Most of us believe we are rational decision makers. But medical decisions are especially complex, thanks to the numerous unknowns and the uniqueness of each person’s body. Suppose you’ve just found out that you or a loved one has prostate cancer, one of the many examples in Jerome Groopman and Pamela Hartzband’s illuminating new book, “Your Medical Mind.” Nearly every urologist would recommend radical surgery to remove the organ. Sounds reasonable, doesn’t it?

But let’s look at the numbers more closely. Prostate cancer is slow-moving; more people die with it than from it. According to one 2004 study, for every 48 prostate surgeries performed, only one patient benefits — the other 47 patients would have lived just as long without surgery. (Groopman and Hartzband discuss the important epidemiological concept “number needed to treat,” which applies to surgeries, prescriptions, therapies, you name it.) Moreover, the 47 who didn’t need the surgery are often left with an array of unpleasant and irreversible side effects, including incontinence, impotence and loss of sexual desire. The likelihood of one of these side effects is over 50 percent — 24 of our 47 will have at least one. This means a patient is 24 times more likely to experience the side effect than the cure.

“Your Medical Mind,” a kind of sequel to Groopman’s 2007 best seller, “How Doctors Think,” aims to empower patients to become active participants, indeed negotiators, in decisions about their health care. “The path to maintaining or regaining health is not the same for everyone,” Groopman and Hartzband write. “Medicine involves nuanced and personalized decision making by both the patient and the doctor.” I suspect insurance companies, H.M.O.’s and more than a few doctors are going to hate this book.

Groopman and Hartzband explore two sets of biases that affect patient decisions. We can be minimalists, preferring to do as little as possible, or maximalists who aggressively pursue treatment. We can be technology enthusiasts, seeking the newest drugs or procedures, or naturalists who believe the body can cure itself, perhaps with the aid of spiritual and plant-based remedies. Of course, these orientations interact: anyone who lives in Northern California knows someone who eagerly takes armloads of herbal supplements while having their chi realigned in between weekly acupuncture sessions (maximalist-­naturalist). And there are minimalist-­technologists, who avoid medical treatment when possible but if surgery is required will ask for the latest high-tech robotic laser surgery. Understanding these biases, the authors argue, can lead to more effective doctor-­patient dialogue.