The study involved 44 North American law enforcement investigators (25 Florida police investigators and 19 investigators from Ontario, Canada) who were affiliated with local police departments. Twenty college students were untrained in interrogation procedures, and 20 other college students were trained in the interrogation techniques of Kassin and Fong. Participants were randomly assigned to view one of two videos, each depicting eight male suspects (four guilty and four innocent) repeatedly denying their involvement in a crime. After completing a brief background questionnaire, investigators were asked to decide whether each individual was lying or truthful regarding involvement in the crime at issue. As in Kassin and Fong's study, investigators were told that between one-fourth and three-fourths of the suspects were lying to protect themselves. In addition to making a truth/deception judgment regarding each suspect, investigators rated their confidence on a scale from 1 (not at all confident) to 10 (extremely confident) and then described in their own words the basis for their judgment. A one-way analysis of variance was performed on each performance variable for the three-way contrast of "naive" students compared with trained students and experienced police investigators. The study found that a participant's higher level of training and experience apparently increased the likelihood that they would judge targets as deceitful rather than truthful. Overall, the investigators did not outperform the student participants in terms of either global accuracy or discrimination performance. The police investigators were also more confident in their judgments of deceit and truthfulness than were the student groups. These findings suggest that the pivotal decision investigators must make regarding whether to further interrogate a suspect under the assumption of the suspect's deceit may be based on prejudgments of guilt confidently made, but often in error. 2 tables and 42 references