Thirty studies comparing competent and incompetent criminal defendants (N = 8,170) were quantitatively reviewed to identify variables associated with judgments about defendants' competency to stand trial. Effect sizes reflecting the magnitude of the relationship between competency status and defendant characteristics were derived and averaged across studies. The characteristics most strongly related to incompetency were (a) poor performance on psychological tests or interviews specifically designed to assess defendants' legally relevant functional abilities, (b) a psychotic diagnosis, and (c) psychiatric symptoms reflective of severe psychopathology. To a lesser degree, traditional psychological tests, previous psychiatric hospitalization, previous legal involvement, marital resources, and demographic characteristics were also related to competency status. Although these results are generally consistent with the legal standard for competency to stand trial, some of the correlates of competency may reflect potential biases in the decision-making process. Limitations of existing research and directions for future inquiry are discussed.