Agriculture is a main source of environmental degradation and biodiversity decline. We investigate the causal effects of culture on pro-environmental behaviors of the agricultural population (farmers), and how policy instruments interact with culture to influence individual behavior. We exploit a unique natural experiment in Switzerland, which consists of two parts. First, there is an inner-Swiss cultural border between German- and French-speaking farmers who share the same natural environment, economy, and institutions, but differ in their norms and values. Second, in 2014, there was an unexpected and vast agri-environmental policy reform that increased the monetary incentive to enroll land into biodiversity conservation. Using a spatial difference-in-discontinuities design and panel data of all Swiss farms between 2010 and 2017, we show the following findings: Before the reform, farmers on the French-speaking side of the cultural border systematically enrolled less land into biodiversity conservation, compared to the German-speaking side. With increased monetary incentives following the 2014 policy reform, the French-speaking farmers enrolled more additional land than the German-speaking farmers, shrinking the discontinuity. These findings indicate that cultural effects on pro-environmental behaviors are more important when external incentives are relatively low, and with increased economic incentives, cultural differences become less important. We discuss the implications for research and especially for policy.