Ethology is relevant to clinical psychiatry for two different reasons. First, ethology may contribute significantly to the development of more accurate and valid methods for measuring the behavior of persons with mental disorders. Second, ethology, as the evolutionary study of behavior, may provide psychiatry with a theoretical framework for integrating a functional perspective into the definition and clinical assessment of mental disorders. This article describes an ethological method for studying the nonverbal behavior of persons with mental disorders during clinical interviews and reviews the results derived from the application of this method in studies of patients who had a diagnosis of schizophrenia or depression. These findings and others that are emerging from current ethological research in psychiatry indicate that the ethological approach is not limited simply to a mere translation into quantitative and objective data of what clinicians already know on the basis of their judgment or the use of rating scales. Rather, it produces new insights on controversial aspects of psychiatric disorders. Although the impact of ethology on clinical psychiatry is still limited, recent developments in the fields of ethological and Darwinian psychiatry can revitalize the interest of clinical psychiatrists for ethology.