Rules of evidence are designed to facilitate trials by controlling what evidence may be presented at trial. Those rules came into being, and evolved over time, due to changes in trial process and structure – especially following the rise of adversarial procedure, whereby control over the marshaling and presentation of facts shifted from judges to lawyers. Refinements and reforms continue to be undertaken to try to improve the scope and clarity of the rules. Trial judges must not only apply the rules, they also have the discretion to make rulings in light of their expectations of the impact they think the evidence will have on jurors. This task involves metacognition: one human trying to estimate the thought processes of others. Thus, evidence rulemakers have been and are, effectively, applied psychologists. The rules of evidence reflect their understanding of the psychological processes affecting, and capabilities and limitations of witnesses, lawyers and jurors. Psychological research and methods provide an additional source of insight and assistance in that endeavor. Better psychological understanding should lead to more effective rules. Psychologists typically employ the scientific method: empirically testing hypotheses in an effort to discover which are valid understandings of how people perceive, store, and retrieve information. To evaluate evidence rules, one could conduct experiments directly on a rule, or borrow from existing knowledge developed in basic research and see whether those understandings support existing or proposed evidence rules.