Introduction

Social cognition is essential for successful human interaction and comprises processes relevant for understanding others’ emotions, perspectives, and mental states in order to interpret, explain, and predict the behavior of others1. The social requirements placed on humans are subject to considerable change across the lifespan. Moreover, cognitive processes that facilitate adaptation to novel and challenging social environments are subject to maturation and degeneration, similar to the patterns observed in other cognitive domains1.

For example, while development of socio-cognitive capacity starts in the first year of life2, significant changes in the ability to attribute mental states to other people (Theory of Mind, ToM) occur around the age of four, when children begin to more accurately interpret emotions and intentions of others3. ToM is a multidimensional construct that can be differentiated into two subcomponents: cognitive ToM, which describes a cognitive understanding of the difference between the speaker’s knowledge and that of the listener, and affective ToM, which describes the empathic appreciation of the observed person’s emotional state4. ToM is necessary in order to understand and predict the behavior of others5 and is a prerequisite for successful social interaction6. However, there is substantial variability in ToM development and other socio-cognitive skills and different mechanisms may underlie the diverse aspects of social cognition7. Neuroanatomical correlates involve for example the medial prefrontal cortex structures, the superior temporal sulcus region, the temporal poles, and the amygdala8. This inter-individual variability is crucially shaped by a range of genetic and environmental factors9,10. Therefore, development and adaptation of socio-cognitive skills across early life is an active process and strongly influenced or trained by the environment children are exposed to11,12. Consequently, there is a need to extend the contexts in which ToM and other socio-cognitive skills are investigated to include settings such as kindergarten and pre- and middle schools13 to properly account for social factors and interactions in children’s development14. This is important as ToM abilities continue to develop into adulthood (e.g.15).

Throughout adolescence and adulthood, humans face considerable social challenges associated with puberty, the end of the schooling period and entering life as an independent adult16. Social demands continue to change throughout adulthood and towards the third age. Advanced age is also associated with cognitive decline, which has marked effects on social functioning1. For example, there is evidence that healthy older adults may have difficulties in inferring mental states in complex social scenarios17 and even basic emotion recognition may decline (for review see Arioli et al. 2018). Furthermore, decline of ToM in older age can occur due to changes in brain structure and function: For example, a recent study showed that older adults exhibit weaker intrinsic connectivity between the right temporoparietal junction and right temporal pole that explained their poorer ToM behavioral performance18.

Importantly, impairment of social functioning in later life has been linked to mental19 and physical problems20, functional disability21, and reduced quality of life22. Moreover, socio-cognitive impairment is a core feature of many neurodegenerative disorders such as frontotemporal dementia or Alzheimer’s disease23 and also an early and salient marker of many neurodevelopmental, neuropsychiatric disorders23,24. Consequently, there is considerable research interest in designing socio-cognitive training approaches that foster socio-cognitive development or prevent age-related decline across the human lifespan.

Despite the importance of social cognition for successful development and aging, no previous study has provided an overview of the different socio-cognitive training approaches currently being used across the healthy human lifespan. This will be addressed in the present review that aims at (a) systematically describing all socio-cognitive trainings for healthy individuals over the lifespan and (b) investigating the effect of socio-cognitive trainings in healthy individuals at different ages using a meta-analytical approach.

Results

Results of the search

The initial search of databases yielded n = 4968 studies and an additional n = 6 studies were identified through scanning of relevant reviews. After removal of duplicates n = 4019 studies were screened. After abstract and title screening, we assessed 38 full-texts for eligibility and included 23 studies in the systematic review. N = 12 studies were included in the pairwise meta-analysis. The PRISMA flow-diagram25 in Fig. 1 provides an overview of the study selection process.

Figure 1
figure 1

PRISMA diagram illustrating the study selection process.

