We empirically study personal norms of sustainability, conceptualized according to the norm-activation theory and operationalized under the notion of strong ecological-economic sustainability. Our case study is commercial cattle farming in semi-arid rangelands of Namibia, a system that is subject to extensive degradation. Using survey data, we characterize farmers’ personal ecosystems and income norms, study their determinants, and analyze their impact on actual management based on the dual-preferences model. We find that ecosystem and income norms are heterogeneous across farmers and independent from each other. Furthermore, farmers with better environmental and financial farm conditions have more demanding norms. We find no evidence for a significant impact of norms on actual management, which provides an explanation for the observed degradation of the system.