In response to a controversy concerning whether the stigma of mental illness has declined significantly in the United States in the past several decades, we assessed changes in public perceptions that mentally ill people are violent. Specifically, we compared answers to an open-ended question regarding respondents' understanding of the term "mental illness" from two nationally representative surveys, one conducted in 1950 and one in 1996. In an earlier paper, we reported the finding that perceptions of violence not only failed to decrease but actually increased significantly between 1950 and 1996. In this paper, we explore the possibility that the dangerousness criterion for involuntary commitment, widely adopted in the United States beginning in the 1960s, has contributed to the unexpected increase in perceptions that mentally ill people are dangerous. We find that, among respondents who mention violence in their description of a mentally ill person, the percentage who use "dangerous to self or others" phrasing to indicate this belief increased substantially, from 4.2% in 1950 to 44.0% in 1996. Moreover, eliminating these respondents from consideration, there was a slight decrease in perceptions of violence between 1950 and 1996. We discuss the possibility that the adoption of the dangerousness criterion, which was intended to protect the civil liberties of mentally ill persons, may also have had the unintended consequence of increasing the stigma of mental illness in the United States.