The use of summer pastures in the European Alps provides much evidence against Hardin's prediction of the tragedy of the commons. For centuries, farmers have kept summer pastures in communal tenure and avoided its overuse with self-designed regulations. During the past decades, however, summer pastures have become less intensely used, which has reduced its agronomic value and the by-production of public goods. However, very little is known about how the various governance incentives affect farmers' use of summer pasture to result in below-sustainable activity. In this study, we develop an empirically informed game theoretical model of farmers' land use decisions, which we validate with survey data from a case study in Switzerland. Our results reveal that farmers weigh the benefit of resource use against the costs of maintaining it and that all major sectoral developments, such as increasing livestock endowment, increasing opportunity costs, and decreasing land use intensity on private plots, result in the reduced use of summer pastures. Based on these insights, we suggest adapting the incentive structure at the local and federal governance levels to increase incentives for stocking at the margin. Our study shows how game theory combines with field validation to identify the contextual behavioral drivers in common pool resource dilemmas for informed and improved policy making.