Definition
The complete agreement of all persons concerned in reaching a single determination on a question. In law, unanimity most often refers to the requirement that every member of a jury concur in a verdict before it may be rendered. A verdict reached without the agreement of all jurors is a nullity; when a jury cannot achieve unanimity, the result is a hung jury and a mistrial.
Unanimity may also apply outside the jury context — to legislative bodies, appellate panels, or deliberative committees — where a rule requiring unanimous agreement means that a single dissenting voice defeats the proposed action or decision.
Common Language
Modern common usage (Wiktionary): The condition of agreement by all parties; the state of being unanimous.
Historical common usage (Webster's 1913): The quality or state of being unanimous.
The gap between common and legal meaning is narrow in concept but significant in consequence. In ordinary usage, unanimity is simply a description of consensus. In law, it is often a formal threshold with procedural and constitutional dimensions: its presence or absence determines whether a verdict is legally valid, whether a conviction stands, and — depending on jurisdiction and era — whether a constitutional right has been satisfied.
Common Confusion
Unanimity is sometimes confused with a mere majority or supermajority requirement. A verdict reached by eleven of twelve jurors is not unanimous, regardless of how overwhelming the agreement appears. The distinction matters acutely: at common law and under the Sixth Amendment as interpreted in federal courts, anything short of complete agreement in a criminal jury fails the unanimity standard. Separately, unanimity should not be confused with the concept of a unanimous opinion in an appellate court — the latter is a descriptive term about how a panel aligned, carrying no binding procedural requirement.
Core Elements
The traditional unanimity requirement in jury proceedings rests on several component ideas:
Complete concurrence: Every sitting juror must agree. In a twelve-person jury, that means twelve votes; in a six-person jury, six.
Genuine agreement: Unanimity must be actual, not coerced. A verdict produced through improper pressure on holdout jurors may be challenged as involuntary.
Expressed in open court: The agreement must be announced through the formal verdict, not merely presumed from the jury's silence or speed of deliberation.
Why It Matters in Research
The unanimity requirement has one of the more turbulent constitutional histories of any procedural concept in American law, and researchers must track it carefully across time and jurisdiction.
For most of American legal history, unanimity in criminal jury verdicts was understood as an unquestioned common law inheritance, applied in federal courts and assumed by most states. That assumption was disrupted in 1972, when the Supreme Court held in Apodaca v. Oregon that the Sixth Amendment did not require unanimous verdicts in state criminal trials, permitting non-unanimous jury convictions. Oregon and Louisiana exploited this opening for decades. In 2020, Ramos v. Louisiana overruled Apodaca, holding that the Sixth Amendment right to a unanimous jury verdict applies to the states through the Fourteenth Amendment. Researchers working on state criminal procedure from 1972 through 2020 must account for this window of non-unanimity — convictions rendered under non-unanimous verdict rules during that period raise retroactivity questions addressed in Edwards v. Vannoy (2021).
Civil jury unanimity is a separate track. Federal civil juries do not have a constitutional unanimity requirement, and many states permit non-unanimous verdicts in civil cases by rule or statute. Researchers conflating the civil and criminal tracks in historical sources will reach erroneous conclusions.
Pre-twentieth-century sources treat unanimity as essentially absolute and uncontested, which reflects the received common law view. This masks the genuine historical complexity Burrill gestures at — that ancient practice did not always require unanimous agreement in civil causes. Researchers mining early American treatises and digests should note that those sources describe the settled rule of their era, not the full sweep of common law history.
For corpus researchers: the Ramos decision makes pre-2020 secondary literature on this topic unreliable as a statement of current constitutional law. Any source describing non-unanimous verdicts as constitutionally permissible in state courts reflects the superseded Apodaca framework.
Historical Dictionary Support
The historical dictionaries converge on a single core definition: unanimity in the jury context means complete agreement of all jurors in their verdict. Burrill and Rapalje & Lawrence both flag that this was the uniform requirement in English and American practice. Burrill is the most historically candid of the shelf sources, noting that ancient common law practice — at least in civil causes — did not always require unanimous agreement, with disagreeing jurors sometimes separated from the majority. This glimpse of pre-modern practice is absent from Black's and Rapalje, which present unanimity as simply settled and universal.
Black's 1st edition entry is corrupted in the source material, importing language from an unrelated definition. The 2nd edition entry is clean and functionally correct: "agreement of all the persons concerned, in holding one and the same opinion or determination of any matter or question; as the concurrence of a jury in deciding upon their verdict." This formulation is accurate as far as it goes but treats unanimity as static doctrine, which the constitutional history does not support.
None of the historical dictionaries address the civil/criminal distinction or the constitutional dimension, reflecting the period in which they were composed — when those questions had not yet been litigated as constitutional matters.
Jurisdictional Note
Federal criminal trials require unanimous verdicts by Rule 31 of the Federal Rules of Criminal Procedure and as a constitutional matter under the Sixth Amendment. After Ramos v. Louisiana (2020), the same requirement applies to serious criminal offenses in state courts. Civil jury unanimity requirements vary by state, and federal civil trials are governed by Federal Rule of Civil Procedure 48, which permits non-unanimous verdicts unless a party demands otherwise.
Encyclopedia Cross-Reference
The Right to a Jury Trial — Size, Unanimity, and Scope (The Law Mind Constitutional Law Encyclopedia)