Modifies provisions relating to expert witnesses

HB 2255 revises Missouri's rules for expert witness testimony in court. For most cases (excluding family, juvenile, probate, and non-jury trials), it requires that expert opinions must be based on reliable methods, sufficient facts, and a reliable application of those methods to the specific case. In criminal cases, the bill prohibits experts from opining on whether a defendant had a mental state that constitutes an element of the crime. This affects all legal proceedings in Missouri that rely on expert testimony, ensuring greater scrutiny of such evidence in most courtroom settings.