Considering the high prevalence of couple distress in Western countries and rising divorce rates, there is a pressing need for accessible and evidence-supported interventions that can strengthen relationship functioning. Although mindfulness has been positively linked to relationship functioning, only few studies have investigated the effects of mindfulness interventions specifically targeted at couples. However, these studies suffer from methodological limitations such as lack of an active control group.

To address this gap, a feasibility trial with progression to a full-scale randomized controlled trial (RCT) will be conducted, depending on whether stoppage criteria are met, to evaluate the effectiveness of a Mindfulness-Based Couple Intervention (MBCI) compared to an online active control intervention (Self-expansion Intervention) and waitlist control group. Moreover, consolidation of intervention effects is examined up to 6 months post-intervention. Finally, possible working mechanisms and the effect of potential moderators are examined.

The current study has three arms: (i) MBCI, (ii) active control intervention (i.e., a self-expansion program) and (iii) waitlist control, with assessments at baseline (T0), immediately post-intervention (T1) and at 6 months post-intervention (T2). After the follow-up assessments, waitlisted participants are offered to take part in (an online version of) either the MBCI or the active control intervention.

The study aims to include 213 couples (426 participants, age >20 years) with a minimum relationship duration of 2 years. Couples will be randomly assigned with a 3:3:2 ratio to MBCI, Self-expansion or waitlist, respectively (with a total of 80 couples in the intervention groups and 53 couples in the waitlist control group). This is a multi-site RCT, with five different centers spread across the Netherlands, based in Nijmegen, Tilburg, Amsterdam, Maastricht and Groningen. In the feasibility phase, 42 eligible couples will be recruited from two sites (Nijmegen and Tilburg). Stratified variable block randomization will be used(block sizes of 8 and 16), with study site as stratification factor.

The MBCI is a close adaptation of the widely used and well-researched Mindfulness-Based Stress Reduction program (MBSR), developed by our research team. Couples learn how to practice mindfulness meditation and how to apply mindfulness in their daily life and in their relationship. As compared to the MBSR, it includes adapted and additional exercises to practice mindfulness within the relationships, such as mindful communication exercises, mindful touch, and mindfully viewing one's partner. The active control intervention encourages couples to invest in self-expansion, by doing a novel activity together every week. This intervention is based on self-expansion theory. According to this theory, couples are satisfied with their relationship as long as they experience a sense of self-expansion. A sense of self-expansion can be promoted by engaging in novel activities together and gaining new insights from each other. Both the mindfulness- and self-expansion interventions consist of 8 weekly sessions, plus a silent day (for MBCI) or a day out (for the self-expansion intervention). Interventions are similar in non-specific factors such as format, structure, time commitment, teacher support, and group dynamics.

The primary outcome is to compare self-reported relationship satisfaction as measured by the Couple Satisfaction Index (CSI-4) from pre-intervention (T0) to immediately post-intervention (T1) between the MBCI group compared to the active control group and between the MBCI group compared to the waitlist control group. Secondary outcomes include the difference in relationship satisfaction from pre-intervention (T0) to 6 month post-intervention (T2) between participants in the MBCI compared to the active and waitlist control group, as well as differences between participants in the MBCI compared to the active and waitlist control group in various relationship behaviors, perceptions of the partner, and individual outcomes measured from pre-intervention (T0) to immediately post-intervention (T1) and pre-intervention (T0) to 6 months post-intervention (T2). In addition, a number of potential moderators, including commitment, attachment style and trait mindfulness measured at T0 will be assessed to determine for whom these programs work.

Unless the trial team determines, based on stoppage criteria, that trial procedures need important modifications thereby re-setting the full-scale trial as a new trial, the outcome data of the participants in the feasibility portion will be utilized in the analyses of the full-scale trial.

Background and objectives

A satisfying romantic relationship strongly predicts mental and physical health, supporting emotion regulation, life satisfaction, and healthy lifestyles. In contrast, chronic relationship distress is linked to anxiety, depression, stress, and negative effects on work and children's wellbeing. Given its high prevalence and rising divorce rates in Western countries, accessible, evidence-based interventions are needed.

Mindfulness is associated with improved relationship outcomes (e.g., satisfaction, acceptance, closeness), but research is limited by methodological weaknesses and a lack of insight into underlying mechanisms. Although mindfulness-based interventions are effective for individual symptoms like depression, anxiety, and stress, few studies have examined their effects in couples.

To address this gap, a feasibility trial with progression to a full-scale randomized controlled trial (RCT) will be conducted, depending on whether stoppage criteria are met, to evaluate the effectiveness of a Mindfulness-Based Couple Intervention (MBCI) to enhance relationship satisfaction compared to an active control intervention (i.e., self-expansion intervention) and waitlist control group. Furthermore, outcomes will be assessed at 6 months post-intervention and investigate possible working mechanisms and potential moderators.