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Characteristics of socio-cognitive trainings

Twenty-three studies (21 RCTs, two quasi-experimental studies) with a total of 1835 participants investigating the effectiveness of socio-cognitive trainings were included. A detailed overview of included studies with information on participants (e.g., group sizes, age, sex), interventions, and investigated outcomes is provided in Table 1. The specific tests for outcome assessments and details of the results of each study are displayed in Table 2.

Table 1 Characteristics of included studies.
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Table 2 Results of the included studies.
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As described, we had initially defined three broad age brackets to cluster different ages ranges (0–18 years: children and adolescents, 18–60 years: young and middle-aged adults, older than 60 years: older adults). Yet, we acknowledge that these age brackets are broadly defined, e.g. substantial changes in socio-cognitive domains are observed between early life and the late teenage years26 and also in individuals aged 60 years and older1. Moreover, there are enormous differences in social challenges individuals face across development, young adulthood, middle and late age. Therefore, we further sub-divided the systematic review and meta-analysis where applicable.

Children and adolescents

In the lowest age bracket, thirteen studies investigated socio-cognitive training in children27,28,29,30,31,32,33,34,35,36,37,38,39. Eight studies were conducted in Europe28,30,33,34,35,36,37,39, two studies in The United States of America27,29, one in Canada38, and two studies in Asia31,32. The mean age of the investigated populations in this age range was 5.98 years (range 43–60 months). N = 10 studies reported training effects in kindergarten age (range:43–60 months); n = 3 studies included older children attending elementary school (range 7–9 years)34,35,39. Socio-cognitive training in both groups was labelled as either ToM-training, storytelling, or metacognitive intervention with the overall aim for children to further develop socio-cognitive skills using tasks where they had to reflect on other person’s mental states and beliefs. Training sessions for children in Kindergarten age were relatively short [between 1527,30,31 and 45 min28] and lead by a trained experimenter. There was substantial variability in training frequency and duration. Two to four training sessions per week were conducted29,30,31,32,33 over five27 to 20 weeks28. Training for children attending elementary school lasted 40–50 min to resemble the duration of a standard school lesson, and training was conducted up to five times per week34,35,39. Kindergarten age children participated in the training either individually33 or in small groups of up to five children27,28,29,30,31,32, children in elementary school attended the training in small groups. Stories used for ToM training in children typically introduced the protagonists first (e.g., animals or children with pleasantly sounding and easy to remember names like Monty and Freddy), followed by a simple story about them. Contents and questions asked by the experimenter varied depending on the goal of the training (e.g., emotion contingencies, false belief tasks). For example, in studies that targeted emotion contingency30, children were told that Monty likes one thing (e.g. apples) but dislikes another (e.g. pears). Monty then puts an apple in a box, leaves and Freddy joins the scene replacing the apple with a pear. Children would then be asked questions about “How Monty feels when he gets an apple”, “What Monty thinks about the box content when he returns” or “How he would feel when he opens the box”. Age-appropriate materials were used in these stories including cartoon pictures32, a storybook with illustrations to engage children27,29,30,31 or puppets30 and the trainers encouraged children to ask and answer questions about the stories and provided feedback. In children in elementary school aged between 7 and 9 years different ToM narratives were told, which were not supported by puppets or picture books, but rather discussed in a group setting with other children and teachers34,35,39. Overall, trainings did not focus on specific developmental challenges posed by kindergarten/school, but rather on basic socio-cognitive processes and skills.

