engaged in fine motor exploration (i.e., fingering and transferring) compared to infants characterized by average activity level, high positive affect, and low negative affect. These findings indicate a link between temperament and exploratory behavior in infancy and toddlerhood.

Although negative emotion expression is common in the toddler period, the field of language development lacks understanding of how negative emotions are integrated within the parent-toddler communicative routines that support language learning. In a sample of 25 one-year-olds (Mage = 16.60 months; 92% White, 8% Biracial), drawing from naturalistic samples of parent-toddler communication characterized by high toddler distress, we identified three toddler vocalization types: (1) non-distressed verbal/preverbal vocalizations (VERB), (2) non-verbal distress vocalizations (CRY), and (3) distressed verbal/preverbal vocalizations (VERBCRY). We found that, on average, 32% of toddlers' distress vocalizations co-occurred with verbal/preverbal expression (i.e., VERBCRY). Older toddlers with more language skills more often integrated distress and verbal/preverbal expression. Parents were most responsive to toddlers' non-distressed verbal/preverbal vocalizations (VERB), and, unexpectedly, were least responsive to toddlers' distressed verbal/preverbal vocalizations (VERBCRY). Moreover, parents' responses to toddlers' distress cues often helped them shift towards more verbal or less distressed communication (i.e., from CRY to VERBCRY or from VERBCRY to VERB). The findings highlight the important role that negative emotions play in everyday parent-toddler communication and the need to understand language learning as a process that is embedded within the emotional fabric of everyday life.

This study examined whether findings from High-Income Countries (HIC) suggesting that (1) maternal postnatal depression (PND) is associated with poorer infant neurodevelopmental outcomes, (2) maternal sensitivity to non-distress (SND) is associated with improved outcomes, (3) SND may buffer the effect of PND, are present in India. Participants were from the Bangalore Child Health and Development Study (BCHADS), a prospective longitudinal cohort study starting in pregnancy (n = 337). Mothers self-reported depression symptoms on the Edinburgh Postnatal Depression Scale; variables reflecting early PND (8 weeks) and chronic PND (factor score created from 8 weeks, 6, 12, 24 months) were examined. SND was coded from a semi-structured play observation at age 6 or 12 months. Infant neurodevelopment was assessed at 24 months using the Bayley Scales of Infant Development. Early PND, chronic PND or maternal SND were not associated with infant cognitive or language development in the whole sample, but in boys increased maternal SND was associated with improved language development. SND did not moderate associations between maternal PND and outcomes. A number of factors, such as parity, birthweight by gestational age, and the presence of an alternate caregiver, demonstrated small but significant sex-specific associations with neurodevelopment in exploratory analyses. Maternal SND was associated with improved language outcomes after accounting for multiple risks, but only in boys. Future work should examine whether broader aspects of the nurturing environment contribute to neurodevelopment in a sex-specific manner.

In everyday perception, expected sounds are predicted by the movement of dynamic objects. Here, we asked if four-to five-month-old infants' understanding of physical events allows them to use motion to predict the timing of collision sounds. We investigated whether infant looking time differed when viewing collision events that resulted in amodal visual or multimodal audio-visual changes. In a moderated, online study, we showed two groups of infants a moving 2-D ball and collision events. In one group, we presented infants with alternating trials from two conditions that varied in the temporal congruence of the ball's motion and collision sounds: one in which sound occurred simultaneously upon collision between the ball and a barrier (audio-visual; AV synchronous), and one where sound came slightly before collision (AV asynchronous). Another group of infants viewed the ball changing color upon collision (visual color change; VCC synchronous), and one in which the color changed slightly before collision (VCC asynchronous). Offline behavioral coding of recorded looking time demonstrated that infants looked longer toward AV asynchronous trials than to AV synchronous ones. This pattern was not seen in the group of infants who viewed color changing collisions. We also observed that infants looked longer toward AV asynchronous trials compared to VCC asynchronous trials. These data demonstrate that young infants have expectations regarding the temporal alignment of collisions and sound, which may provide a foundation for physical inference and learning in natural sensory environments.

