BACKGROUND: Efficient, equitable health research depends on understanding why people decide to take part. The aims of this overview were to update the version published in 2020, identifying psychosocial influences on participation and mapping them to recruitment research and psychological theory. METHODS: Searches were undertaken in February 2024. Qualitative, quantitative, and mixed-methods systematic reviews were identified, without language or date limits. Methodological quality was rated using AMSTAR-2, and low-quality reviews were excluded. Barriers and facilitators were identified inductively and mapped to the Theoretical Domains Framework (TDF) and COM-B model, and to empirical recruitment research. RESULTS: The update included 70 reviews, including 44 new reviews, covering a breadth of populations and settings, and drawing on 1940 primary studies (1428 unique). We identified 15 facilitators, most commonly: altruism, potential for personal benefit and trust. Incentives and convenient, low-burden research were also facilitators. Another 10 facilitators were new to this update. There were 16 barriers, most commonly: perceived risk, practical difficulties, and distrust of researchers. Many barriers applied to specific designs, particularly randomised trials. Factors that were barriers or facilitators include the influence of others and information quality. Barriers and facilitators were coded to the Motivation and Opportunity components of the TDF, particularly knowledge and social influences; only two factors were coded to a Capability. Psychosocial influences and empirical recruitment research had some overlap, but some barriers and facilitators had not been evaluated. CONCLUSIONS: Common barriers and facilitators to research participation were identified, some new to this update, which could be addressed through targeted recruitment strategies to increase the efficiency and generalisability of primary research. Factors affecting participation are not only personal; they are also normative and social. The priorities are to change the ways we recruit to research (perhaps tested in SWATs) and identify barriers and facilitators in areas not well covered in current research. TRIAL REGISTRATION: PROSPERO CRD42017062738. Registered on April 2017.