The present study is, to our knowledge, the first to assess the effects of a mindfulness-based socio-emotional curriculum on preschool-aged children in the French school context, and the largest RCT to date evaluating a mindfulness-based intervention in preschoolers. Teachers were highly satisfied with the curriculum and motivated to implement the program in their classes. The number of delivered sessions and duration were consistent with our recommendations, and stood in the same range as previous studies evaluating MBIs (including SEL components or not) in preschool children (Bockmann & Yu, 2022). Implementation fidelity assessed using audio-recordings of activities was judged very satisfactory.
Effects of the intervention on mental health
In accordance with our hypotheses, this curriculum had a positive impact on children's mental health compared to teaching-as-usual. Teachers reported improvements in emotional, conduct and peer relationship problems, with effect sizes in the 0.2–0.3 range. Similar results were found in a recent literature review assessing the effects of MBIs in preschool children, in which MBIs reduced behavioral problems and enhanced emotion regulation (Bockmann & Yu, 2022). More specifically, some studies suggest that MBIs lead to decreased externalizing and internalizing behaviors such as anxiety and aggression (Crooks et al., 2020). Of note however, one previous study also found a positive effect of a MBIs on SDQ hyperactivity score, which was not found in the present trial (Janz et al., 2019). Overall, our findings of positive effects on different dimensions of mental health are consistent with the Healthy Outcomes from Positive Experiences (HOPE) framework. HOPE focuses on positive experiences that prevent mental health issues in children. In this framework, establishing positive relationships with adults and other children and developing self-awareness and self-regulation through peer interactions are considered key positive childhood experiences that promote children's mental health (Burstein et al., 2021).
Subgroup analysis revealed some heterogeneity in the impact of intervention on mental health between subgroups. Boys, older children, children with higher conduct problems at baseline, and children with higher negative mental health at baseline showed greater improvement in mental health than the sample as a whole. Increased effects on children with low mental health are consistent with previous studies highlighting that MBIs have greater effects on diverse indicators, e.g., self-regulation, prosocial behaviors or hyperactivity for preschool children with initial deficits in these indicators (Flook et al., 2015; Viglas & Perlman, 2018). Results for older children are not consistent with the literature. In a previous study, the effect sizes of interventions that target behavioral problems in children were larger for younger children compared to older children (Wilson & Lipsey, 2007). It is possible that in our study, program content was more adapted to older children. In the first evaluation study of the KC, children were on average 5 months older than our sample.
Effects of the intervention on positive and negative self-management
Despite its positive effects on mental health, we failed to detect a significant impact of our intervention on family estimates of positive or negative self-management in the whole sample. Still, intervention decreased conflictuality in the teacher-child relationship as measured through teacher questionnaires. This is in line with recent reviews (Blewitt et al., 2020; Cipriano et al., 2023) that suggest that SEL programs designed toward children also contribute to enhancing responsive and nurturing teacher-child interactions, with reduced teacher-child conflicts (Blewitt et al., 2020).
Children with low mental health at baseline specifically displayed improvement in negative self-management. In addition, boys showed specific improvements in positive self-management, as assessed both through teacher questionnaires and observations by independent evaluators. This stands in contrast with a previous SEL study that highlighted that although girls outperformed boys on SEC both before and after SEL intervention, boys did not demonstrate more or less SEC growth than girls during intervention (Mondi & Reynolds, 2021). Finally, intervention prevented teacher-child conflict from increasing, and enhanced positive self-management compared to control for children in classes with teachers with low experience. It is possible that teachers in their first years of activity face more challenges in implementing strategies to help children manage themselves, and that our intervention helps provide some of these strategies. It is also possible that teachers with low experience tend to implement SEL programs with higher adherence to the objective, which could contribute to better results on children's outcomes, including self-management.
Effects of the intervention on positive relationships with teachers and peers
We did not find a significant effect of our intervention on positive relationships with teachers and peers when considering the whole sample. These results stand in contrast with previous research, as previous SEL programs reported coherent positive effects of SEL programs on these outcomes (Cipriano et al., 2023; Durlak et al., 2011). In addition, these specific outcomes were directly targeted by the KC activities implemented in our program, and a previous study assessing the efficacy of the KC showed positive effects on prosocial behaviors (Flook et al 2015). Compared to the study of Flook et al. (2015), discrepancies could be explained by the fact that the measure of prosocial behavior used in this study was different from the one used in our study, or that this previous study was carried out on a much smaller sample of 68 children. Another possible explanation is that our population differs from the study of Flook et al (2015) in terms of socio-economic status: it is possible that our intervention, although sufficient to decrease behavioral problems, was insufficient to enhance prosocial behaviors in a socio-economically disadvantaged population.