Infant development is a dynamic, cross-domain process. Infants acquire new skills, integrate them with well-established behaviors, and do so simultaneously. New skills can bolster development of existing behaviors or create moments of temporary instability for the developing system. How do multiple skills interact? To address this question, we mapped longitudinal change in infant vocalization against the backdrop of infants' evolving locomotor skills. We followed 22 infants across the 6 months surrounding the emergence of walking and examined changes in the production of vocalizations and in their coordination with locomotion during everyday activity at home. As a group, infants displayed positive linear growth in vocalization frequency, duration, and coordination with locomotion. However, a majority of individual infants showed evidence of a temporary decline in growth-a "dip"-in all three measures during the 2 months around walk onset. Moreover, although infants coordinated vocalizations with crawling (a well-established skill) across most of the observation period, above-chance coordination of vocalization with walking (a new skill) only emerged after 1 month of walking experience. Thus, learning to walk creates opportunities for infant communication, but is also a moment in development when progress slows, albeit temporarily.

During early vocabulary growth, infants use single words to express complete ideas. Requests ("milk") and protests ("no!") are fundamental ways that infants communicate their needs. However, understanding what an infant wants from a single word can be challenging. Caregivers likely rely on infants' gestures and actions to interpret single-word utterances. How common are requests and protests during everyday activities? To what extent do infants accompany their requests and protests with gestures (e.g., points to request) and/or actions (e.g., flailing arms to protest)? Are caregivers more likely to respond to such "multimodal" requests and protests than to infants' words alone (i.e., unimodal speech)? We observed 31 toddlers (12-24 months) with their mothers during everyday interactions at home. In aggregate, infants expressed 2580 single-word utterances, with nearly half being requests (35%) or protests (8%). Most requests and protests (84%) were multimodal, occurring in the context of actions (mostly) or gestures. Unexpectedly, although infants' gestures/actions often "aligned" with their words in meaning or intent (e.g., saying "no" while pushing a toy away), infants also displayed "misaligned" gestures/actions (e.g., saying "no" to one toy while reaching for another). In particular, infants' aligned' multimodal requests and protests were more likely to elicit a response from mothers than were their words alone. Findings underscore the embodied and embedded nature of early communication in the natural home setting: Infants express their intentions by coupling words with gestures/actions, and certain types of multimodal communications are more effective than others in fulfilling their goals.

Familiar songs are often more effective than unfamiliar songs at capturing infant attention, regulating emotion, and eliciting rhythmic movements, but how infants rely on lyrics and melody to recognize well-known music from their regular repertoire is not well understood. Here we investigate whether infants use these musical features selectively or interactively during song recognition across early development. Six- (n = 40), 11- (n = 47), and 20-month-old infants (n = 41) participated in an online preferential listening paradigm. We measured infants' attentive listening to a cappella recordings of Wheels on the Bus, an original composition, and two songs mismatched in melody and lyric familiarity (i.e., melody from Wheels paired with lyrics from the novel song, and vice versa). Confirmatory analyses showed that, while 6-month-olds behaved indiscriminately across conditions, the 20-month-olds listened longest to all conditions with at least one unfamiliar component. Exploratory analyses of infant rhythmic movements revealed that the 11-month-olds produced more rhythmic movements to familiar lyrics. Some infants, mostly the older infants, also produced lyrically-relevant gestures more often when those lyrics were present than when they were not (e.g., rolling arms during "wheels" lyrics), across both familiar and unfamiliar songs. Results suggest that by late infancy, infants integrate lyrics and melody when recognizing familiar music.

