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The article of Schueller, Kashdan and Parks (2014) provides us with the opportunity to further clarify some aspects of the design and the choices we made in our meta-analysis "Positive psychology interventions: A meta-analysis of randomized controlled studies" (Bolier et al., 2013). We appreciate their commentary and endorse the useful discussion of defining positive psychological interventions for future meta-analyses. Their main concern is that we were too narrow in our inclusion strategy and should have been more inclusive by integrating effectiveness studies of related areas in positive psychology. In this reply, we argue that our strategy is equally legitimate: science is often a piecemeal effort in which the researcher limits the scope and the research question. Defining the criteria of a positive psychological intervention (PPI) can be done in a broad or a more narrow way. We acknowledge that our meta-analysis has limitations. Limitations are inherent in all meta-analyses, especially when they are published as a journal paper, which limits the scope of any work. That said, the focus of our meta-analysis was based on a conscious choice and we presented a clear description of our search strategy in order to be transparent and produce a replicable review of the literature. 1. Scope and inclusion criteria The aim of our study was to make a meta-analytical overview of positive psychological interventions, that is, those interventions that were primarily aimed at increasing positive feelings, positive cognitions and positive behaviour as opposed to interventions aiming to reduce symptoms, problems or disorders. We took this definition from an earlier meta-analysis of PPIs (Sin & Lyubomirsky, 2009). In defining inclusion criteria, we opted to include only those studies that examine interventions that have been explicitly developed in line with the theoretical focus of positive psychology. This is usually reported in the introduction section of an article. We were not so strict that the exact phrase ‘positive psychology’ should be used. According to our criteria, a focus on wellbeing enhancement or positive aspects of human functioning should be made explicit in the article, as opposed to a focus on problems and disease. For example, in the article of Fava, Rafanelli, Cazzaro, Conti and Grandi (1998) on the effectiveness of wellbeing therapy, 'positive psychology' is not mentioned, but they remark in the introduction: “The route of recovery lies not exclusively in alleviating the negative, but in engendering the positive” (p. 475). Therefore, this study was included (as it also met our other inclusion criteria). We agree with Schueller and colleagues (2014) that to differentiate between PPIs and other interventions, an intervention should meet both a goal criterion and a pathway criterion. By focussing on the above-mentioned criterion, both goal and pathway criteria were guaranteed in the selection of studies in our meta-analysis. Looking at the bowl of fruit or focussing on the apples? Bolier, Haverman, Westerhof, Riper, Smit, & Bohlmeijer www.internationaljournalofwellbeing.org 100 However, not all 'pathways' were included, which was a conscious choice and was explained in the 'selection of studies' section of our meta-analysis. We tried to learn from the meta-analysis of Sin and Lyubomirsky (2009). The Sin and Lyubomirsky meta-analysis has value in its own right: it gives a first, broad, but obviously not complete, overview of the effectiveness of PPIs. They decided to include a number of studies (not all) on mindfulness and reminiscence and life review, while at the same time not including other interventions like Acceptance and Commitment Therapy or the values affirmations interventions, which Schueller et al. (2014) suggest as an intervention of choice. We decided not to include interventions that were developed from a different theoretical background, even though their commonalities with and differences from positive psychology have been spelled out in recent years (e.g. Kashdan & Ciarrochi, 2013). Reminiscence and life review stem from psychogerontology (see Westerhof & Bohlmeijer, 2014 for a recent review). Mindfulness stems from Buddhist traditions whereas Acceptance and Commitment Therapy is positioned as a third wave of cognitive behavioural therapy. In this way, the included interventions are much more homogeneous as they are derived from the same theoretical stream within psychology. Another reason why we did not include interventions that use all kinds of positive psychological pathways – actually supporting the reasoning that they were developed from other traditions – is that they have been meta-analytically reviewed in their own right, as stated in our study. For example, Pinquart and Forstmeier (2012) conducted a meta-analysis of 128 studies on reminiscence and life review and showed that these interventions were not only successful in alleviating depressive symptoms, but also in enhancing positive psychological