Outcome measures described below will all be measured with questionnaires. Notably, the trial also involves additional methodologies, such as a computer-based task, Experience Sampling Methodology (ESM), and a conflict study utilizing physiological measures and behavioral observations. These will not be described further in the current registered report, as they are intended to elucidate the working mechanisms of the interventions rather than to evaluate the general effectiveness of the MBCI. All of these additional sub-studies will be preregistered prior to data analysis.

Recruitment

This is a large-scale multi-site RCT, with five different centers spread across the Netherlands, based in Nijmegen, Tilburg, Amsterdam, Maastricht and Groningen. The study aims to include a total of 213 couples (426 participants, >20 years) with a minimum relationship duration of 2 years. In the feasibility phase of the study, 42 eligible couples will be recruited from two sites (Nijmegen and Tilburg). Participants will be recruited via social media, flyers in public spaces, adds in local newspapers and magazines and through our own network. Interested couples can send their contact details via a Qualtrics form, after which they will be contacted to schedule a screening interview and send a short pre-screening questionnaire on intimate partner violence (which is an exclusion criterium). In the screening interview, participants will be given more information about the study and the interventions and assess eligibility. If couples are eligible and willing to participate, they will be sent the information letter and informed consent form.

Progression from feasibility to full-scale trial

In the feasibility phase of the proposed study, the trial team will collect data to improve several aspects of the trial's processes. At the completion of the feasibility portion, the trial team plans to proceed directly to the full-scale trial, including results from the feasibility stage in the full-scale trial, unless one or more of the stoppage criteria are met. In that case, the trial team will address issues that are identified and begin the full-scale trial anew, separate from the feasibility trial.

The stoppage criteria are the following:

Recruitment and enrolment of trial participants: If the study fails to enroll sufficient eligible participants to fill the three arms within the two months prior to start of the intervention programs. The study will need at least 15 couples (30 participants) at each location (i.e., Tilburg and Nijmegen) in order to have a minimum of five couples in every intervention group, which is an acceptable group size to deliver the interventions to. The trial team will discuss whether it is feasible and sufficiently productive to improve the recruitment strategy and if so, adapt the recruitment strategy accordingly and progress to the full-scale trial. If not, the trial team will consider eliminating the active control intervention (i.e., proceeding with a two-arm trial) and progress to the full-scale trial without data from participants assigned to the active control intervention.

Fidelity of the delivery of the intervention: All sessions of both interventions will be recorded, such that only the teacher is visible. Per location (i.e., Tilburg and Nijmegen), recordings of two sessions of each intervention (25%) will be randomly picked for evaluation. For the MBCI, the MBI-TAC will be used and an additional adherence checklist specifically developed to assess the relationship-related and adapted components of the intervention. For the self-expansion intervention, an adherence checklist will be developed. Two researchers will independently assess if the components of the sessions were delivered according to the pre-specified programs. If more than 20% of the components are not delivered as specified. The trial team will seek to identify barriers to the delivery of the intervention, and modify how teachers are trained prior to delivering the interventions or the interventions themselves accordingly, before proceeding to the full-scale trial. After completion of the data collection, a sensitivity analysis will be conducted to evaluate the influence of including feasibility data in our final analyses.

Intervention drop-out: If more than 20 percent of participants assigned to the MBCI or active control program drop out of the intervention. Drop-out is defined as completing less than five sessions (including the full-day session). The trial team will investigate to identify reasons why participants drop out and will discuss if the intervention needs to be modified accordingly, before proceeding to the full-scale trial. In case the intervention needs to be substantially modified, the full-scale trial would need to commence as a new trial.

Measurement drop-out: If more than 20 percent of the post-intervention measurements of the primary outcome of the study are missing. Additionally, missing data on all other outcome measures will be kept track of (although these are not stoppage criteria). The trial team will investigate to identify reasons why participants drop out of the measurement(s) or why there is missing data and will modify or eliminate the particular measurement(s) accordingly, before proceeding to the full-scale trial. The trail team will need to commence anew with adapted measurement(s) in the full-scale trial or exclude the measurement(s) from the full-scale trial in case it needs to be eliminated.

At the end of the feasibility trial, the investigators will use the collected information and evaluation of the stoppage criteria to decide whether to: (1) Continue to the full-scale RCT without modifications to the trial procedures, (2) Modify trial procedures based on participant and teacher feedback before continuing to the full-scale trial, or (3) Modify the trial procedures before re-setting them and conducting the full-scale RCT as a new trial (in this case, outcome data collected during the feasibility phase will not be included in the analyses of the full-scale trial).

Randomization

After baseline assessment, couples will be randomly assigned with a 3:3:2 ratio to one of three groups (with a total of 80 couples in the intervention groups and 53 couples in the waitlist control group). The research team will use block randomization with block sizes of 8 and 16 and stratified by site. Randomization will be done in Castor EDC, which has full randomization concealment.

