Understanding the behavioural factors that influence farmers' intentions to participate in agri-environmental schemes is crucial for delivering sustainability in agricultural landscapes. Drawing on a qualitative synthesis approach, we seek to understand the underlying motivations behind farmers' decisions to engage with individual as well as collective agri-environmental schemes. We systematically map qualitative evidence on behavioural factors in farmers' decision-making using an expanded Theory of Planned Behaviour framework, incorporating trust and legitimacy elements. Our analysis highlights the role of farmer attitudes in individual schemes. Subjective norms influenced by the farming community, and trust in policy-making processes, were crucial factors determining participation in collective schemes. Normative legitimacy, contract complexity and inflexibility, as well as financial and non-financial outcome beliefs, were key barriers to participation in both types of schemes. Based on our findings, we recommend prioritizing interventions that foster institutional and relational trust. Low levels of trust are linked to barriers caused by subjective norms in both collective and individual schemes. Creating opportunities for social interactions and learning can be essential to foster social capital and trust. Policy development should acknowledge the potential relevance of the broader community context in shaping farmer's attitudes, and particularly its relevance in overcoming barriers linked to cognitive legitimacy, to improve both individual and collective participation in agri-environmental schemes.