I know that a petit (trial) jury in a criminal case requires 12 jurors and I understand why the number 12 was chosen. But why does our U S Constitution, which is modeled after the Magna Carta, require 23 members of the grand jury when the Magna Carta only required 12 members of a grand jury?
Why was the number of members of the grand jury set at 23 instead of 12 by the framers of the Constitution?
I completely agree with Jen's answer and I am writing here to discuss some deeper historical dimensions to the question.
Historically, notion of using a 23 member grand jury dates to the reign of King Edward III whose reign started in the year 1368 in England.
It is not derived from the 23 members of the Lesser Sanhedrins in ancient Israel. The grand jury was changed from having 12 members to having 23 members at least seventy-eight years after King Edward I expelled all Jews from England in the year 1290, at a time when the expulsion of Jews from England was still in effect.
The Grand Jury can also be traced to the time of the Norman conquest of England in 1066. There is evidence that the courts of that time summoned a body of sworn neighbors to present crimes that had come to their knowledge. Since the members of that accusing jury were selected from small jurisdictions, it was natural that they could present accusations based on their personal knowledge.
Historians agree that the Assize [court session or assembly] of Clarendon in 1166 provided the ground work for our present Grand Jury system. During the reign of Henry II (1154-1189), to regain for the crown the powers usurped by Thomas Becket, Chancellor of England, 12 "good and lawful men" in each village were assembled to reveal the names of those suspected of crimes. It was during this same period that juries were divided into two types, civil and criminal, with the development of each influencing the other.
The oath taken by these jurors provided that they would carry out their duties faithfully, that they would aggrieve no one through enmity nor deference to anyone through love, and that they would conceal those things that they had heard.
By the year 1290, these accusing juries were given the authority to inquire into the maintenance of bridges and highways, defects of jails, and whether the Sheriff had kept in jail anyone who should have been brought before the justices. "Le Grand Inquest" evolved during the reign of Edward III (1368), when the "accusatory jury" was increased in number from 12 to 23, with a majority vote necessary to indict anyone accused of crime.
A grand jury is an ex parte proceeding run by a prosecutor in secret with only the most minimal judicial involvement. Counsel for the suspects can represent the suspects as witnesses if they are called to testify before the grand jury, but cannot make arguments on applying the law to the facts or legal arguments to a grand jury.
To indict, twelve members of a grand jury must vote to indict (no matter how many members the grand jury has), and a majority of the grand jury must support the indictment (which is why 23 grand jurors is the upper limit). So, for example, in a federal grand jury with 16 members, three quarters of them must vote in favor of doing so to indict a suspect.
The increase in the size of the grand jury from twelve to twenty-three made it easier to indict criminal defendants by relaxing the unanimity requirement for an indictment that had existed when there were only twelve jurors on a grand jury.
The grand jury is supposed to vote to indict if it finds that there is probable cause to find that a defendant committed a crime under the law explained to it by the prosecutor and the evidence that is presented to it by the prosecutor. One grand jury's refusal to indict a suspect does not preclude a prosecutor from presenting the same charges against the same suspect to a different grand jury later on.
Federal grand juries almost always vote to indict. At the state level, indictment rates vary widely from one county to another, and from state to state.
The grand jury requirement exists in the U.S. under state law mostly in states in the Eastern U.S., while most Western states allow grand juries to be convened but only do so in exceptional cases (usually for political reasons or as a secret means for prosecutors to gather evidence).
In the early decades of the United States grand juries played a major role in public matters. During that period counties followed the traditional practice of requiring all decisions be made by at least twelve of the grand jurors, (e.g., for a twenty-three-person grand jury, twelve people would constitute a bare majority).
Any citizen could bring a matter before a grand jury directly, from a public work that needed repair, to the delinquent conduct of a public official, to a complaint of a crime, and grand juries could conduct their own investigations. In that era most criminal prosecutions were conducted by private parties, either a law enforcement officer, a lawyer hired by a crime victim or their family, or even by laymen. A layman could bring a bill of indictment to the grand jury; if the grand jury found there was sufficient evidence for a trial, that the act was a crime under law, and that the court had jurisdiction, it would return the indictment to the complainant.
