Invitation architecture and inequitable access to health research: A large cross-sectional study of 43,005 UK adults

Background Under-representation in health and social care research threatens the validity, equity and legitimacy of evidence. Although individual barriers to participation are well described, less is known about how structural exposure to research opportunities, particularly healthcare-mediated invitation, relates to participation. We examined sociodemographic, motivational and exposure-related factors associated with participation, satisfaction and prior refusal among UK adults. Methods We conducted a cross-sectional online survey of 43,005 UK adults aged ≥ 16 years between July 2024 and June 2025 using non-probability digital recruitment. Multivariable logistic regression examined prior participation and refusal, and ordinal logistic regression examined satisfaction among previous participants. Missing covariate data were addressed using multiple imputation by chained equations. Geographic variation was assessed across UK regions. Results Overall, 39.4% reported previous participation and 71.9% of previous participants were somewhat or extremely satisfied; only 16.2% recalled healthcare-mediated invitation. Invitation showed the strongest association with participation (aOR 8.63; 95% CI 8.03–9.27). Perceived benefit was associated with greater participation (aOR 1.79; 95% CI 1.54–2.07), greater satisfaction (aOR 1.61; 95% CI 1.30–1.99), and lower odds of refusal (aOR 0.76; 95% CI 0.63–0.91). University education (aOR 1.96) and previous work in health, care or research (aOR 1.67) were also associated with participation. Black respondents had lower odds of participation than White respondents (aOR 0.75; 95% CI 0.61–0.92), while Asian, Mixed and Other ethnic groups reported lower satisfaction and higher odds of refusal. Participation and refusal varied across UK regions. Conclusions Healthcare-mediated invitation was more strongly associated with research participation than motivational or sociodemographic characteristics. Differential exposure to structured invitation pathways may contribute to inequalities in research engagement. Strategies to broaden participation should address not only willingness and awareness, but also how opportunities are distributed through healthcare and research systems. Trial registration Not applicable. This study was a cross-sectional observational survey and did not involve assignment of participants to a health-related intervention.