The KIND Challenge community intervention to reduce loneliness and social isolation, improve mental health, and neighbourhood relationships: an international randomized controlled trial

Loneliness and social isolation are established risk factors for poor health and mortality. Despite widespread implementation of interventions to foster social connection, evidence for their effectiveness remains mixed, with many approaches showing small effects and requiring substantial resources or trained personnel. There is a need for scalable, low-intensity, community-based interventions that can be adopted outside clinical settings. Acts of kindness may strengthen community relationships and reduce loneliness by enhancing social support (emotional, tangible, informational, belonging, and companionship). Prior smaller studies suggest kindness improves wellbeing, relationship quality, and perceived social support, yet its impact as a standalone strategy on loneliness has not been rigorously tested. This study evaluates whether encouraging community members to perform weekly acts of kindness toward neighbours over four weeks can reduce loneliness and social isolation, improve mental health, and enhance neighbourhood relationships across three countries during the COVID-19 pandemic.

Systematic reviews and meta-analyses indicate that clinical interventions can reduce loneliness and improve survival, but with heterogeneous and often small effects, and substantial resource requirements. Community-based interventions are common but seldom rigorously evaluated, partly due to logistical constraints. Digital tools may exacerbate loneliness in some contexts but can also facilitate offline connections and have shown promise in both non-clinical and clinical samples. Experimental evidence indicates that performing acts of kindness can increase general wellbeing; smaller studies suggest benefits for relationship quality, relational functioning, social support, and happiness. The functional components of social support (emotional, informational, tangible, belonging, companionship) align with typical kindness acts and may contribute to reduced loneliness by building social capital, self-esteem, and reducing self-focus. However, the standalone effect of kindness on loneliness has not been examined in large-scale trials. This study addresses that gap by testing a kindness-based community intervention across multiple countries.

Design: Three parallel two-arm randomized controlled trials (USA, UK, Australia) conducted July–September 2020, following CONSORT guidelines and preregistered (ClinicalTrials.gov NCT04398472). Ethics approvals obtained from Swinburne University of Technology (Australia and UK) and Brigham Young University (USA). Participants and Recruitment: Community adults aged 18–90 using the Nextdoor platform were recruited via in-app advertisements. Interested individuals consented online and completed baseline surveys in Qualtrics. Exclusion: insufficient English proficiency. Target n per country was 1452; total n randomized across countries was 4284. Randomization and Blinding: Simple 1:1 randomization via Qualtrics to KIND Challenge (intervention) or waitlist control. Blinding participants was not possible due to intervention nature; research personnel were blind until after data collection. Intervention: Participants in the KIND Challenge were encouraged to perform at least one self-selected act of kindness each week for four weeks, adhering to local COVID-19 safety guidelines. Suggested acts mapped onto social support types: emotional (e.g., check-ins), informational (e.g., local tips), tangible (e.g., errands), belonging (e.g., neighbourhood projects), and companionship (e.g., regular contact). Weekly email reminders supported adherence. Measures: Primary outcome: loneliness (UCLA Loneliness Scale Version 3). Secondary outcomes: social isolation risk (Lubben Social Network Scale), depression (PHQ-8), social anxiety (Mini-SPIN), quality of life (EUROHIS-QOL-8), positive affect (PANAS-SF), stress (PSS-4). Neighbourhood indicators: social cohesion and trust (Social Capital Scale), neighbourhood perception of stability/change and importance, social relationship quality (Social Relationship Index—neighbourhood modified). Bespoke measures assessed neighbourhood conflict, number of neighbourhood contacts, and COVID-19 social restrictions. Statistical Analysis: Country-specific intention-to-treat analyses using Mplus 8.4. Baseline balance assessed per CONSORT. For continuous outcomes, linear regression models with covariates (e.g., gender, age, baseline outcome) and intervention allocation (waitlist as reference). For categorical outcomes, binomial/multinomial regression. Non-normality robust estimators (MLR) used. Missing data handled via FIML multiple imputation, assuming missing at random. Standardized beta coefficients indicated effect magnitude, Cohen’s d estimated pretest–posttest controlled group differences. Sensitivity analyses adjusted for additional explanatory variables (e.g., baseline psychosocial metrics, Nextdoor membership length, baseline neighbourhood measures, and COVID-19 restrictions). Feasibility, safety, compliance, and acceptability were assessed descriptively.

