What can a perception-memory expert tell a jury?

Witnesses to crimes and other forensically relevant events sometimes describe memories in which they express confidence that, on the basis of research in perception and memory, may be inappropriately high, given the circumstances of the case. The most effective way of conveying this observation and the reasons for it to a jury is via an expert in perception and memory. In any given case, a mosaic of factors having to do with perception and memory are relevant. I briefly sketch the most common of these factors and categorize them as to the degree to which they may be unambiguously and/or quantitatively applied to a specific case. I discuss one such factor, witness-object distance, in some detail, showing how an expert might describe distance effects on perception and providing examples from actual cases.