On a spring afternoon several years ago, Evan McKinley was hiking in the woods near Halifax, Nova Scotia, when he felt a sharp pain in his chest. McKinley (a pseudonym) was a forest ranger in his early forties, trim and extremely fit. He had felt discomfort in his chest for several days, but this was more severe: it hurt each time he took a breath. McKinley slowly made his way through the woods to a shed that housed his office, where he sat and waited for the pain to pass. He frequently carried heavy packs on his back and was used to muscle aches, but this pain felt different. He decided to see a doctor.

Pat Croskerry was the physician in charge in the emergency room at Dartmouth General Hospital, near Halifax, that day. He listened intently as McKinley described his symptoms. He noted that McKinley was a muscular man; that his face was ruddy, as would be expected of someone who spent most of his day outdoors; and that he was not sweating. (Perspiration can be a sign of cardiac distress.) McKinley told him that the pain was in the center of his chest, and that it had not spread into his arms, neck, or back. He told Croskerry that he had never smoked or been overweight; had no family history of heart attack, stroke, or diabetes; and was under no particular stress. His family life was fine, McKinley said, and he loved his job.

Croskerry checked McKinley’s blood pressure, which was normal, and his pulse, which was sixty and regular—typical for an athletic man. Croskerry listened to McKinley’s lungs and heart, but detected no abnormalities. When he pressed on the spot between McKinley’s ribs and breastbone, McKinley felt no pain. There was no swelling or tenderness in his calves or thighs. Finally, the doctor ordered an electrocardiogram, a chest X-ray, and blood tests to measure McKinley’s cardiac enzymes. (Abnormal levels of cardiac enzymes indicate damage to the heart.) As Croskerry expected, the results of all the tests were normal. “I’m not at all worried about your chest pain,” Croskerry told McKinley, before sending him home. “You probably overexerted yourself in the field and strained a muscle. My suspicion that this is coming from your heart is about zero.”

Early the next evening, when Croskerry arrived at the emergency room to begin his shift, a colleague greeted him. “Very interesting case, that man you saw yesterday,” the doctor said. “He came in this morning with an acute myocardial infarction.” Croskerry was shocked. The colleague tried to console him. “If I had seen this guy, I wouldn’t have gone as far as you did in ordering all those tests,” he said. But Croskerry knew that he had made an error that could have cost the ranger his life. (McKinley survived.) “Clearly, I missed it,” Croskerry told me, referring to McKinley’s heart attack. “And why did I miss it? I didn’t miss it because of any egregious behavior, or negligence. I missed it because my thinking was overly influenced by how healthy this man looked, and the absence of risk factors.”

Croskerry, who is sixty-four years old, began his career as an experimental psychologist, studying rats’ brains in the laboratory. In 1979, he decided to become a doctor, and, as a medical student, he was surprised at how little attention was paid to what he calls the “cognitive dimension” of clinical decision-making—the process by which doctors interpret their patients’ symptoms and weigh test results in order to arrive at a diagnosis and a plan of treatment. Students spent the first two years of medical school memorizing facts about physiology, pharmacology, and pathology; they spent the last two learning practical applications for this knowledge, such as how to decipher an EKG and how to determine the appropriate dose of insulin for a diabetic. Croskerry’s instructors rarely bothered to describe the mental logic they relied on to make a correct diagnosis and avoid mistakes.

In 1990, Croskerry became the head of the emergency department at Dartmouth General Hospital, and was struck by the number of errors made by doctors under his supervision. He kept lists of the errors, trying to group them into categories, and, in the mid-nineties, he began to publish articles in medical journals, borrowing insights from cognitive psychology to explain how doctors make clinical decisions—especially flawed ones—under the stressful conditions of the emergency room. “Emergency physicians are required to make an unusually high number of decisions in the course of their work,” he wrote in “Achieving Quality in Clinical Decision Making: Cognitive Strategies and Detection of Bias,” an article published in Academic Emergency Medicine, in 2002. These doctors’ decisions necessarily entail a great deal of uncertainty, Croskerry wrote, since, “for the most part, patients are not known and their illnesses are seen through only small windows of focus and time.” By calling physicians’ attention to common mistakes in medical judgment, he has helped to promote an emerging field in medicine: the study of how doctors think.

There are limited data about the frequency of misdiagnoses. Research from the nineteen-eighties and nineties suggests that they occur in about fifteen per cent of cases, but Croskerry suspects that the rate is significantly higher. He believes that many misdiagnoses are the result of readily identifiable—and often preventable—errors in thinking.

Doctors typically begin to diagnose patients the moment they meet them. Even before they conduct an examination, they are interpreting a patient’s appearance: his complexion, the tilt of his head, the movements of his eyes and mouth, the way he sits or stands up, the sound of his breathing. Doctors’ theories about what is wrong continue to evolve as they listen to the patient’s heart, or press on his liver. But research shows that most physicians already have in mind two or three possible diagnoses within minutes of meeting a patient, and that they tend to develop their hunches from very incomplete information. To make diagnoses, most doctors rely on shortcuts and rules of thumb—known in psychology as “heuristics.”

Heuristics are indispensable in medicine; physicians, particularly in emergency rooms, must often make quick judgments about how to treat a patient, on the basis of a few, potentially serious symptoms. A doctor is trained to assume, for example, that a patient suffering from a high fever and sharp pain in the lower right side of the abdomen could be suffering from appendicitis; he immediately sends the patient for X-rays and contacts the surgeon on call. But, just as heuristics can help doctors save lives, they can also lead them to make grave errors. In retrospect, Croskerry realized that when he saw McKinley in the emergency room the ranger had been experiencing unstable angina—a surge of chest pain that is caused by coronary-artery disease and that may precede a heart attack. “The unstable angina didn’t show on the EKG, because fifty per cent of such cases don’t,” Croskerry said. “His unstable angina didn’t show up on the cardiac-enzymes test, because there had been no damage to his heart muscle yet. And it didn’t show up on the chest X-ray, because the heart had not yet begun to fail, so there was no fluid backed up in the lungs.”