Definition
The quality that renders a witness's testimony worthy of belief. Credibility is the jury's assessment of whether a witness should be believed — how much weight, if any, to give to what that witness says. It is distinct from competency: competency is the threshold legal question of whether a witness may testify at all; credibility is the substantive question of how far that testimony should be trusted once the witness is permitted to speak.
Credibility encompasses several overlapping considerations: whether the witness has the capacity and opportunity to have perceived what they claim; whether their memory is reliable; whether they are being truthful; and whether their account is internally consistent and consistent with other evidence. Demeanor, bias, interest in the outcome, prior inconsistent statements, and character for truthfulness are all conventional grounds on which credibility may be attacked or supported.
Common Language
Modern common usage (Wiktionary): A reputation impacting one's ability to be believed; the believability of statements as measured by whether they are probable or improbable when judged by common experience.
Historical common usage (Webster's 1913): The quality of being credible; credibleness — applied broadly to facts and witnesses alike.
The common and legal meanings are close but not identical in a way that matters for research. Ordinary usage treats credibility as a general reputational quality — something a person or institution either has or lacks. Legal credibility is a narrower, more structured concept tied specifically to testimonial evidence and the fact-finder's role. It is always the jury's domain (or the judge's in a bench trial), not a free-floating judgment. A witness can be personally unreliable in life and still have their testimony credited on a specific point, or vice versa. The legal concept also carries procedural scaffolding — rules about what evidence may be introduced to attack or bolster credibility — that has no counterpart in everyday usage.
Common Confusion
Credibility and competency are the two terms most consistently conflated, in both historical sources and modern practice. Competency is a legal prerequisite: a witness who lacks competency (due to mental incapacity, privilege, or disqualification) cannot testify at all, and the court decides the question. Credibility presupposes competency has already been resolved in the witness's favor. The distinction matters procedurally: challenges to competency are raised before or at the moment of testimony and ruled on by the judge; challenges to credibility are presented to the fact-finder through cross-examination, impeachment evidence, and argument. Historical sources, including the dictionary entries below, are careful to maintain this distinction, but practitioners and older opinions sometimes blur it.
Why It Matters in Research
The credibility/competency boundary is the primary navigational issue in this area of the corpus. Pre-twentieth-century sources devote substantial attention to competency rules that have since been largely abolished — disqualifications based on interest in the outcome, criminal conviction, religious belief, and spousal testimony were live issues in nineteenth-century evidence law. When historical sources discuss "credibility" they sometimes slide into territory that modern law would classify differently, or discuss what is now treated as credibility as if it were still a competency question. Researchers reading Greenleaf, Best, or Taylor on evidence should keep this shift in mind.
The jury's exclusive province over credibility is a constitutional and common-law fixture, but what that means in practice has evolved. Modern corpus materials will show the rise of expert testimony on credibility (particularly in cases involving child witnesses or eyewitness identification), which generated significant litigation over whether such testimony improperly invaded the jury's function. Earlier corpus materials will not reflect this tension at all.
Impeachment doctrine — the mechanism for attacking credibility — is its own dense subdomain. Prior inconsistent statements, bias, prior convictions, and reputation or opinion evidence of untruthfulness each carry different procedural rules, and those rules changed substantially with the adoption of modern evidence codes. Federal Rule of Evidence 607, 608, and 609 govern in federal court; state rules vary and their history in the corpus may reflect pre-codification common law approaches.
Credibility of documents and out-of-court statements appears in the corpus under authentication and hearsay frameworks rather than under "credibility" as a heading. A researcher searching "credibility" will surface witness-focused materials; hearsay reliability doctrine lives elsewhere.
Historical Dictionary Support
All three historical sources — Black's, Bouvier's, and Burrill's — converge on a single, concise definition: worthiness of belief, and specifically the quality in a witness rendering testimony worthy of belief. The formulation is nearly identical across all three, and Burrill and Black's share near-verbatim language drawn from the same English authority (1 Burrow 414, 417 and 3 Blackstone's Commentaries 369).
Bouvier's adds the key procedural allocation — credibility is a question for the jury — and cites Greenleaf and Taylor on Evidence for support. This was not a contested point in the period; it was settled common law that the jury weighs the witness, not the court. The historical sources do not address the modern complications of expert credibility testimony, social science research on eyewitness reliability, or the evidentiary rules that now structure impeachment in detail. Researchers should treat these entries as capturing the foundational common-law baseline, not the full complexity of current doctrine.
None of the three historical sources defines credibility as applied to documentary or physical evidence, hearsay declarants, or scientific methodology — uses the word now sometimes carries in modern opinions and briefs.
Jurisdictional Note
The core principle — credibility is for the jury, competency is for the court — is uniform across American jurisdictions. Variation appears in impeachment rules: which prior convictions may be used, whether rehabilitation is permitted after attacks on credibility, and the standards for expert testimony on witness reliability differ meaningfully between federal courts and some state systems. Researchers working across jurisdictions should not assume uniform rules from federal sources.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia: Impeachment of Witnesses; Competency of Witnesses; Burden of Proof and Standard of Proof.