Millions of custodial suspects waive their rights each year without the benefit of legal counsel. The question posed to psychologists in disputed Miranda waivers is whether this waiver decision was, knowing, intelligent, and voluntary. Mental health professionals must be aware of potential barriers to Miranda comprehension to provide expert opinions regarding a defendant's competency to waive rights. The current study examined how Miranda warning reading level, length, and method of administration affects Miranda comprehension. Recently arrested detainees at Grayson County Jail were administered oral and written Miranda warnings from the Miranda Statements Scale (MSS; Rogers, 2005) to measure their comprehension of the warnings. Surprisingly low levels of Miranda comprehension were found for most warnings. For all warnings at or above 8th grade, a substantial minority (27.1% - 39.6%) of defendants exhibited failed (i.e., < 50% understanding) Miranda comprehension. Regardless of other variables, oral administrations resulted in a substantially larger number of defendants with failed Miranda comprehension. Implications for public policy and clinical practice are discussed.

Each year, an estimated 318,000 defendants who do not comprehend the Miranda warnings waive their rights and provide incriminating evidence without the protection of counsel (Rogers, 2008), which make Miranda-related competencies one of the most pervasive pretrial issues. A wide range of issues could potentially affect an individual's capacity to provide a knowing and intelligent waiver. Previous Miranda research has focused narrowly on the effects of cognitive and developmental factors. The current study added to the Miranda literature by examining the impact of two highly prevalent conditions found in correctional populations, attention deficits and substance abuse. Adult defendants in custody (N = 118) were evaluated within 36 hours of arrest in order to assess both chronic psychological disorders and situational variables. Results indicate that attention deficits have a significant impact on defendants' ability to provide a knowing Miranda waiver, whereas substance use profoundly affected their reasoning about Miranda waiver decisions. This study represents the first systematic investigation of the effect of transient mental states on Miranda-related abilities with criminal defendants. Important implications for forensic practice are addressed.