Young and middle-aged adults

Six studies investigated effects of socio-cognitive training in young and middle aged adults [middle age bracket: mean age in the investigated studies ranged from 21.10 to 41.10 years40,41,42,43,44,45]. Four studies were conducted in The United States of America40,41,42,43, two studies in Europe44,45. Four of them used online or computerized socio-cognitive training40,41,43,45, focusing on one43 or several socio-cognitive tasks40,41,45 that were trained repeatedly. Other trainings were conducted in an offline group setting42,44,45. Duration of training sessions for young and middle aged adults ranged from 2043 to 45 min40,41. There was substantial variation in training frequency, yet, training in this age bracket was delivered at a higher frequency than in children and ranged from six40 to 42 sessions42. The only exception was the study conducted by Santiesteban and colleagues, who conducted only two training sessions44. Unlike in children, where trainings focused mainly on development of rather basic socio-cognitive processes and skills using simple tasks, there was no clear pattern of specific ToM-tasks used in the different trainings in young and middle-aged adults. Several more complex socio-cognitive tasks were trained including emotion recognition by the Reading the Mind in the Eyes task (in which participants can only see the eyes of a person and have to decide which emotion the eyes express out of a list of four emotions41) or a modified version of the Iowa Gambling Task, which practices social decision making40. The specific tasks were trained in an adaptive manner depending on the participant’s performance. Three of the studies used additional didactic presentations on socio-cognitive processes and strategies to enhance socio-cognitive abilities40,42,45, to provide theoretical background, and to introduce strategies to adapt to new situations. One study43 also trained working memory in addition to socio-cognitive tasks. Both studies that investigated ToM demonstrated significant improvement after the training (either on an broad ToM task45 or by using an Imitation-Inhibition Task44, in which participants were either asked to imitate a specific behaviour or inhibit its imitation). Three studies investigated emotion recognition42 or perspective-taking as primary outcomes41,43 and reported significant improvements after the end of the training period. No beneficial training effects were demonstrated in a study that used social decision making ability as primary outcome40. For additional information on these results see Table 2.

Older adults

In the upper age bracket, four studies investigated socio-cognitive training in adults older than 60 years [mean age range between 68.30 and 71.50 years46,47,48,49]. All four studies were conducted by the same Italian group and used similar socio-cognitive training approaches, designed as RCTs and one quasi-experimental design without a control group48. All ToM trainings focused on tasks and extended conversations about mental states. ToM tasks were practised in small groups with experienced experimenters as group leaders, who provided feedback on task performance. Specifically, a trainer presented one or two written ToM stories in each session and then asked several questions about the main character’s mental state, one character’s belief about the other character’s mental state, mental states underlying specific social behaviour and what the main character could do or say in order to change the other character’s mental state. ToM stories varied in content and complexity including misunderstandings, double bluffs, or sarcasm. Answers were then discussed in a group setting. Two47,48 to four sessions were completed46,49, with each session lasting up to two hours. Two studies reported that the training sessions were conducted weekly47,48, whereas two studies did not provide information on training frequency46,49.

Results of the meta-analysis

Twelve RCTs provided data on our primary outcome (ToM) and were included in a random-effects pairwise meta-analysis. The forest plot of the main analysis is displayed in Fig. 2. Based on the outcome of the systematic review, we post-hoc decided to further cluster our analysis for the lowest age bracket (0–18 years) in two groups: children between 3 and 5 years and children aged between 7 and 9 years. In total, 875 participants were included in this analysis (n = 140 children aged between 3 and 5 years in the ToM training; n = 134 children aged between 3 and 5 years in the control group; n = 168 children aged between 7 and 9 years in the ToM training; n = 154 children aged between 7 and 9 years in the control groups; n = 146 adults older than 60 years in the ToM training; and 133 adults older than 60 years in the control group). Studies eligible from the middle age bracket did not provide sufficient data to justify inclusion into the meta-analysis, as a minimum of n ≥ 2 studies with comparable data is needed to calculate a meta-analysis (Deeks et al., 2021).

Figure 2
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Forest plot of meta-analysis on the effectiveness of theory of mind training compared to control group, arranged by age groups.

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The overall effect size, the standardized mean difference (SMD), was 3.40 (95%CI: 1.19–4.88), indicating a large effect size, which was statistically significant (p < 0.01). However, I2 was high (98%), pointing to substantial heterogeneity between studies. Subgroup analysis also showed a large effect size (SMD = 2.51 (95%CI: 0.48–4.53)) for children aged between 3 and 5 years and an even larger effect size for children aged between 7 and 9 years (SMD = 2.71 (95%CI: − 0.28 to 5.71)) . We also identified a strong training effect for adults older than 60 years (SMD = 5.90 (95%CI: 2.77–9.02)). However, there was substantial heterogeneity in all comparisons (children aged 3–5 years: I2 = 97%, children aged 7—9 years: I2 = 98%, adults older than 60 years: I2 = 97%).

