The development of emotion processing of body expressions from infancy to early childhood: A meta-analysis

Abstract

Body expressions provide important perceptual cues to recognize emotions in others. By adulthood, people are very good at using body expressions for emotion recognition. Thus an important research question is: How does emotion processing of body expressions develop, particularly during the critical first 2-years and into early childhood? To answer this question, we conducted a meta-analysis of developmental studies that use body stimuli to quantity infants' and young children's ability to discriminate and process emotions from body expressions at different ages. The evidence from our review converges on the finding that infants and children can process emotion expressions across a wide variety of body stimuli and experimental paradigms, and that emotion-processing abilities do not vary with age. We discuss limitations and gaps in the literature in relation to a prominent view that infants learn to extract perceptual cues from different sources about people's emotions under different environmental and social contexts, and suggest naturalistic approaches to further advance our understanding of the development of emotion processing of body expressions.

Introduction

The ability to discriminate and recognize other people's emotion is important for social interactions. Adults process a rich combination of perceptual cues from people's facial, vocal and body expressions to recognize emotions quickly and accurately so they can take appropriate actions (de Gelder, 2009; Belin et al., 2011; Keltner et al., 2016). These cues also include changes in body odor and temperature (Robinson et al., 2012; Rosen et al., 2015; Salazar-López et al., 2015; de Groot and Smeets, 2017). For emotion body expressions, adults seem to focus on perceptual cues in the upper body, including the arms and hands (Pollux et al., 2019; Ross and Flack, 2020). Bodies can provide more diagnostic information about emotions than other perceptual cues under certain circumstances, such as when a person is far away (de Gelder, 2009; Bhatt et al., 2016; Enea and Iancu, 2016). Thus, an important research question is how emotion processing develops, particularly during the critical first 2-years and into early childhood. For example, we recently showed that the focus on the upper body shown by adults may emerge as early as 7-months (Geangu and Vuong, 2020). Developmental research, however, has focused predominantly on facial expressions (Geangu et al., 2016a; Bayet and Nelson, 2019).

Our aim in this mini-review is to synthesize evidence from developmental studies of emotion processing of body expressions from infancy until early childhood to address the research question. We have two goals toward this aim. First, we highlight the importance of environmental and social contexts for learning perceptual cues to emotion expressions. As infants grow, different visual information related to faces and bodies become more prevalent in the visual field during their daily activities (Smith et al., 2018), and they experience more and more varied emotion expressions under different social contexts. Second, we present a meta-analysis of developmental studies that use body stimuli to quantity infants' and children's ability to discriminate and process emotion expressions at different ages. The evidence suggests that there is a shift from faces being prevalent in the visual field toward other parts of the body (e.g., hands; Fausey et al., 2016; Ausderau et al., 2017; Jayaraman et al., 2017; Smith et al., 2018), and so the meta-analysis may help us relate laboratory-based studies to infants' and children's natural learning environment. We conclude with suggestions for future research directions.

Emotion body expressions in context

A prominent view of the development of emotion processing is that infants learn to extract perceptual cues from different sources about people's emotions and their communicative value (Campos et al., 1994; Leppänen and Nelson, 2009; Widen, 2013; Smith et al., 2018; Walle and Lopez, 2020). With respect to body expressions, infants frequently have people (e.g., parents, siblings) in their visual field view throughout the first year of life (Ausderau et al., 2017; Jayaraman et al., 2017). Importantly, the prevalence of different body parts that are present in the visual field changes during development. For example, faces are more prevalent than other body parts during the first 4 months after birth (Jayaraman et al., 2017). This prevalence shifts to other body parts after this age. Fausey et al. (2016) used head-mounted camera recordings in infants' home environment to demonstrate an increase in the proportion of hands in infants' visual field with a corresponding decrease in the proportion of faces, with a larger proportion of hands emerging between 6 and 9-months-old. The changes in prevalence of different body parts are observed across the first 2-years of life, and are likely due to cognitive and motor development that allow infants to more actively explore and interact with their environment and people (Flavell, 1982; Fischer and Silvern, 1985; Ausderau et al., 2017).

