Globally, dramatic declines in wildlife populations continue due to anthropogenic pressures. Many of these declines are linked to unsustainable human behaviours. In developing countries, community-based conservation programs are essential for arresting these declines. However, many programs are not effective and require substantial improvements if they are to deliver sustainable conservation outcomes. The social sciences, including conservation psychology, provide an opportunity to understand the social dimension of conservation problems and improve the effectiveness of community-based conservation programs. The Sumatran orangutan (Pongo abelli) is a suitable case study for this approach. The species is critically endangered due to habitat loss and degradation, the illegal wildlife trade, hunting, and human-wildlife conflict. The future of the species relies heavily upon effectively engaging with local communities to support the conservation of the orangutan and its habitat.This thesis aims to inform community-based conservation strategies that effectively incentivise and motivate local communities in the context of developing countries to adopt conservation behaviours. To achieve this, I ask: What are the socio-psychological factors and processes that determine how community-based conservation programs influence behaviour change and community support? How can this knowledge be utilised to more effectively design, implement and manage community-based conservation programs? The thesis addresses these research questions using a socio-psychological approach to investigating secondary data from developing countries and also by conducting a comparative case study of community-based conservation programs for the Sumatran orangutan. This research is the first in-depth socio-psychological investigation for Sumatran orangutan community-based conservation programs.First, I conduct a realist synthesis focused on community-based conservation programs in developing countries that measured changes in community behaviour in relation to conservation objectives. A realist synthesis identifies the critical mechanisms operating within a program, the outcome caused by this mechanism and how the context affects these mechanisms. The synthesis highlights three main mechanisms that explain the reasoning of individuals to engage in conservation behaviours: 'conservation livelihood provides economic value,' 'conservation provides benefits that outweigh losses of curtailing previous behaviour' and 'local authority over resources creates empowerment.' The success of each mechanism was affected by various contexts including: relative significance of income, capacity and cultural acceptability. The findings from the synthesis advance the understanding of the decision-making processes of communities subject to iii community-based conservation programs and highlight how different contexts alter the reasoning process.Second, utilising the comparative case studies, I investigate the effect of differing motivations on community's decisio...