The Terrorist Radicalization Assessment Protocol (TRAP-18) is a structured professional judgment instrument for threat assessment of the individual terrorist. It is a rationally derived theoretical model comprising eight proximal warning behaviors and 10 distal characteristics. Empirical research on the TRAP-18 is reviewed, including both nomothetic and idiographic studies of individual terrorists in both the United States and Europe. Mean interrater reliability is 0.895 (Cohen's kappa),

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... from 0.69 to 1.0. Evidence of criterion validity has been demonstrated, including usefulness of the instrument across various extremist ideologies (jihadism, ethnic nationalism, and single-issue), and its ability to discriminate between thwarted and successful attackers. The instrument appears to advance the domain recommendations of Monahan (2012 Monahan ( , 2016 for the risk assessment of the individual terrorist. The TRAP-18 is further discussed as a threat assessment instrument for mental health clinicians. The limitations of the current research provide direction for further studies to assess its reliability and construct, discriminant, and predictive validity.