Thus as infants mature and explore their environment, they are likely to extract and process different body parts that become more prevalent in their visual field to recognize different emotion expressions, and possibly relate body parts to perceptual cues in other modalities such as vocal expressions or odor changes. The prevalence of bodies in the visual field may also be relevant for other social tasks. For example, infants as young as 6-months-old fixate on the hands of people who reach and grasp objects, and look less at other body parts that are in view (Falck-Ytter et al., 2006; Kochukhova and Gredebäck, 2010; Geangu et al., 2015). These changes in the availability of different body cues to emotions and social interactions also increase the opportunities infants have to learn the relation between body expressions and the social and non-social contexts in which they occur, further contributing to the development of emotion processing of body expressions (Campos et al., 1994; Leppänen and Nelson, 2009; Widen, 2013; Walle and Lopez, 2020). These experiences during maturation may lead to appropriate neuro-physiological responses associated with emotion processing (e.g., Krol et al., 2015; Rajhans et al., 2015; Ross et al., 2019).

Visual information for emotion processing of body expressions

By adulthood, research suggests that combinations of body postures and movements define signature cues for recognizing emotions from body expressions (Atkinson et al., 2004; Atkinson, 2013; Poyo Solanas et al., 2020). For example, signature cues for happy expressions may include an upright posture with raised arms. The cues for anger expressions may include a forward-leaning posture and shaking fists, contrasted to a backward-leaning posture and hands in front of the body for fear expressions. Sad expressions have the most subtle cues that tend to include a dropped position of the head, with arms brought near the body. The existent evidence indicates that adults rely on visual information contained in the upper body (e.g., torso, arms and hands) to recognize emotions expressed in static body images (Pollux et al., 2019; Ross and Flack, 2020).

The naturalistic studies discussed in the previous section provide evidence that bodies are prevalent in infants' visual field from very early on (Fausey et al., 2016; Jayaraman et al., 2017; Smith et al., 2018). The results from these studies are complemented by behavioral and neural evidence that, from birth, infants are sensitive to body postures and movements (e.g., Hirai and Hiraki, 2005; Geangu, 2008; Simion et al., 2008; Geangu et al., 2015; Bhatt et al., 2016; Gillmeister et al., 2019). This initial sensitivity may help them orient and attend to bodies. Infants seem to also attend to visual information in the upper body like adults, in line with the increased prevalence of body parts in infants' visual field (Geangu and Vuong, 2020). Thus infants and young children's reliance on signature cues based on body parts for emotion processing of body expressions may reflect changes to the prevalence of different body parts in the visual field under different contexts during infancy and early childhood (Ausderau et al., 2017; Smith et al., 2018). There is currently no direct evidence for this possibility. Furthermore, developmental studies on the emotion processing of body expressions use different emotions, body stimuli and outcome measurements across different age groups, leading to gaps in the literature.

Review of emotion processing of body expressions

To address this issue and our overarching aim, we synthesize published studies on emotion processing of body expressions by infants and children. This synthesis can provide a holistic view to identify gaps and motivate future research. We conducted a literature search on PUBMED, Scopus, Medline, Embase and PsycInfo in October 2022 for articles which investigated emotion processing of body expressions in typically developing infants and children up to ~7.5-years-old. Although studies may include older age groups or developmental groups, we focused on typical development and body stimuli (or stimuli that included the body) within our age range. The electronic searches were complemented with hand citation searches. There were 1,787 unique articles, with 3 additional articles from hand searches. QV undertook the searching and screening processes. See the Supplementary material for details.

Study characteristics

Table 1 summarizes the 38 articles included in the review. The studies are ordered by the youngest age group (mean age in months), and range from 3.4-months-old to 87.1-months-old (7.3-years-old). Most studies balanced the number of male and female participants. Several studies included comparisons to older age groups (e.g., adults) or developmental conditions (e.g., hearing impairments or mental disabilities). We include developmental milestones from Ausderau et al. (2017) to illustrate some known developmental changes occurring at different ages.

A few studies considered psychological (Rajhans et al., 2015), social (Krol et al., 2015) and cultural factors (Tuminello and Davidson, 2011; Yang et al., 2022) in emotion processing of body expressions. Anger, fear, happy and sad expressions were tested the most, and ~29% (11/38) included an emotionally neutral condition as recommended by Hepach and Westermann (2016). Other expressions included, for example, disgust, surprise, pride and irritation. The body stimuli ranged from abstract representations (e.g., point-light displays or schematic line drawings) to videos and real-time interactions with experimenters (Quam and Swingley, 2012). Thus the stimuli could include static (e.g., body posture), dynamic (e.g., body movements) information (or both), and they could be combined with other perceptual cues such as faces and voices.

