The overall aim of this observational study is to investigate how individual differences influence the effects of mindfulness meditation to uncover for whom mindfulness is beneficial and for whom it may be harmful. The first objective is to identify the mechanisms underlying the effects of mindfulness meditation on mental health. The second objective is to examine how three candidate factors, namely trauma symptoms, tendency to dissociate, and repetitive negative thinking, influence the effect of mindfulness meditation on mental health.

Adults who enrolled for a Mindfulness-Based Intervention (MBI) at the participating sites (n=120 in total) will be invited to participate. Before the start of the MBI, after half of the sessions, at the end of the MBI and at 3-months follow-up, participants will complete self-report questionnaires. The main outcomes are symptoms of anxiety and depression, quality of life, wellbeing, and adverse effects resulting from the MBI. A subset of participants will be invited for a semi-structured interview after the end of the intervention.

Update December 2024:

Instead of analysing all subsamples recruited from the different sites separately, the investigators will now analyse all participants from all sites jointly. This amendment has two reasons. First, analysing all participants jointly allows to statistically compare the differences in effects across sites by including a variable that indicates from which site a participant was recruited. If the subsamples are analysed separately, the investigators can only compare the results at face-value but cannot determine whether the effects are statistically different across sites. Second, recruitment could not start at one site because of a restructuring of the mindfulness interventions there and the investigators are experiencing recruitment difficulties in a second site (recruited 5 participants within 1.5 years). Thus, it will not be feasible to recruit 120 participants per site. For those two reasons, the investigators decided to analyse all participants jointly and only recruit 120 participants in total for the quantitative part of this study (see updated study protocol).

This project aims to investigate how individual differences influence the effects of mindfulness meditation to gain a first understanding of the personalised effects of mindfulness and uncover for whom mindfulness is beneficial and for whom it may be harmful. In a first step towards a better understanding of the effects of mindfulness mediation on each individual, the investigators aim to identify the mechanisms underlying the effects of mindfulness meditation on mental health and wellbeing (first objective). Based on prior research, the investigators hypothesise that mindfulness meditation exerts its effects via internal awareness, decentering, and non-judgment, but the investigators will also explore other mindfulness skills as potential mechanisms. Knowing the underlying mechanisms will help understand why mindfulness meditation leads to improved mental health in some individuals while it may lead to harm in other individuals. In a second step, the investigators aim to examine specific characteristics of individuals that may influence whether mindfulness meditation has beneficial or possibly harmful effects. Specifically, the investigators aim to examine how three candidate factors, namely trauma symptoms, tendency to dissociate, and repetitive negative thinking, influence the effect of mindfulness meditation on mental health and wellbeing (second objective). Knowing how these individual characteristics influence the effect of mindfulness meditation will clarify for whom mindfulness works best and for whom it may lead to undesired effects.

For both objectives, mental health and wellbeing will be measured using self-report questionnaires to determine the effects of the mindfulness intervention on participants' mental health and wellbeing. To achieve the first objective, the investigators will measure change of different mindfulness skills (the hypothesised mechanisms) with self-report questionnaires across the mindfulness intervention in order to test whether the mindfulness intervention leads to change in mindfulness skills, which in turn leads to change in outcomes. To achieve the second objective, the investigators will measure baseline levels of candidate factors with self-report questionnaires in order to test whether these candidate factors influence in what way the mindfulness intervention affects mental health and wellbeing. Candidate factors are trauma history and symptoms, tendency to dissociate, and repetitive negative thinking. Additionally, the investigators will measure obsessive-compulsive disorder related beliefs as potential candidate factors, as these beliefs influenced the effects of mindfulness in our own clinical practice. Meditation practice-related variables such as frequency, intentions and previous experience will be measured in order to control for potential practice-related effects on mental health.

Inclusion Criteria:

• Enrolled in a mindfulness-based intervention at one of the participating sites

Exclusion Criteria:

• Insufficient knowledge of the Dutch or English language (depending on the study site)
• No internet access

Adults who enrolled for a Mindfulness-Based Intervention (MBI) at one of the participating study sites.

Moderately sensitive information including age, demographic data, all other data coming from questionnaires and interview transcripts will be pseudonymized and shared with other researchers upon request. This dataset will only comprise data of those participants who agreed to their data being shared on the consent form.

The coded, pseudonymized dataset and supporting information (see above) will be available beginning from 3 months and ending 10 years after publication of the research results.

The receiving party will be bound by contractual agreement to keep the transferred data confidential at all times, to only process the data for the purpose of the Study or for improving scientific knowledge about mindfulness and is not permitted to link the data with other data which might render the information more identifiable. To this end, appropriate Data Transfer Agreements (DTAs) will be established.