Modifies provisions relating to expert witnesses

HB 263 revises Missouri's rules for admitting expert witness testimony in court. It creates two tiers: for specific cases (like family court, juvenile proceedings, or non-jury trials), it allows expert opinions without strict foundational requirements. For all other cases, it requires experts to demonstrate their testimony is (1) helpful to the judge or jury, (2) based on sufficient facts/data, (3) derived from reliable methods, and (4) reliably applied to the case. The bill also prohibits experts in criminal cases from opining on whether a defendant had a mental state that defines the crime. This directly affects lawyers, experts, and courts handling testimony across most civil and criminal trials.