Heterogeneity analyses revealed a positive effect of our intervention on teacher-child closeness and positive relationships with peers for children in classes with teachers having low experience. Positive relationships with teachers, including teacher-child closeness, were also positively impacted by intervention for children with low mental health at baseline. Finally, intervention had a positive impact on agreeableness with peers for older children.
Effects of the intervention on emotional processing and executive functioning
We failed to detect effects on task-based measures of emotional processing both in the whole sample and in all subgroups, and on task-based measures of executive functioning that were evaluated in the whole sample only. Concerning executive functioning, previous MBIs and mindfulness-based SEL programs reported improvement in executive functioning in young children using experimental tasks (Crooks et al., 2020; Flook et al., 2015; Janz et al., 2019). However, these effects were detected on smaller samples and tasks that were not the same as the tasks completed in this study. The absence of an effect on emotional processing is unexpected, as our program directly targets competencies that relate to this indicator. Of note, executive functioning and emotional processing outcomes were assessed using experimental tasks only, and methodological issues described in the methodological considerations section below could explain the absence of positive results for these outcomes.
Effects of the intervention on teachers’ well-being
We failed to detect an effect of our intervention on teachers' well-being. It should be noted that these analyses were conducted in the context of a lack of statistical power, as Table 3 shows an ex-post minimum detectable effect size ranging between 0.67 and 0.86. Children's emotional and behavioral problems and teacher stress and negative affect are mutually associated (Narea et al., 2022). Therefore, we could expect that SEL programs directed toward children’s SEC would lead to increased teacher well-being. While positive teacher-child relationships (that can be promoted through SEL programs) have positive impacts on teacher occupational well-being (Nwoko et al., 2023), only a few studies have explored the impact of a SEL program designed for children on teacher well-being. One recent study assessing the impact of the mindfulness-based SEL program OpenMind for preschool children showed no effects on teachers' mindfulness-trait and perceived stress, even if this program included a short mindfulness course for teachers (Jackman et al., 2019).
Methodological considerations
In the present study, effects of intervention were detected using teacher-report questionnaires or family-based standardized estimates integrating teacher-report questionnaires. No significant effects were found using blinded-observation or experimental task measures alone. Different interpretations of this discrepancy can be proposed.
First, one may consider that questionnaire measures are biased, whereas blinded-observations and experimental task measures are not, as teachers in the intervention group may be prone to “reference bias” where they overestimate children's progress following intervention (Boon-Falleur et al., 2022). It is also possible that use of the curriculum led to decreased stress levels for teachers (which were not directly measured in our study) who became more tolerant toward children's problem behaviors (Bockmann & Yu, 2022). In this view, questionnaire-based significant changes may not reflect true changes. On the contrary, one may consider that teacher-reported questionnaires evaluate central tendencies in the behavior of children that are not easily captured by observational tools and tasks, which measure behavior at the time of the experiment or observation, and not the average level of performance over a longer period of time (Boon-Falleur et al., 2022). Recent evidence indeed suggests that behavioral tasks fare poorly compared to standard questionnaires to measure individual differences in behavior (Palminteri & Chevallier, 2018). In the field of SEC, one study found that teacher-rated questionnaires were more accurate predictors of self-regulation and academic outcomes compared to behavioral tasks (Boon-Falleur et al., 2022).
Of note, in the present study, correlations between task-based and questionnaire measures on the one hand, and observational and questionnaire measures on the other were all in the weak range, even when related constructs (for example, negative self-management) were evaluated. In particular, despite evaluating children’s behavior in the same context, inCLASS variables and teacher-rated measures were all weakly correlated. Here, inCLASS measures were all collected on the same day. Future studies should investigate whether adding more time points for data collection for inCLASS measures results in stronger correlations with teacher-rated measures of similar constructs.
