Primed by the ongoing political and academic debate about rising health costs and the over-provision of medical care in the developed world, this research project strives at gaining an in-depth understanding about how the provision of social comparison information influences physician behavior, adherence to clinical guidelines and quality of care. To this end, we are planning to run a pragmatic randomized trial in which general practitioners (GPs) from the universe of all GPs active in the outpatient sector in Switzerland are randomly assigned to either an intervention group receiving social comparison letters or the control group (no letter). The information in the social comparison letters is extracted from routinely collected insurance claims data and contains information about the health care costs or practice style behavior (e.g. vaccination rate, generic drug use) of a GP in comparison to peers with a comparable risk pool of patients. Previous studies have shown that peer comparison information has the potential to significantly influence individual decision-making in various contexts thereby motivating the extension to the proposed physician population (e.g. Meeker et al., 2016; Allcott & Rogers, 2014).