Investigating the effectiveness of interventions intended to reduce loneliness using psychological strategies and a theory of change: a systematic review of interventional studies and meta-analysis

Abstract Background The adverse effects of chronic loneliness on mental health are well-established. Psychological interventions are more likely to be effective if based on a Theory of Change (ToC) that clarifies intended mechanisms. We aimed to inform development and implementation of psychological interventions for loneliness by synthesising the literature evaluating the effectiveness/cost-effectiveness of psychological strategies with the primary aim of reducing loneliness, specifying their underlying ToC. Methods We searched five electronic databases (CINAHL, PubMed, PsycINFO, Scopus, and Web of Science) from 2016 to August 2023 for studies evaluating the effectiveness and/or cost-effectiveness of psychological interventions that targeted loneliness as a primary outcome and were based on a discernible ToC. We conducted quality appraisal using the Cochrane Risk of Bias tool and used the approach of narrative synthesis to report findings. Results Twenty two studies met eligibility criteria, including 2,567 participants, and ten theoretical approaches: Cognitive behavioural therapy (CBT) (n = 10), mindfulness (n = 4), reminiscence therapy (n = 3), social identity theory (n = 1), behavioural activation (n = 1), self-management of wellbeing theory (SWB) (n = 1), imagined interaction theory (n = 1), Orem’s self-care deficit theory (n = 1), logotherapy (n = 1) and interpersonal therapy (n = 1). All trials were judged to be at some/high risk of bias. None assessed cost-effectiveness. The ToCs for CBT included i) cognitive reappraisal of maladaptive cognitions, ii) promoting social engagement, and iii) developing coping skills to break reinforcing patterns that maintain loneliness. Pooled effect size (g = -0.463,95%CI:[-0.765 to -0.162]) shows CBT moderately reduces loneliness. The ToCs presented for other approaches included targeting self-awareness, improving emotional regulation, and enhancing group belonging. We identified medium strength evidence to support interventions based on CBT, social identity theory, reminiscence therapy, Orem’s self-care deficit theory and logotherapy. We identified very weak evidence to support interventions based on behavioural activation or mindfulness. We found no evidence to support interventions based on SWB theory, interpersonal psychotherapy, or imagined interaction theory. Conclusions We identified a range of theoretical psychological mechanisms of reducing loneliness. Our summary of these psychological approaches and their underlying ToC provides a valuable resource to researchers involved in intervention development. Methodological limitations of included studies guide the design of larger-scale trials.