The studies used different outcome measurements, including accuracy, facial muscle activities from electromyography (EMG), eye-tracking measurements (e.g., fixations or pupil dilations), and event-related potentials (ERPs) in electroencephalography (EEG) related to different neural markers of emotion processing. One study measured facial thermal-imaging responses to body expressions (Nicolini et al., 2019). The studies also tested emotion processing of body expressions under different experimental conditions, such as body inversion. Several studies also compared emotion processing between different developmental conditions.

Meta-analysis

The studies in this review highlight the rich variety of body stimuli, outcome measurements and experimental manipulations used to test whether and how infants and children recognize emotion body expressions. Although this richness allows for a broad generalization, there is no quantification of infants' and young children's overall ability to discriminate between different emotion pairs (given differences in these studies). Thus, the goals of the meta-analysis is to combine effect sizes across studies to determine: (1) whether there is an overall ability to discriminate between different expression pairs; (2) whether this ability differs between different pairs; and (3) whether this ability varies with age.

For 22 of the 38 articles, we could derive mean and standard deviation for each body expression from graphs and/or tables to be included in the meta-analysis. We focused on anger, fear, happy, sad and neutral expressions as most studies used one or more of these expressions, resulting in 10 possible pairs (~14% [3/22] included a neutral condition). We calculated Hedges' g as the effect size and took the absolute value to quantify participants' ability to discriminate expression pairs. We log-transformed any effect sizes calculated from sample proportion data (Nelson et al., 2013; Witkower et al., 2021). For each study included in the meta-analysis, the effect size was calculated separately for each outcome measurement, within-subject experimental condition and age group. The effect sizes were averaged across outcome measurements and within-subject conditions resulting in 2 (sad vs. neutral) to 21 (anger vs. happy) effect sizes for each pair. A random-effects model with restricted maximum likelihood estimation (REML) was used to test whether the overall effect size for each emotion pair was greater than zero. Lastly, we conducted a meta-regression between effect size and mean age (in months) for each pair. The meta-analysis was conducted using the meta (v6.1-0; Schwarzer et al., 2015) package for R-Studio (v1.4.1106). See Supplementary material for details. The data and scripts are available at the Open Science Framework (https://osf.io/tyg6n/).

Figure 1 presents a forest plot for the 10 expression pairs, with studies ordered chronologically by the mean age in months. For the 6 pairs including two emotions (Row 1 in Figure 1), combining the effect sizes across all studies showed consistent evidence for small to medium effects (g = 0.36 to 0.68; ps < 0.001). The meta-regression showed inconsistent evidence that effect size varied with age for these pairs (ps > 0.05).

Figure 1

A similar but weaker pattern was found when each emotion was compared to the neutral condition (Row 2 in Figure 1; 4 pairs). The mean effect size also ranged from small to medium effects. It was significantly >0 for anger and happy expressions (g = 0.69 and 0.28, respectively; ps < 0.02) but not for fear and sad expressions (g = 0.20 and 0.34, respectively; ps > 0.07). There was a significant correlation between effect size and age for anger expressions (p < 0.001) but not for the other expressions (ps > 0.61 for fear and happy expressions; no solution for sad expressions). However, there was a small number of effect sizes that included a neutral condition (e.g., N = 2 for sad, N = 4 for the other emotions) and so we do not make any strong conclusions from these results.

Discussion and future directions

Our review identified a wide range of laboratory-based developmental studies of emotion processing of body expressions. We also note that researchers use different terms for similar or related emotions, such as joy vs. happy (e.g., Lagerlöf and Djerf, 2009), as well as more ambiguous cases such as win and lose (Nguyen and Nelson, 2021) which can be associated with happy/excitement and disappointment. Many individual effect sizes in these studies had confidence intervals that included 0. However across these studies, the evidence suggests that infants and children can discriminate between emotion expressions across a variety of body stimuli and experimental paradigms, and that infants and children can integrate perceptual cues across bodies, faces and voices. A similar pattern was seen for discriminating emotion from neutral body expressions, but this finding is limited by the small number of effect sizes.