The grand jury would then appoint the complaining party to exercise the authority of an attorney general, that is, one having a general power of attorney to represent the state in the case. The grand jury served to screen out incompetent or malicious prosecutions. The advent of official public prosecutors in the later decades of the 19th century largely displaced private prosecutions.
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Grand juries were popular in the Eastern states because in the Revolutionary War period and the period leading up to the Revolutionary War in the United States, grand juries were used as a tool of resistance to what was perceived as oppressive British rule by citizens of the American colonies.
But, that fervor had faded and the difficulty of finding grand jurors on the frontier discouraged the use of this institution of people migrated to the west to form new states, and as the prosecution of crimes came to be the sole or predominant province of professional prosecutors employed by the state.
In states that don't require grand jury indictments in all felony cases, an adversarial preliminary hearing before a judge to screen for probable cause is used in in lieu of a grand jury in some felony cases.
The only country other than the United States which still uses grand juries is Liberia, which was founded by freed slaves from the U.S.
The Constitution does not require a 23-member grand jury; it does not specify a size:
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, ...
If there is a constitutional minimum, it is not apparent from the text, and I am not familiar with any judicial pronouncement on this issue.
The Federal Rules of Criminal Procedure allow for grand juries of 16 to 23.
ohwilleke's answer presents a brief history of grand jury composition and explains why 23 makes practical sense.
The United States Supreme Court has held that the grand jury requirement is not incorporated against the states: Hurtado v. California, 110 U.S. 516 (1884). Therefore, the procedure for indictment in state courts, including grand juries if they are used, is a matter of state law.
Here are a few minor historical details regarding juries. Although it was commonly thought that the Magna Carta guaranteed the right to trial, it did not. This is discussed extensively in an article by Walter Clark (CJ, North Carolina Supreme Court). There are multiple versions of the Magna Carta, the first of which (1215) has clause (39)
No free man shall be seized or imprisoned, or stripped of his rights or possessions, or outlawed or exiled, or deprived of his standing in any way, nor will we proceed with force against him, or send others to do so, except by the lawful judgment of his equals or by the law of the land.
The 1225 version, reissued by Edward I in 1297, restates this as (29)
No free man shall in future be arrested or imprisoned or disseised of his freehold, liberties or free customs, or outlawed or exiled or victimised in any other way, neither will we attack him or send anyone to attack him, except by the lawful judgment of his peers or by the law of the land. To no one will we sell, to no one will we refuse or delay right or justice.
This is the Due Process clause, sort of.
Before the Magna Carta, the Assize of Clarendon 1166 established a procedure for seizing those suspected of crimes:
In the first place the aforesaid King Henry, on the advice of all his barons, for the preservation of peace, and for the maintenance of justice, has decreed that inquiry shall be made throughout the several counties and throughout the several hundreds through twelve of the more lawful men of the hundred and through four of the more lawful men of each vill upon oath that they will speak the truth, whether there be in their hundred or vill any man accused or notoriously suspect of being a robber or murderer or thief, or any who is a receiver of robbers or murderers or thieves, since the lord king has been king. And let the justices inquire into this among themselves and the sheriffs among themselves.
Subsequently, the “grand assize” (1179) (Magna Assisa, Assize of Windsor) – said to be the origin of the grand jury, created a method for resolving property disputes. The original text is lost, but is summarized in a contemporary work by Rannulf Glanvill (book 2).
This article fills in more detail. There was a requirement under the Magna Assisa that 12 knights agree as to a dispute over property rights. The sheriff would summon 4 (local) knights who would choose twelve lawful knights who were most cognizant of the facts, there would be “voir dire” to determine if a knight knew the facts, until twelve were found who knew and agreed upon the facts. If the jurors could not all agree, others were added to the number, until twelve at least agreed in favor of one side or the other. That is, there would be an increase in the size of the "jury" until the required number of agreements was found. 23 is the number of jurors required to get 12 knights to agree.