To provide additional confirmatory information for these results, additional sensitivity analyses with fixed-effects models were conducted and results are displayed in Fig. 2, showing lower effect sizes than the random-effects model (overall SMD = 1.24 (95%CI: 1.06– 1.42)). A sensitivity analysis dividing the studies according to different theory of mind tests used (e.g. false-belief task, strange story test, socio-cognitive composite score) are reported in the Supplementary Materials, Fig. 1. However, these sensitivity analyses did not reduce the heterogeneity of the studies and could only be conducted for children (including all studies, regardless of age-clusters within in the group) because not enough data was available for the other comparisons (when considering three studies as minimum per subgroup). A funnel plot that was conducted over all studies further showed an uneven distribution, indicating a possible publication bias (see Supplementary Material, Fig. 2) for studies with higher effect sizes. Yet, it should be noted that we could only include a small number of studies and the funnel plot does not consider the quality of included studies and should therefore be interpreted with caution.

Risk of bias analysis

The results of the risk of bias assessment are summarized in Table 3. Overall, most studies were rated as “with some concerns”, mainly because studies and/or analyses were not pre-registered and the randomization process and blinding was not clearly described. Two studies28,48 were rated as high risk of bias because they were not properly randomized, but quasi-experimental studies.

Table 3 Risk of Bias (RoB) assessment of included studies.
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Discussion

This is the first systematic review and meta-analysis of socio-cognitive training approaches that are currently used in healthy individuals across the human lifespan. Results of the 23 studies included in the systematic review showed that socio-cognitive trainings differed across age groups with regard to their settings, methods, frequency, duration, and training elements. Notably, 10 of the 23 included studies focused on very young healthy children around four years of age. Results of the meta-analysis for studies that specifically targeted ToM outcomes and included twelve studies with 875 participants demonstrated that (a) socio-cognitive trainings yielded large effect sizes and, (b) effects were most pronounced in healthy older adults (> 60 years of age). The middle age bracket could not be included into the meta-analysis, because there was not enough data available for the different outcomes. However, five of the six studies included in the systematic review also reported positive effects of socio-cognitive training targeting ToM44,45, emotion recognition42, and perspective-taking41,43. Risk of bias assessment revealed some methodological concerns in all included studies, mostly because of failure to pre-register studies or missing information on the randomization process.

Different stages of life pose unique challenges to social interaction and socio-cognitive abilities: as children, we start to develop the abilities to understand and communicate with others’, during puberty, adolescence and adulthood, we are constantly confronted with life changes that challenge our socio-cognitive skills and our ability to adapt these to new circumstances16. Finally, as we age, (socio-) cognitive abilities may start to decrease leading so social difficulties and reduced quality of life17. Therefore, socio-cognitive training may be helpful at every stage of life to either develop (new) socio-cognitive abilities, adapt our behaviour to new environments and challenges or to maintain a high-level of socio-cognitive functioning. Consequently, an important precondition for designing suitable training approaches for individuals of different ages is knowledge about specific challenges posed by different stages of life47, as well as adapting the training to individual needs. Training for younger children typically aimed at developing certain socio-cognitive skills and therefore focused explicitly on examples of every-day situations relevant for such young age in rather short training sessions (i.e., 5–20 min per session) in which ToM-stories were told by an experienced group leader, frequently an adult with a pedagogical education. For example, the group leader read a story, asked interactive questions about the stories and children were required to adopt the mental perspectives of other people within the story. Training was frequently supported by age-appropriate playful components such as hand puppets, drawings and/or funny names of the main characters to ensure compliance. For children, socio-cognitive trainings thus provided opportunity to actively think about specific age-appropriate socio-cognitive tasks to develop awareness of mental states as they start to learn to reflect about other people’s perceptions and thoughts. These highly adapted training environments effectively aid ToM development and likely explain the relatively high effect sizes found in our meta-analysis demonstrating that even “healthy” developing children can benefit from socio-cognitive training. Furthermore, there was a difference between socio-cognitive trainings for children in kindergarten age and children at elementary school: training for younger children included age-appropriate materials such as the use of comic books and puppets, whereas training for children at elementary school did not use these but focused their training on group discussions and short-stories.