The ability to recognize emotions is often inferred from infants' and children's ability to discriminate emotion pairs. Several studies in our review measured neuro-physiological outcomes while participants viewed different emotion body expressions, such as ERP components (e.g., Krol et al., 2015; Rajhans et al., 2016), EMG responses (Geangu et al., 2016b; Addabbo et al., 2020), pupil dilations (Geangu and Vuong, 2023) and facial temperature (Nicolini et al., 2019). Importantly, these measurements are related to emotion processing in adults (Robinson et al., 2012; Kret et al., 2013; Yeh et al., 2016). They suggest that infants and children can process the emotional content of body expressions using static (e.g., body posture) and dynamic (e.g., body movements) cues, rather than discriminating emotion pairs (Ross and Atkinson, 2020). A second finding is that emotion-processing abilities do not vary with age (as indicated by the meta-regression for the 6 emotion pairs), which is surprising given the developmental milestones and changes in visual information that are prevalent in infants' and children's visual field as they mature (Ausderau et al., 2017; Smith et al., 2018).

These 2 main findings should be considered in light of emotion processing in adults. Although body postures and gestures contribute to emotion processing in adulthood, body cues do not necessarily convey all emotions equally (Atkinson et al., 2004; Atkinson, 2013; Poyo Solanas et al., 2020) and may need to interact with other perceptual cues for effective emotion processing in the natural environment. For example, body expressions may be important for disambiguating fear and surprise, which can be easily confused with facial expressions (Smith and Schyns, 2009; Actis-Grosso et al., 2015). Thus our review and meta-analysis underscores the importance of investigating the development of emotion processing from multiple perceptual cues.

The 2 main findings should also be considered in light of potential limitations highlighted by our review. First, the sample size for young infants tend to be less than for older infants and children resulting in more variability for the younger group. Second, young infants were not tested with as many emotion pairs compared to the older age groups leaving a gap in understanding the early development of emotion processing of body expressions. This younger age group also tended to be tested with fewer emotion expressions within a study (e.g., typically 2 expressions) than older age groups. There was also a smaller proportion of studies that included a neutral condition (~29%; Hepach and Westermann, 2016). Third, there is a relatively small number of body-stimulus databases used across all studies (see Table 1). Nearly all studies with infants younger than 9-months used the stimuli from Atkinson et al. (2004). For other age groups, several studies used static and dynamic body-stimulus databases that have only been validated by adults. A few studies recorded their own body expression videos with different expressivity (e.g., expressive dance movements; Boone and Cunningham, 1998). Finally, few studies presented naturalistic stimuli that combined body, facial and vocal cues. Those that did manipulated the congruency of the emotion expression between different cues, leading to stimuli that were not necessarily naturalistic.

Given these limitations, we suggest several future research directions. The first is to test young infants with a larger variety of emotion body expressions, including neutral expressions (Hepach and Westermann, 2016). It would also be important to test infants longitudinally to map out the developmental trajectory for emotion processing of body expressions. Future work can also combine different outcome measurements (e.g., pupil dilation, EMG and EEG), use naturalistic dynamic multi-sensory perceptual cues (e.g., Geangu et al., 2011; Poulin-Dubois et al., 2018; Quadrelli et al., 2019), test different cultures (e.g., Geangu et al., 2011, 2016a; Geangu, 2015; see Parkinson et al., 2017; Poulin-Dubois et al., 2018; Quadrelli et al., 2019, for adults), and investigate factors contributing to observed individual differences (e.g., Crespo-Llado et al., 2018). One key limitation is that the body stimuli used in laboratory studies are visually impoverished and may not capture many of the perceptual cues that infants and children may experience in their daily activities (e.g., Smith et al., 2018). Given the importance of the maturing infants' environmental and social contexts, future studies can be conducted in the real world and focus on, for example, the frequency of different facial and body emotion expressions in the infants' visual field, parenting behaviors, and the context in which emotion expressions occur (e.g., Fausey et al., 2016; Jayaraman et al., 2017; Smith et al., 2018). These directions will be highly challenging but will be important to address the gaps in understanding the development of emotion processing of body expressions—and emotion processing more generally—highlighted by our mini-review.

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