Primary outcome (Loneliness): Significant reductions post-intervention relative to waitlist were observed in the USA (B = -0.06, SE = 0.02, p = 0.003; Cohen’s d = -0.13) and UK (B = -0.05, SE = 0.02, p = 0.026; Cohen’s d = -0.21); no effect in Australia (B = -0.02, SE = 0.02, p = 0.331; Cohen’s d = -0.12). Effects in USA and UK were maintained in sensitivity analyses. Secondary outcomes: - Social isolation: USA intervention participants reported lower social isolation (B = 0.05, SE = 0.02, p = 0.033; Cohen’s d = 0.46; moderate effect). - Social anxiety: Reductions in USA (B = -0.06, SE = 0.03, p = 0.027; d = -0.12) and Australia (B = -0.05, SE = 0.02, p = 0.031; d = -0.01). - Stress: Reduction in Australia (B = -0.05, SE = 0.02, p = 0.031; d = 0.17; small effect). - No significant intervention effects for depression, quality of life, or positive affect. Neighbourhood outcomes: - UK: Higher neighbourhood importance (B = 0.13, SE = 0.05, p = 0.039; OR = 1.62) and greater likelihood of feeling in a stable vs. declining neighbourhood (B = 0.25, SE = 0.11, p = 0.022; OR = 1.97). - Australia: Less likelihood of reporting an aversive neighbourhood (B = -0.20, SE = 0.08, p = 0.011; OR = 0.46); improved social relationship index. - USA: Greater likelihood of reporting no neighbourhood conflict (B = 0.09, SE = 0.05, p = 0.039; OR = 1.62). - USA and Australia: More likely to know six or more neighbours post-intervention (USA: B = 0.09, SE = 0.05, p = 0.017; OR = 1.66; Australia: B = 0.08, SE = 0.04, p = 0.045; OR = 1.47). Feasibility and acceptability: Retention was low (>40% dropout in intervention arm). No adverse events reported. Average number of kind acts completed: USA 3.01, UK 2.78, Australia 2.58. Acceptability ratings exceeded 6/10 across connectedness, meaningfulness, safety, positivity, and comfort.

The RCTs demonstrate that a low-intensity, low-cost kindness-based community intervention can produce small but meaningful short-term reductions in loneliness (USA, UK), social isolation (USA), social anxiety (USA, Australia), and stress (Australia), while improving aspects of neighbourhood relationships. These effects are comparable in magnitude to more resource-intensive interventions and suggest a scalable public health strategy that leverages social support provision and community engagement. Country differences likely reflect varying compliance, platform familiarity, and differing COVID-19 social restrictions during data collection. Autonomy in selecting kindness activities may have enhanced feasibility and impact. Although assessments focused on actors providing kindness, potential reciprocal benefits may extend to recipients, potentially amplifying community-level effects over time. Findings advance loneliness intervention research by shifting focus from therapy/support for recipients to empowering individuals to provide support through everyday acts, which may build social capital and strengthen neighbourhood ties.

This study is the first large-scale international RCT to evaluate a kindness-focused community intervention for reducing loneliness. Encouraging weekly acts of kindness toward neighbours yielded small but significant improvements in social and neighbourhood outcomes and offers a promising, scalable, low-cost approach for public health initiatives addressing loneliness and social isolation. Future research should assess long-term durability, optimal dosage, mechanisms (e.g., reciprocity, belonging, purpose), potential benefits to recipients, and effectiveness outside pandemic contexts and across diverse populations.

- Short follow-up (four weeks) limits conclusions about long-term durability. - Participant blinding was not possible. - Conducted during COVID-19; social restrictions may have constrained the range and frequency of kind acts and influenced outcomes despite statistical control. - Sample may overrepresent older adults and individuals predisposed to community engagement (Nextdoor users), limiting generalizability. - High attrition (~50% from baseline to follow-up); although missingness was handled via multiple imputation assuming MAR, retention remains a concern. - No correction for multiple testing across secondary outcomes; findings should be interpreted with caution regarding Type I error. - Potential recruitment biases inherent in volunteer online samples. - Intervention dosage may have been suboptimal; higher recommended frequency (e.g., five acts/week) was not feasible during early pandemic constraints, possibly attenuating effects.