A colleague and I recently got into a heated discussion over health care spending. It wasn't that he disagreed with me about the need to rein in costs; but he said he was frustrated every time he tried to do so. Earlier that week, for example, he had tried to avoid ordering a costly M.R.I. scan for a patient who had been suffering from headaches. After a thorough examination, my colleague was convinced the headaches were the result of stress.