Using a simulated child sexual abuse case, our study investigated the impact of expert testimony about cognitive development in children on jury decisions. Twenty-four gender-balanced panels, each of 6 mock jurors, viewed a videotaped simulation of a criminal court trial in which age of the child victim/witness (6, 9, or 12 years), sex of the child, and the presence or absence of expert testimony were manipulated. There were significant main effects for exposure to expert testimony on jurors' ratings of the child on the factors addressed by the expert: memory expertise, susceptibility to suggestion, and reality monitoring ability. Jurors rated the expert testimony highly in terms of its helpfulness and impartiality. There were no significant main effects for child credibility ratings according to age or sex of the child victims, but juror gender polarization effects were noted, with females significantly more likely to rate the child's credibility higher and to find the defendant guilty. © 1994 Plenum Publishing Corporation.