Lyon, T. D. & Koehler, J. J. (1996). The Relevance Ratio: Evaluating the Probative Value of Expert Testimony in Child Sexual Abuse Cases. Cornell Law Review. 82, 43-78.

ABSTRACT

The Federal Rules of Evidence (and most states) define "relevant evidence" as evidence that has "any tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable than it would be without the evidence." Evidence that a person suffered particular symptoms is relevant for proving that abuse occurred if the presence of those symptoms increases the chance that abuse actually did occur. One can determine whether the presence of symptoms increases the chance that abuse occurred by considering two percentages: the percentage of abuse cases in which the symptoms occur, and the percentage of nonabuse cases in which the symptoms occur. If the percentage of abuse cases that exhibit the symptoms is greater than the percentage of nonabuse cases that exhibit the symptoms, then the symptoms are relevant for proving that abuse occurred. We refer to the ratio of these two percentages as the "relevance ratio," and offer this ratio as the appropriate standard against which to measure the relevance of evidence in general and expert testimony in cases of alleged child abuse in particular.

For example, suppose a prosecutor wishes to introduce expert psychological testimony that a particular child suffered from low self-esteem in support of his contention that the child was abused. Is low self-esteem relevant for proving abuse? Even if data are available which indicate that many abused children suffer from low self-esteem, it is also true that some nonabused children suffer from low self-esteem. But this latter fact alone does not establish that low self-esteem is irrelevant. What is needed to make the determination are data that compare the percentage of abused children with low self-esteem with the percentage of nonabused children with low self-esteem. If a higher percentage of abused children exhibit low self-esteem than nonabused children, then this symptom is relevant in proving that abuse occurred.