In contrast, trainings designed for adolescents and adults do not require development of new socio-cognitive abilities from scratch but rather to apply and adapt already existing skills to new situations. Training in these groups also comprised longer sessions (between 30 and 60 min, as our results indicate) and rather complex tasks were used (e.g., reading the mind in the eyes task or social decision making in complex social dilemma scenarios). Furthermore, training for younger and middle-aged adults frequently comprised of psychoeducation with the focus on understanding, perceiving, managing, and using of socio-cognitive tasks. For example, participants were provided information about different emotions, social situations and learning materials about different mental states with the aim to reflect on their own socio-cognitive abilities. Notably, training approaches for young and middle-aged adults were highly standardized with regard to structure, tasks and materials and surprisingly, individual challenges that participants may face in their everyday life requiring specific behavioural adaptations were not addressed. The overall training duration was rather long, ranging from eight and 39 weeks and training sessions were no longer than 60 min to accommodate work schedules. Results indicate that trainings were effective in improving ToM, emotion recognition, as well as perspective taking in middle-aged adults.

Because socio-cognitive abilities may start to decline with more advanced age17, the aim of socio-cognitive training in individuals over the age of around 60 years was to assist maintenance or improvement of these abilities. Training for older adults mainly comprised practicing specific ToM-tasks (e.g., reading a complex false-belief story that includes misunderstandings, double bluffs or sarcasm and conversations about different perspectives in stories) as well as conversations about mental states (e.g., what do the people in the false-belief story think about the situation and about the mental states of the other characters in the story?). Conversations about mental states were described as particularly important in older adults to address lack of social interaction due to reduced social network sizes50,51. Surprisingly, none of the reviewed studies in older adults trained specific cognitive processes that are frequently impaired in aging and interact with socio-cognitive processes (e.g. working memory, attention, processing speed52). Duration of training sessions was two hours in all investigated studies. A longer duration was chosen to provide older participants with enough time to complete the respective tasks without feeling rushed. Importantly, the meta-analysis suggests that socio-cognitive training was even more effective in healthy older adults than in children. Future research needs to clarify the effective elements in the respective training approaches responsible for this effect (e.g., differences in duration, frequency, content).

Overall, socio-cognitive abilities are multi-componential and involve several processes at a number of levels from cognitive, perceptual to conceptual53, which can all be targeted in effective interventions. Overall, different training studies highlight the need to include a variety of training tasks to promote generalization46. Given the fact that one of the most desirable outcomes of a specific training are transfer effects to other untrained tasks, research should focus on designing trainings that are able to produce transfer or generalization to several socio-cognitive (sub-) domains46, e.g. by incorporating other cognitive domains that are known to support socio-cognitive skills. Furthermore, as most of the included studies in our review focused on ToM-training, a number of socio-cognitive functions such as emotion recognition, (visual) perspective-taking and social decision making are underrepresented, even though all of these domains are essential for social interaction in everyday life1. Yet, it is conceivable that training more complex tasks like ToM-exercises and interpretation of complex social situations leads to more pronounced generalization than the training of single socio-cognitive functions. However, direct comparisons of these approaches are currently not available. Still, our review suggests that there is a gap in socio-cognitive training research and training studies focusing on emotion recognition, visual perspective taking, and social decision making are currently lacking.