Finally, methodological issues may also explain the discrepancy between questionnaire-based and other measures findings. First, in our study design, children that displayed severe behavioral issues were excluded from the blinded observations and experimental tasks protocol. This may have prevented us from capturing the changes occurring in this subgroup of children with these measures. Second, whereas our a priori power calculation was based on an expected MDES of 0.35, all effects detected in this study were below this threshold, indicating that our initial sample should have been increased. Table 3 shows an ex-post minimum detectable effect size for InCLASS measures ranging between 0.37 and 0.50 SD. It may be the case that the intervention has some smaller but meaningful effects on observational outcomes that we are not able to detect with this sample size. Third, and in line with the previous point, several of our task-based and observational measures suffered from attrition, which was particularly severe for the last tasks of our experimental protocol, possibly due to children’s tiredness. This resulted in limited post-hoc power for these measures, which may have prevented us from detecting intervention effects. As an example, in the whole sample, considering the standard errors found for inCLASS positive engagement with tasks, our study was only powered to detect a group difference superior to .45, without correction for multiple testing. Fourth, some of our observational and experimental task measures also suffered from differential attrition, which could have contaminated our findings in an unmeasurable way.
Strengths and limitations
This study displayed several strengths, as it used a RCT design with a pre-published analytic plan and a larger sample size than previous studies of mindfulness-based interventions in preschool-aged children. It included multi-informant reports, with teacher-rated questionnaires, standardized observations by observers blinded to study allocation and objectives, and behavioral tasks. Implementation fidelity was assessed through audio-recordings that were all double-coded by evaluators, and teacher implementation showed great fidelity over the school year. Limitations mostly concerned statistical power for task-based and observational measures, with issues of attrition and differential attrition.
Future directions
Considering the promising results found in this study, this mindfulness-based SEL program deserves further investigations to strengthen its evidence-base.
First, here, treatment-effects were only assessed over a school year, and mental health was evaluated through teacher-rated questionnaires only. Follow-up data is currently being collected to assess the effects of the intervention 6 months after the end of the curriculum using teacher-rated measures. Potential effects of our intervention on academic outcomes will also be assessed for children during the first year of elementary school, using French national evaluations. Future studies should evaluate this curriculum over a longer period, including parent-rated measures of mental health and self-rated measures of psychological well-being in older children, to provide a more complete picture of intervention impact.
Second, in the last decade, a new wave of studies has highlighted the importance of systemic SEL, which involves the whole school and parents in SEL interventions to establish consistent practices across school grades and environments (classroom, whole school, and home) (Liew & Spinrad, 2022; Mahoney et al., 2021). Although the precise added-value of parental involvement in SEL programs is still not clear (Durlak et al., 2011), lack of parental involvement could prevent the generalization of learned strategies outside the school environment. However, to date, only a few studies using mindfulness-based SEL programs have included parental components (Bockmann & Yu, 2022). In the future, sharing digital content related to curriculum activities with parents could be an interesting add-on, as well as training all the other school professionals on related SECs.
Third, in this study, program training was delivered by a research team in a typical RCT efficacy measurement approach. Future studies should investigate the optimal processes to disseminate this intervention in France, involving key stakeholders (Soneson et al., 2022), and evaluate the efficiency and sustainability of the intervention when delivered by local trainers after dissemination (Porzsolt et al., 2015).
Fourth, our intervention was implemented in a socio-economically disadvantaged area. As higher socio-economic status is associated with fewer behavioral difficulties (Poulain et al., 2019), it would be interesting to explore if its effects are similar in French areas with higher socio-economic status. A previous meta-analysis found that behavioral problems prevention programs’ effect sizes were larger for children with low socio-economic status (Wilson & Lipsey, 2007). When considering SEL programs more specifically, reports of differential effects of programs on children from diverse socio-economical family status are inconsistent (Garner et al., 2014; Mondi & Reynolds, 2021). Future studies should also investigate whether program delivery to all children is relevant or whether it should be restricted to schools located in disadvantaged areas.
Finally, although total time dedicated to the curriculum was consistent with our recommendations, there were individual preferences in the implementation of the different activities between teachers, with some teachers implementing more of one type of activity and somewhat neglecting another activity over the course of the year. The given reasons were often teachers’ lack of interest in implementing one activity, or a lack of interest and comprehension difficulties from their students for this activity. These challenges were mostly evoked for the emotion circle time activity. It is possible that this activity, targeting mostly interpersonal skills, was more difficult to implement at the beginning of the year, as previous meta-analysis showed that the most efficient SEL programs are the ones that develop intrapersonal skills before interpersonal skills (Cipriano et al., 2023). However, we lacked the statistical power to calculate if there were disparities in efficacy between each type of activity; i.e, if some activities were more effective than others in improving children’s outcomes, or to try to identify the most active components of our program, which could be an interesting topic for a future study (Bockmann & Yu, 2022).