Applying these four factors shows that some types of expert testimony should be admissible and others should be inadmissible. An expert psychological diagnosis offered to prove that if abuse occurred, it should not be admitted because the testimony is not demonstrably reliable and may be hard to cross-examine. Expert testimony that the complainant is telling the truth should be inadmissible for the same reasons. Expert testimony that children rarely fantasize or fabricate a claim should also be inadmissible because it tends to overwhelm the jury on the crucial issue. However, expert testimony explaining the complainant's unusual behavior should be admissible because the defendant has generally made the testimony necessary. A further type of testimony has received little use, but should be used more. This testimony would explain either the capabilities of the particular child or the capabilities of child witnesses in general. Discussions of judicial decisions, recommendations for research, and 326 footnotes. (Author summary modified)