There is considerable controversy about what causes (in)effectiveness of physician performance pay in improving the quality of care. Using a behavioral experiment with German primary-care physicians, we study the incentive effect of performance pay on service provision and quality of care. To explore whether variations in quality are based on the incentive scheme and the interplay with physicians’ real-world profit orientation and patient-regarding motivations, we link administrative data on practice characteristics and survey data on physicians’ attitudes with experimental data. We find that, under performance pay, quality increases by about 7pp compared to baseline capitation. While the effect increases with the severity of illness, the bonus level does not significantly affect the quality of care. Data linkage indicates that primary-care physicians in high-profit practices provide a lower quality of care. Physicians’ other-regarding motivations and attitudes are significant drivers of high treatment quality.