Mexico’s national forest protection program, Pago por Servicios Ambientales (PSA), is one of the largest Payments for Ecosystem Services (PES) programs worldwide, but its funding has declined sharply recently. This project aims to study whether the program’s cost-effectiveness can be increased by modifying design features. Specifically, we will pilot test the impact of requiring PES participants to enroll all of their eligible forested landholdings ("full enrollment" requirement), relative to the status-quo used by PSA and many other programs that participants can choose which subset of their forest to enroll in the program. We will pilot a full enrollment PSA modified contract among a cohort of PSA applicants who were rejected due to budget constraints in one municipality of Selva Lacandona (Chiapas). We will assess the treatment’s impact on the program take-up rate, avoided deforestation, and participants’ economic well-being.