Abstract: Given the asymmetry of information between patients and physicians, and the limited ability of the National Health Insurance to monitor physicians’ decisions regarding diagnostic tests and treatment, the quality of care provided by physicians is difficult to observe directly. Yet their decisions have important implications for public healthcare expenditure. We plan to conduct a field experiment among General Practitioners in the Ile de France region using standardized patients. We will assess how closely physicians’ diagnostic effort, recognition of disease and treatment choices conform to medical guidelines, and whether these measures differ according to patients’ gender or skin color. We will also examine whether the gender gap varies across diseases depending on whether patient gender carries relevant epidemiological information.