Even as we train massive machines to collect, store, and manipulate data for us, humans are the final observers, interpreters, and arbiters of that data. In medicine, the biases are particularly acute for two reasons. The first is hope: we want our medicines to work. Hope is a beautiful thing in medicine—its most tender center—but it is also the most dangerous. Few stories involving the mix of hope and illusion in medicine are more tragic, or more long-drawn, than that of the radical mastectomy. By the early 1900s, during the brisk efflorescence of modern surgery, surgeons had devised meticulous operations to remove malignant tumors from the breast. Many women with cancer were cured by these surgical “extirpations”—yet, despite surgery, some women still relapsed with metastasis all over their bodies. This postsurgical relapse preoccupied great surgical minds. In Baltimore, the furiously productive surgeon William Halsted argued that malignant tissue left behind during the original surgery caused this relapse.