Interventions

The RCT includes three arms: treatment, active control and waitlist control. Waitlisted participants are offered the option to follow an online version of either the MBCI or the active control intervention after completion of the follow-up measurements. In the RCT, both the MBCI and self-expansion intervention consist of eight weekly group sessions and are designed to be comparable in non-specific factors such as structure, time commitment, teacher support, group dynamics, and expectations. Both interventions were designed by our research team and were modified according to feedback from couples taking part in a pilot of the intervention, (for preregistration of the pilot see: https://osf.io/3c4a6/overview?view\_only=6e19f4a6303f426fa3229a508546c999 ).

Outcome measures

Outcome assessment for the trial will occur at baseline (T0), immediately post-intervention (T1) and 6 months post-intervention (T2). The primary outcome is the difference in self-reported relationship satisfaction as measured by the Couple Satisfaction Index (CSI-4) between participants in the MBCI compared to the active and waitlist control group from pre-intervention (T0) to immediately post-intervention (T1). Secondary outcomes include the difference in relationship satisfaction from pre-intervention (T0) to 6 months post intervention (T2) between participants in the MBCI compared to the active and waitlist control group, as well as the difference between participants in the MBCI compared to the active and waitlist control group in various relationship behaviors, perceptions of the partner, and intrapersonal outcomes measured at from pre-intervention (T0) to immediately post-intervention (T1) and from pre-intervention (T0) to 6 months post-intervention (T2). In addition, a number of potential moderators will be assessed to determine for whom these programs work best. Notably, the trial also involves additional methodologies, such as a computer-based reaction time task, Experience Sampling Methodology (ESM) and a conflict study utilizing physiological measures and behavioral observations. These will not be described further in the current registered report, as they are intended to elucidate the working mechanisms of the interventions rather than to evaluate the general effectiveness of the MBCI. All of these additional sub-studies will be preregistered prior to data analysis.

Data analysis

Consistent with the feasibility trial design and small sample size, no hypothesis tests are planned for the feasibility portion of the trial. Instead, the investigators will present a description of the feasibility elements, including recruitment and eligibility of the participants, and numbers and percentages of participants who completed the intervention and responded to the different measures.

Only the main statistical analyses will be described below. A more detailed statistical analyses plan will be uploaded before analyzing the data. In the full-scale trial, primary analyses will be run in an intent-to-treat sample. Two linear mixed-effects models will be run to assess differences in treatment effects from pre-intervention to immediately post-intervention to 6 months post-intervention, using the lmer function in lme4. One model will estimate the difference between MBCI and the waitlist control group, and the other between MBCI and the active control group. Random intercepts and random slopes will be included for couples and random intercepts for individuals within a couple. The timepoints will be entered as categorical variables, and estimated marginal means will be used to assess group differences at each time point, specifically to determine whether any treatment effects emerging between pre-intervention and immediately post-intervention are maintained or further enhanced at the 6 months post-intervention measurement. By using time as a categorical variable, no assumptions will need to be made on whether to expect the hypothesized linear effect from pre- to post-intervention to continue or to stabilize at 6 months post-intervention. The primary analysis would concern the fixed effect of the time x condition interactions from pre- to immediately post-intervention (MBCI compared to waitlist and MBCI compared to control). Stratification variable (i.e., 'site') will be added as a covariate. Effect sizes will be reported as standardized mean differences (Cohen's d) with 95% confidence intervals (CIs). Similarly, for secondary outcome measures a linear mixed-effects model will be employed, using the lmer function in lme4 to estimate differences in treatment effects from pre-intervention to immediately post-intervention to 6 months post intervention between MBCI and active control, and separately between MBCI and waitlist control, with time as a categorical variable. The investigators will not adjust for multiple analyses since a single primary outcome was identified a priori. All outcome analyses will be conducted in R. All analyses will be 2-sided with alpha = 0.05.

Mediation and moderation

To examine potential mechanisms underlying intervention effects on relationship satisfaction, additional mediation analyses will be conducted focusing on psychological processes theorized to be specifically targeted by the mindfulness intervention. The following mediators will be examined: changes in rumination, emotion regulation, stress trait mindfulness, and self-expansion. To examine whether intervention effects differ as a function of individual and relationship differences, moderation analyses will be conducted using pre-intervention measures of commitment, attachment style and trait mindfulness. Each moderator will be added separately to the primary analysis. The primary test of moderation will be the three-way interaction between time, treatment and moderator, indicating whether changes in relationship satisfaction differ between intervention conditions as a function of the moderator. These mediation and moderation analyses are considered secondary and exploratory, and results will be interpreted cautiously with respect to multiple comparisons.

Inclusion Criteria:

• age >20 years
• relationship duration of >2 years

Exclusion Criteria:

• One or both partners have followed an MBSR- or MBCT training or similar mindfulness program for at least four sessions in the past three years.
• One or both partners are currently in relationship therapy.
• One or both partners are unable to sufficiently speak and understand the Dutch language.
• Presence of severe mental disorders in one or both partners, namely addiction, suicidality (in the past 12 months) or psychosis (in the past 12 months?).
• Presence of partner violence (in the last 12 months)

Upon request

12 months after the collection of the primary outcome

Approval of proposed purpose for data access