In 1952, the American Psychiatric Association tried to standardize the definitions of mental illnesses, including depression, by creating a taxonomy of mental illnesses. In the first edition of the Diagnostic and Statistical Manual, depression was listed under the broad category of “disorders without clearly defined physical cause,” which also included schizophrenia, paranoia, and mania. The DSM-III, published in 1980, was the APA’s first attempt to clarify the definitions of specific disorders by listing their symptoms; the new edition included guidelines for differentiating depression from other disorders like schizophrenia, dementia, and uncomplicated bereavement, and outlined eight symptoms of depression, included “poor appetite or significant weight loss” and “complaints or evidence of diminished ability to think or concentrate.” If an adult met four of the eight symptoms, the manual counseled, he or she would meet the criteria for clinical depression. In the DSM-V, published in 2013, depressive disorders were finally allocated their own chapter. The diagnostic criteria were mostly unchanged, with the exception of one additional symptom: “Depressed mood most of the day, nearly every day, as indicated by either subjective report (e.g., feels sad or empty) or observation made by others (e.g., appears tearful).”

Some scientists believe that the DSM-V definition is still too vague. As the psychiatrist Daniel Goldberg noted in the journal World Psychiatry in 2011, many of the DSM symptoms are opposites, which can make it difficult for researchers working to develop a more precise understanding of the condition. “A patient who has psychomotor retardation, hypersomnia, and gaining weight is scored as having identical symptoms as another who is agitated, sleeping badly, and has weight loss,” Goldberg wrote.

Many recent studies have corroborated Goldberg’s concerns. In 2000, for example, a group of researchers at Johns Hopkins University attempted to identify subtypes of depression by studying the symptoms of nearly 2,000 patients. However, the researchers were unable to find much of a pattern connecting gender, family history, symptoms, and the degree of the condition (mild to severe). “Depression is heterogeneous,” they concluded, adding that “the severity of an episode appears to be more informative than the pattern of symptoms.” And in 2010, researchers in Germany testing the validity of the DSM-IV definition found that the criteria captured a huge population of patients with “widely varying associations with the pattern of co-morbidity, personality traits, features of the depressive episode and demographic characteristics.” The results, they argued, “challenge our understanding of major depression as a homogeneous categorical entity.”

Part of the problem, said Scott Monroe, a professor of psychology at the University of Notre Dame, is that in medical parlance, depression is considered a syndrome rather than a disease. (While a disease is a specific condition characterized by a common underlying cause and consistent physical traits, a syndrome is a collection of signs and symptoms known to frequently appear together, but without a single known cause.) In a paper published in June in the journal Current Directions in Psychological Science, Monroe called for scientists to begin defining depression with more precision. “It is in this vague and imprecise realm that problems can arise,” he wrote, “and vague insights based on imperfect similarities and differences eventually may prove to be clear oversights.”