And here’s a couple of examples: If the police and the state have DNA evidence and they say it raises a strong probability that it’s you and nobody else who did it; if that’s the strength of their DNA, you need a DNA expert to show why that is not as reliable as they say, or maybe the evidence wasn’t handled in the proper fashion — that’s one thing. Another thing — and this is the big one — people, DAs, prosecutors always ask this question: Why would a child lie? Why would a person make this up?
Experts can explain the things at work in both a child’s mind and in an accuser’s mind, either based upon general understanding of their thinking processes or something in their background that makes them prone to believe things that aren’t true and can explain from how, say, a psychologist’s point of view or an expert interrogator’s point of view, the answers they gave were coached or were trained or were unreliable. And they can point to things they say and things they do to show that to the jury. That’s admissible evidence.
They can’t say whether the allegation is true or not, but sometimes it’s necessary to remind the jurors that yes, indeed, people lie; yes, indeed, children lie. More importantly, they are mistaken and sometimes those mistaken beliefs are coached into them so deeply that they come to believe they’re true when they’re not. Jurors don’t want to believe that could happen because they don’t think they would do it themselves. So sometimes, you have to call an expert to explain how the mind works to get there.