This paper deals with presumptions that shift the burden of persuasion on some issue in a civil case and attempts to explain why lawyers and judges treat these rules as having great importance. If the persuasion burden applied is 'more probable than not', such rules should affect outcomes only in those rare cases when the evidence is in equipoise. Yet in practice, these rules form an important and vigorously contested part of doctrinal law. The paper attempts to account for the prominence of these rules by considering them from the perspective of behavioral theory, particularly studies of anchoring and adjustment effects. Behavioral theorists have demonstrated that the precise way in which various questions are framed can have a profound impact on the answers given, particularly when the questions involve probability or judgments under uncertainty. This paper suggests that the perceived importance of persuasion burden-shifting rules may be due to their role in framing questions for the legal decisionmaker, particularly by causing decisionmakers to focus (or 'anchor') on one factor in a multi-factored decision, and the implicit recognition of judges and lawyers of the importance of such framing effects.