Although some studies have examined juror perceptions of child witnesses and the factors that influence credibility inferences, research to date has not examined the effects of one potentially powerful variable, expert psychological testimony, that may influence jurors' perceptions of the credibility of a child witness in a child sexual abuse trial. This chapter first examines research on the effects of expert psychological testimony in two legal domains other than child sexual abuse (eyewitness identification and rape trauma syndrome), domains wherein researchers have begun to examine the effects of expert testimony on juror and jury decisionmaking. The authors then discusses key findings from their own research on expert psychological testimony in child sexual abuse trials. Using a hypothetical technique and testing for where the child is perceived as unprepared or with a demeanor that leads to inferences of being unprepared, the child is viewed by jurors as less credible. Expert psychological testimony can inform jurors about the demeanor of the child and is a way to counteract these perceptions. 86 references