Esther Mertens

| 65 Effectiveness of a Psychophysical Intervention preferred. Research has shown that participants who benefit from an intervention often show improvement early in the intervention regardless of its time span (e.g., Lutz et al., 2014; Tadić et al., 2010). Hence, the finding that the strongest effects were shown in the first part of the intervention might represent a typical trajectory of intervention effects in general. The decline in intervention effects is possibly related to a decrease in students’ motivation, as motivation is found to be a moderator of intervention effects (Philips &Wennberg, 2014). It could be that the second year of R&W contains too much repetition and not enough deepening of previously learned skills or new topics. Thus, implementing only the first year of the intervention appears to minimize the burden on the students without jeopardizing the effectiveness of the intervention. The second aim was to determine the influence of the width of the intervention’s ecological focus on its effectiveness. The results showed that the condition with the most narrow ecological focus (i.e., only a core team of teachers was involved in the intervention) appeared to be most effective, suggesting that for some interventions “less is more”. This finding is in line with the results of the meta-analysis of Durlak and colleagues (2011) who showed that the positive effect of involving more people and systems in interventions is not always found. Additionally, Taylor, Mumford, Liu, and Stein (2017) examined a school-based intervention with different levels of involved people and found that the intervention effects did not increase as more students and teachers were involved. An intervention with a narrow ecological focus might benefit from trainers’ feelings of responsibility for implementation. In an intervention with a narrow ecological focus only a few people are involved and thus solely responsible for properly and effectively implementing the intervention. In an intervention with a broad ecological focus many people are involved and can share the responsibility for implementation. This diffused responsibility might trigger a ‘bystander effect’. A bystander effect refers to one’s diminished feelings of responsibility to act in a situation when more people are present (Fischer et al., 2011). Perhaps the R&W trainers and other teachers in the Standard and Plus condition had a more passive attitude in the implementation, waiting for the other to act, whereas R&W trainers in the Light condition had a more active attitude in the implementation as they were the only ones that could act regarding the intervention. Future research should focus on the possible influence of bystander effects when responsibility for implementation is shared among a relatively large number of people. Furthermore, interventions with a broad ecological focus may have an increased risk of sending mixed messages to the students due to the large number of people that are involved. These mixed messages could decrease intervention effects. How the intervention lessons should be implemented is explicitly described in the manual (Ykema, 2002; 2018). How the intervention techniques can be applied during regular lessons or at home is not described nor structured. This lack of structure can be especially challenging with techniques based on a physical approach. Hence, outside the intervention lessons the techniques and skills of the intervention can be applied 3