Studies comparing the effectiveness of medical treatments have the potential to reduce health care costs by helping purchasers, such as Medicare, eliminate low‐​value services. Health care analysts generally agree that current institutions underproduce comparative‐​effectiveness research, and economists agree that private sector tends to underproduce such public goods. Many, therefore, want Congress to fund such research. But is market failure really the culprit? And would taxpayer‐​funded research solve the problem, or would it lead to government rationing? Or would it have no effect on health care costs?