themes like wellbeing, meaning in life, and ego-integrity. It would not be conducive to furthering the field if we were to merely summarise the evidence already (superbly) summarised in Pinquart and Forstmeier’s study. Similar arguments can be made for other domains that have similarities with positive psychology, such as forgiveness therapy. We thus decided to make a further delineation by excluding those areas that have already been extensively reviewed. This is not a wrong choice, as there is a conscious and grounded strategy being applied: it is just a different approach to that proposed by Schueller and colleagues (2014). A separate issue that Schueller et al. (2014) raise is the inclusion in our meta-analysis of an old study: Lichter, Haye and Kammann (1980), while they assume that we included only studies from 1998 and further. This assumption is wrong: we searched for studies from 1998 in different databases, but being published in or after 1998 was not an inclusion criterion. The references in the meta-analysis of Sin and Lyubomirsky that were considered positive psychology interventions were also a starting point. From that meta-analysis, in which several studies from before 1998 were included, we selected only study 2 of Lichter et al. (1980) because study 1 uses an intervention that challenges irrational beliefs based on cognitive therapy, an intervention that did not fit our definition criterion. 2. Lower pooled effect sizes A finding in our meta-analysis was that the overall effect size was lower than in the meta-analysis of Sin and Lyubomirsky (2009). Schueller et al. (2014) rightly stated that this may be due to our specific selection of studies. For example, the effect sizes for studies examining gratitude interventions that were included in our meta-analysis were much lower than the effect sizes for studies investigating forgiveness therapy that were included by Sin and Lyubomirsky. This could have resulted in a lower overall effect size. Schueller et al. did not mention a second important factor that might have influenced the effect size in our study. We included only studies that were randomized at the individual level. Individual randomization is one important Looking at the bowl of fruit or focussing on the apples? Bolier, Haverman, Westerhof, Riper, Smit, & Bohlmeijer www.internationaljournalofwellbeing.org 101 criterion for the quality of a study design. This is common practice in conducting meta-analyses, but may have lowered the pooled effect size in our meta-analysis. The effects of psychotherapy for adult depression, for instance, might have been overestimated in earlier studies, because the higher quality of later studies coincided with lower effect sizes (Cuijpers, van Straten, Bohlmeijer, Hollon, & Andersson, 2010). This finding might also apply for positive psychology and was indeed corroborated in our analysis of the moderating effects of study quality for the outcome depression. For example, the famous first happiness intervention studies (Fordyce, 1977; 1983) were excluded from our meta-analysis for this reason, as were the studies of MacLeod (2008). These studies have rather high effect sizes, but score low on methodological rigour. This methodological criterion, the inclusion of only randomized controlled trials, also partly explained why 'only' 19 studies in 17 articles (not 16 studies as Schueller et al., 2014 have stated) overlapped in the Sin and Lybomirsky meta-analysis and our meta-analysis. Schueller and colleagues (2014) question the finding that less intensive self-help interventions were less effective than more intensive face-to-face interventions. However, it is in line with the finding of Sin and Lyubomirsky that self-administered interventions are less effective. This is, moreover, supported by reviews of online self-help interventions that are grounded in positive psychology (Bolier & Martin Abello, 2014; Mitchell, Vella-Brodrick, & Klein, 2010). It is questionable if this result would have been different if a more comprehensive inclusion strategy had been applied. 3. Best possible self Schueller and colleagues' (2014) comments also concern the supposed incomplete status of interventions that were included in our meta-analysis. They set an example with the 'Best Possible Self' (BPS) intervention. We indeed included studies that examined the BPS intervention (Boehm, Lyubomirsky & Sheldon, 2011; King, 2001; Layous, Nelson, & Lyubomirsky, 2013; Lyubomirsky, Dickerhoof, Boehm, & Sheldon, 2011; Peters, Flink, Boersma, & Linton, 2010; Sheldon & Lyubomirsky, 2006). Other studies were excluded or did not emerge in our search strategy (Austenfeld et al., 2006; Austenfeld & Stanton, 2008; Hanssen, Peters, Vlaeyen, Meevissen, & Vancleef, 2013; King & Miner, 2000; Meevissen, Peters, & Alberts, 2011; Oyserman, Bybee, & Terry, 2006; Peters, Meevissen, & Hanssen, 2013). We now look at these studies in more detail: 1) Both the Austenfeld et al. (20

Looking at the bowl of fruit or focussing on the apples? Reply to the comments of Schueller et al.