Several factors influence the development of socio-cognitive abilities during childhood, among them e.g. social factors as family discourse about emotions54, number of siblings55, and interaction styles among family members56. However, cross-cultural studies have demonstrated that these social factors are not identical in different cultures and children may develop socio-cognitive abilities via different pathways (e.g. by talking about other people’s mental states instead of their own mental states) depending on their cultural background31. Indeed, except for studies in the upper age-bracket which were all conducted in Italy, the reviewed socio-cognitive studies were conducted in different cultural backgrounds. While the meta-analysis demonstrated that all studies were effective in improving socio-cognitive abilities, it is worth noting that training materials were culturally-sensitive and adapted to the specific culture they were used in (e.g. European/American/Asian faces in Emotion Recognition Tasks). This highlights the utmost importance to not only consider social and aging contexts, but also the cultural aspects in training designs57.

Importantly, the results of our meta-analysis showed substantial positive effects of socio-cognitive training on ToM in all investigated age-groups. However, we were not able to consider effects on affective (i.e., inferences regarding others’ emotions) ToM and cognitive ToM (i.e., inferences concerning others’ beliefs and knowledge58) separately, because not enough data was provided on these two constructs. However, this would be relevant as cognitive ToM shows greater age-related decline than affective ToM51. Table 4 provides an overview of identified areas for future research in the field of social cognitive trainings, as well as general recommendations for training studies. In addition, data long-term maintenance of training effects was rarely reported or comprised different timings of the assessments and could not be considered in the meta-analysis. Also, all our analyses showed a high heterogeneity, which did not decrease in our sensitivity analysis where we divided the studies according to different theory of mind tests used (e.g. false-belief task, strange story test, socio-cognitive composite score), thus limiting our meta-analytic results. Contributors to this unexplained heterogeneity may include variability in socio-cognitive training content and duration (which we therefore described in detail in our included tables), as well as potentially different socio-demographics and personality factors in the studied samples, which may not be discernible from published results. To account for this, future studies may try to conduct an individual patient data meta-analytic approach in which original data from all included studies will be compared and detailed mediation or moderation analysis are possible.

Table 4 Recommendations for future studies.
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Transfer effects have only been assessed in few studies. Consequently, more research in these areas is needed. In addition, many studies were not pre-registered and information about participant randomization was frequently lacking, which needs to be addressed in the future to reduce risk of bias.

In sum, effective and adaptive socio-cognitive functioning is crucial throughout the human lifespan and identifying effective training elements may contribute to preventing mental and physical disease and to increase quality of life in various populations. The present systematic review and meta-analysis highlights the specific characteristics of training approaches in different age groups and confirms the effectiveness of these approaches across the healthy human lifespan.

Methods

The present systematic review and meta-analysis was pre-registered. The review protocol is available at www.crd.york.ac.uk/PROSPERO/ (ID: CRD42020193297). Reporting follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline25. See Supplementary Material Tables 1 and 2 for the “PRISMA for Abstracts Checklist” and the “PRISMA checklist for systematic reviews”.

Systematic review

A systematic literature review was conducted to describe and examine the characteristics and effects of social cognition training approaches in healthy participants across the human lifespan, using the highest reporting standards in this field. In the following, the search and study selection processes for the systematic review are described, as well as eligibility criteria for study inclusion, data selection processes and quality assessment procedures.

Search and study selection

MEDLINE Ovid, Web of Science Core Collection, CENTRAL, and PsycInfo databases were searched for social cognition training studies up to 15th January 2021. An update search was performed up to the 1st September 2021. We also searched for additional studies in reference lists of relevant reviews (e.g. Hofmann et al.61). In cases where no full text could be obtained, we contacted the authors and requested full text publications within a 10-day time frame. As an example, the full search string for MEDLINE is described in the Supplementary Materials, Table 3.