Less is known about the development of joint attention in late preterm infants (34 - < 37 weeks of complete gestation). We aimed to examine the effect of late prematurity on the developmental trajectory of joint attention and to investigate whether maternal interactive style could moderate a possible detrimental effect of prematurity on the ability. Participants were 43 late preterm and 29 full-term infants, assessed at 12- and 15-month uncorrected chronological age, and their mothers. Both groups responded more to joint attention over time; however, late preterm infants presented significantly lower levels at both time points. Furthermore, late preterm and full-term infants seemed to display different initiating joint attention trajectories from 12 to 15 months. Maternal behaviors did not influence joint attention nor moderate the effect of prematurity. Results suggest that late prematurity may have a differential impact on distinct behavioral dimensions of joint attention, potentially reflecting the specific mental processes involved in RJA and IJA.

Early mother-child conversational initiation is increasingly understood as a developmentally dynamic, embodied, and socially embedded process in which caregivers and children coordinate multimodal cues, including gaze, gesture, touch, and speech, to establish joint attention and meaning. Drawing on an existing audio-visual corpus of Korean mother-child interactions from 35 dyads aged 8-27 months, sampled across stages of language and motor development, we examine how nonverbal cues preceding verbal conversational onset vary with child age, sex, and speaker role. Results show that initiatory multimodal cues decline with age, especially in conversational blocks where mothers initiate the vocal exchanges, a pattern consistent with a developmental shift toward more verbally-driven interactional strategies. Look cues and object-oriented tactile cues were the most frequent signals preceding conversational blocks, highlighting their central role in initiating exchanges. In addition, initiators produce more initiatory multimodal cues than their partners in the moments preceding conversational onset, pointing to a strategic use of multimodal signals to capture attention and frame the upcoming verbal interaction. Overall, the findings highlight the key role of gaze and object-oriented action in early interaction and the adaptability of communicative strategies in mother-child exchanges as children take on a greater role in verbal initiation.

As infants acquire postural control, they learn to keep balance over a progressively smaller base of support. We systematically tested the development of postural control from a floor sit, where infants' thighs and lower legs are supported by the ground, to a bench sit, where infants' lower legs are unsupported. Six- to 9-month-old infants (N = 25) began in a floor sit with their thighs on a stationary platform and lower legs on a platform that was lowered in 2-cm increments until infants were in a bench sit with feet dangling in the air or they lost balance and fell. Each decrease in the height of the bottom platform decreased infants' base of support and made postural control more challenging. Although bench sitting was presumably novel, all infants maintained balance in a bench sit if their feet rested on the ground. However, infants' trunk control predicted their ability to maintain balance with their feet dangling in the air. Infants with more trunk control compensated for decreased support by progressively increasing their trunk-thigh angle (leaning backward rather than forward) and hence keeping their center of mass within their decreasing base of support. Thus, improvements in trunk control provide infants with more efficient postural-control strategies to cope with novel threats to balance. We suggest that flexibility and adaptability are integral to learning to sit and prepare infants for the next challenges in motor development.

Infants manually contact objects differently depending on their body position and the physical properties of objects. Previous studies have examined these factors separately, in structured laboratory settings, and laid the foundation for the investigation of the interaction of these elements. The current study revisits ideas from structured research in less controlled conditions by investigating how body position and the type of object shape manual contact with objects during free-flowing play. Nine-month-old infants (N = 80) participated in a semi-structured dyadic play session with their caregiver, interacting with four objects of two different kinds - two small and graspable objects; and two larger and stationary objects requiring specific actions to elicit an outcome (e.g., button pressing to make balls spin). The dyad was not constrained in terms of positioning, location on the floor, or movement around the room. We examined how the duration and frequency of infants' manual contact with objects varied based on the real-time interaction between object type and body position. The results indicate that infants contacted objects more frequently while sitting independently compared to other positions. Stationary objects were contacted more frequently but for shorter durations than graspable objects, regardless of the body position. Infants contacted graspable objects longer while sitting independently (compared to other positions), however, stationary objects were contacted with similar durations for all body positions. We found that durations of object contact depend on the interaction between object type and body position, which is more complex than what could be predicted by studies that examined these factors in isolation. These results highlight the need to study multiple interacting elements in less controlled settings.