The investigators would like to find out if Mentalization-Based Therapy (MBT) is effective for people aged 60+ who struggle in relationships. The study aims to understand whether MBT helps older people to build better relationships and feel better about themselves. There is currently no research with people over 60 and MBT, yet MBT is being offered to the older adult population by NHS trusts throughout the UK without evidence for its effectiveness. As well as developing knowledge about how MBT can help at this point in life, the study aims to improve the quality of care offered.

MBT targets mentalization, which is the ability to make sense of one's own and other people's thoughts, feelings, actions and beliefs. Current research suggests that the ability to mentalize changes over the lifespan and may be influenced by many factors, some of which are specific to later life. For example, changes in relationships during later life and biological changes in the brain may impact mentalization systems.

The investigators would also like to understand what difficulties MBT may be effective for in later life. The diagnosis of borderline personality disorder (BPD), which MBT was developed to treat, was, until recently, assumed to disappear with age. However, growing evidence suggests that symptoms change, rather than disappear.

Given these unknowns, the study will use a Hermeneutic Single-Case Efficacy Design (HSCED). Up to six participants, up to six people who know the participants, and clinicians delivering the MBT interventions will be recruited. Data in the form of questionnaires, self-report and therapy documents will be gathered, and everyone will be interviewed. For each participant, the data will be used to compile both an affirmative (yes, MBT was effective) case, and a sceptic case (no, MBT was not effective). Cases will then be reviewed by an adjudication panel comprising one service user expert by experience, one MBT expert and one expert in another therapeutic modality. For each case, each expert will decide if the affirmative or sceptic case was more likely. Finally, findings will be synthesised and used to draw conclusions about the effectiveness of MBT.

The HSCED lends itself to theory-building, as it gathers in-depth data from individuals and facilitates comparison within and between cases. Further, participants' contribution to their own 'rich case record' through change interviews recognises people as taking an active role in their own healing.

Study Design The study will involve single-centre convenience-sampling recruitment from a Later Life MBT Group run within Older Adult Community Mental Health Teams (CMHT) within Derbyshire Healthcare NHS Foundation Trust. The MBT intervention groups have been established for approximately 12 months and constitute an existing referral pathway within the Older Adult CMHT service. Given this, the study takes an observational design, as although participants receive a therapeutic intervention (MBT) the investigators will not assign a specific intervention to participants, rather they will recruit participants from an already established MBT group. The MBT intervention that participants will be receiving is therefore 'treatment as usual'.

The study will use a Hermeneutic Single-Case Efficacy Design (HSCED), an interpretative framework for evaluating causality between processes and outcomes in psychotherapeutic interventions (Elliott, 2001; Elliott, 2002). Based on several of Wall et al.'s (2017) HSCED adaptations, the study will recruit four to six participants (depending on drop-out rate), informants (people who know the participants), and four clinicians delivering the MBT intervention. This will allow for case-wise comparison and provide greater evidence for considering efficacy theory. Data collected will be both quantitative (questionnaires) and qualitative (free-form questionnaires, therapy documents and interviews).

Affirmative and sceptic cases will be developed and reviewed by the research team to ensure a fair presentation of the data. In addition, the HSCED design limits bias by the inclusion of the adjudicational panel, comprising judges who are independent of the research team. One judge will be an expert in MBT, one an expert in another therapeutic modality, and the final judge will be a service user who identifies as an older adult with attachment difficulties, offering the perspective of someone who might be eligible for the MBT intervention. For each participant, each expert will decide if the affirmative or sceptic case was more likely. Finally, findings will then be synthesised and used to draw conclusions about the effectiveness of MBT.

Inclusion Criteria:

Participants - people who:

• Have been referred into and assessed as suitable for a Derbyshire Healthcare NHS Foundation Trust's Later Life MBT Group
• Have persistent (i.e. 1 year+) and pervasive (i.e. multiple relationships) difficulties in relationships and so stand to benefit from the intervention
• Those aged 60+
• Have the ability to complete measures
• Can communicate comfortably in spoken and written English

Informants - people who:

• Are identified by the participant
• Are aged 18+
• The participant considers themselves to have a close relationship with
• Have the ability to complete appropriate measures
• Can communicate comfortably in spoken and written English

Clinicians - people who:

• Are involved in delivering/facilitating the MBT interventions with participants

Exclusion Criteria:

Participants - people who:

• Are undertaking any other psychological intervention
• Do not have the capacity to give informed consent; since capacity is dynamic, this will be assessed by clinicians and the independent outcome assessor on an ongoing basis according to their clinical judgement. If lack of capacity is identified during an interview, clinical judgement would be used around rescheduling and the information would be fed back to the individual's clinical team or escalated as necessary

Informants - people who:

• Do not have the capacity to give informed consent; since capacity is dynamic, this will be assessed by the independent outcome assessor according to their clinical judgement. If lack of capacity was identified during an interview, clinical judgement would be used around rescheduling and the information would be escalated as necessary

Clinicians - people who:

• n/a

Up to six participants will be recruited via purposive convenience sampling from people enrolled on one of Derbyshire Healthcare Foundation Trust's (DHCFT's) Later Life MBT interventions. The MBT group interventions take place in a secondary care outpatient setting.

Once participants have been recruited, they will be asked to invite someone with whom they have a close relationship to participate in the study, known as the informant. Up to six informants will be recruited in this way via purposive convenience sampling.

Up to four clinicians running the Later Life MBT intervention will be recruited via purposive convenience sampling, since they will be the people already running the group.