In federal diversity cases applying state law, which rule governs the use of witness credibility?

• A. Dead Man's Act
• B. Burden of proof
• C. Privileges
• D. Hearsay

Answer: (B)

The rule is the burden of proof. In federal diversity cases applying state law, credibility matters only as a tool for meeting the party’s required burden of proof. The trier of fact weighs witness testimony to decide whether the party has persuaded them to meet the applicable standard (for example, a preponderance of the evidence). Hearsay and privileges concern whether testimony or evidence is admissible at all, not how credibility is used to meet the proof standard. Dead Man’s Act is a specific rule about testimony related to transactions with a decedent and does not govern credibility in general. So, the key idea is that credibility is evaluated in light of whether the party has met the required burden of proof.