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The question might seem basic, but why do juries in the United States consist of 12 members?

Was there an experimental determination of this number?

Would the addition or removal of a juror operate more effectively?

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    The premise is false. Juries must have 12 unanimous jurors in felony criminal cases (recently modified from SCOTUS precedent that allowed 10 or 11 in state court felony criminal cases with non-unanimous verdicts allowed in juries of 11 or 12 in 2 states), only 6 are required in federal civil cases and in misdemeanor cases, but 7 (letting an alternate juror deliberate) isn't unusual. Smaller juries (e.g. 3 or 5) are sometimes used in traffic cases and eminent domain cases. Juries aren't constitutionally required at all in state court civil/misdemeanor cases. Grand juries have 12-23 members.
    – ohwilleke
    Jun 2, 2022 at 7:15
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    I would guess it's simply because twelve is an ancient and enduring symbol of completeness and symmetry.
    – Anton Sherwood
    Feb 5 at 19:12
  • For the same reason there are a dozen eggs in a carton?
    – Michael Hall
    Apr 15 at 14:05

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The origin of the jury is a complex mix of Saxon, Danish and Norman custom which morphed and melded along with English Common Law, which is the basis of the law in all ex-British colonies including the USA.

Danish towns in the north and east of England had hereditary “law men”, often 12 in number who decided legal disputes. In parallel the West Saxons (Wessex) in the south and west charged 12 theigns in each area with keeping the peace. When the Normans conquered (who were also of Viking origin) they adopted and adapted the existing legal structures.

In the 12th century, Henry II established that a jury of 12 should decide land disputes. Meanwhile other juries of various sizes were formed to investigate crimes and bring charges - this is the origin of the Grand Juries that still live on in some jurisdictions in the USA although they have been replaced by a judge in other common law jurisdictions (and some states in the USA).

Ultimately the jury that brought charges and the jury that decided guilt were split and the size was standardised. However, it is incorrect to say that it is always 12: some jurisdictions have different numbers for different purposes. For example, rule 48 of the US Federal rules for civil procedure sets the number at not less than 6 and not more than 12.

The jury system continues to evolve with various jurisdictions adopting different numbers, majority verdicts and judge only trials.

There is nothing special or “scientific” about 12: it is what it is because it is what it is.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – feetwet ♦
    Jan 4, 2018 at 1:40
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    Worth noting that the U.S. Supreme Court just changed the law to require a jury of 12 in felony cases as a matter of constitutional law at the state level in the last year or so. Prior to that, smaller, and non-unanimous juries were allowed. Juries of less than 12 are allowed in misdemeanor and civil cases. Grand juries are required to have 23 members in many jurisdictions, and the size of a coroner's inquest jury is also not always 12. I have tried many civil cases to juries of 7.
    – ohwilleke
    Oct 30, 2020 at 23:33
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It isn't 12, for federal trial courts it's at least 6 and no more than 12. https://www.law.cornell.edu/rules/frcp/rule_48 For grand juries it's more https://www.uscourts.gov/services-forms/jury-service/types-juries. In az it's 6,8, or 12 depending on the matter being considered. https://law.justia.com/codes/arizona/2005/title21/00102.html. I can't go through all the states but it's not always 12.

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  • Rule 48 applies in civil cases. In criminal felony cases in the U.S. the requirement is 12 as established by U.S. constitutional law. In some misdemeanor cases where a jury is required at all, the required size is often 6. Some states have juries of less than 6 in cases where there is no constitutional right to a jury trial (e.g. in eminent domain cases).
    – ohwilleke
    Oct 30, 2020 at 23:37
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    @ohwilleke can you cite your sources?
    – Sam
    Oct 31, 2020 at 13:05
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It would not suprise me if the historical establishment of 12 jurors was rooted in Judeo-Christian tradition, based upon the Biblical establishment of the 12 judges of the twelve tribes of Israel in the Old Testament Book of Judges, to wit the New Testament Book of Revelations declares that the 12 Apostles of Jesus will sit upon 12 thrones judging the 12 tribes of Israel. The purpose of both references was to show how sin and evil can bring bondage into your life, yet showing God’s faithfulness to deliver His people when they turn to Him. I cannot help but think the number 12 seated in judgment over the affairs of an accused is rooted in Biblical text.

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    Then why does Scotland, with its strong Catholic heritage, have 15 jury members?
    – user35069
    Apr 15 at 16:05
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Historic tradition mentions a king of Gwent and Glywysing (i.e., Morgannwg) in South Wales, Morgan of Glamorgan (c. 725), as the first to have a jury of 12 to decide on legal cases. His motive, according to tradition, was to take the model of the 12 apostles (the jury) and Jesus (the Judge).

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  • This would be a great answer if you could add a link to a credible source for this "historic tradition." You can edit the answer to do so. I would however note that the number 12 has been popular in ancient pre-Christian cultures, for example in Hebrew scriptures and ancient Greek mythology. It's therefore possible that the link with the 12 apostles was identified after the number 12 was settled on.
    – phoog
    Apr 24, 2019 at 17:38
  • The New York Times is not an affordable source for history, but it might provide a place to start: nytimes.com/1995/06/11/weekinreview/…
    – Krazy Glew
    Jun 2, 2022 at 20:05
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I don't have any legal schooling or knowledge, however the first thought that springs to my mind is that the removal or addition of a jury-member would create an odd number. This in turn might create less hung juries.

So to answer your last question, would the addition or removal of an jury member make the jury more effective, I think it might.

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    In criminal cases juries must be unanimous. It is not an issue of 6-6 vs 6-7. Both would be hung juries.
    – George White
    Oct 30, 2020 at 23:29
  • In practice, all civil juries must be unanimous too although this is not a constitutional requirement in the U.S. at the state level where there is no constitutional right to a jury trial in civil cases.
    – ohwilleke
    Oct 30, 2020 at 23:35

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