Previous research suggests that "direct" reforms to the liability system-reforms designed to reduce the level of compensation to potential claimants-reduce medical expenditures without important consequences for patient health outcomes. We extend this research by identifying the mechanisms through which reforms affect the behavior of health care providers. Although we find that direct reforms improve medical productivity primarily by reducing malpractice claims rates and compensation conditional on a claim, our results suggest that other policies that reduce the time spent and the amount of conflict involved in defending against a claim can also reduce defensive practices substantially. In addition, we find that "malpractice pressure" has a more significant impact on diagnostic rather than therapeutic treatment decisions. Our results provide an empirical foundation for simulating the effects of untried malpractice reforms on health care expenditures and outcomes, based on their predicted effects on the malpractice pressure facing medical providers.