Eligibility criteria

The “Participant, intervention, comparison, outcome” (PICO)-system was used62 to define eligibility criteria. Studies were considered eligible if they had included healthy female and male participants of all ages (P). Studies that had included patients with diagnosis of any psychiatric or other medical diseases were excluded. Randomized controlled trials (RCTs) as well as controlled studies investigating interventions focusing on training social-cognitive skills were included in the systematic review (I). No further pre-specifications about details of the interventions were made. Control Groups in eligible studies were required to have received either a different type of socio-cognitive intervention in healthy participants of all ages or participated in passive control and/or waitlist control group (C).

Behavioural changes in ToM tasks were defined as primary outcome (O). ToM is defined as the ability to attribute mental states to others or the ability to understand and predict others’ behaviour based on their mental states and is the most frequently studied socio-cognitive process across development and in healthy and pathological aging63,64. Secondary outcomes were chosen to represent three additional major socio-cognitive domains: social perception1 (recognizing others as “living persons” via the analysis of perceptual information including e.g. emotion recognition and visual perspective taking65), social understanding (using the social perception input for higher-level processes, e.g. theory of mind66), and social decision-making (using the obtained social information for social decision making67). Studies had to be published in English or German to be included. Only direct pre-post intervention outcome data was considered because the timing of reported long-term effects was too heterogeneous to allow for reasonable comparison between studies. However, Table 2 provides an overview of all assessment time-points for each study, wherever applicable.

Data extraction

Only studies that fulfilled all of the above-mentioned inclusion criteria were included in the present review. From these studies, data was extracted using a standardized extraction form by two reviewers (MR, KH). If no consensus could be reached, a third author (MM) was contacted for a final decision. Authors of specific studies were contacted for additional information, if required68.

Risk of bias

Risk of Bias was assessed using the Revised Cochrane risk-of-bias tool for randomized trials (RoB2 tool69, which implements signalling questions for five domains leading to low/high/medium concern for risk of bias). Two review authors independently assessed risk of bias for each study (MR, KH).

Meta-analysis

A random-effects pairwise meta-analysis was conducted to calculate the overall effect of socio-cognitive training compared to passive control interventions. Direct comparison of different socio-cognitive trainings was not possible because data was too sparse and heterogeneous to perform a pairwise meta-analysis. Data analysis was conducted using R70. For all analyses, the alpha level was set at 0.05. Our primary outcome “theory of mind” was the dependent variable for the meta-analysis. Secondary outcomes could not be considered in the meta-analysis because there was not enough data available. Three broad age-subgroups were defined comprising studies which had included participants between 0 and 18 years, participants between 19 and 59 years, and participants older than 60 years for a first overview and clustering of studies. Yet, in the lowest age bracket, we identified studies in children between 4 and 5 years and 7–9 years old children who have started attending school (i.e., reflecting two major challenges for socio-cognitive development). No studies on socio-cognitive trainings in adolescents were identified. In addition, no studies investigating participants aged between 40 and 60 years could be identified. Consequently, the middle age bracket included only young adults. Only RCTs were considered for the meta-analysis. Data were independently extracted by two reviewers (MR, KH). The mean change from baseline to post-intervention, standard deviation of the mean change, and the number of evaluated participants in each intervention group were used to calculate standardized mean differences. As for the systematic review, only immediate effects were considered because data on long-term effects was limited and heterogeneous timings. We used the I2 statistic to address heterogeneity of the included studies. As recommended in the Cochrane Handbook for systematic reviews of interventions68, we interpreted heterogeneity as follows: 0–40%: not important/low heterogeneity; 30–60%: moderate heterogeneity; 50–90%: substantial heterogeneity; 75–100%: considerable heterogeneity. A funnel plot for identifying possible publication bias was calculated.

Sensitivity analyses were conducted using fixed effect models. In addition, we further divided our sample according to different theory of mind tests used in the studies to investigate whether different ToM tests (e.g., false-belief task, strange story task) have an influence on the effects. Results of the sensitivity analysis are illustrated in the Supplementary Materials, Fig. 1. The data that support the findings of this study are available from the corresponding author upon request.