Under Scots law, a criminal trial may end in one of three verdicts: one of conviction ("proven") and two of acquittal ("not proven" and "not guilty"). Historically, the two verdicts available to Scots juries were that the case had been "proven" or "not proven". However in a dramatic case in 1728 the jury asserted "its ancient right" to bring in a "not guilty" verdict even when the facts of the case were proven (see jury nullification). As the "not guilty" verdict gained wide acceptance amongst Scots juries, Scots began to use "not guilty" in cases where the jury felt the "not proven" verdict did not adequately express the innocence of the defendant. Shrewd defence then further encouraged this interpretation in order to persuade juries unwilling to bring in a "not guilty" verdict that the "not proven" could be brought in as a lesser or "third verdict".
The result is the modern perception that the "not proven" verdict is an acquittal used when the judge or jury does not have enough evidence to convict but is not sufficiently convinced of the defendant's innocence to bring in a "not guilty" verdict. Essentially, the judge or jury is unconvinced that the suspect is innocent, but has insufficient evidence to the contrary. However technically under Scots law (though not in the perception of the public) there is no difference between "not proven" and "not guilty" and both are equivalent to the "Not Guilty" verdict of English Law and of other jurisdictions.
Out of the country, the "not proven" verdict may be referred to as the Scottish Verdict or the Scotch Verdict, although in Scotland itself it may be referred to colloquially as the bastard verdict, which was a term coined by Sir Walter Scott, who himself was sheriff in the court of Selkirk.
On one account, advanced two hundred years ago by the historians Hume and Arnot, the third and distinctively Scottish verdict was rooted in religious oppression. The Crown persecuted the Covenanters but popular support made it impossible to convict them in a jury trial. To pare the power of the jury, the Scottish judges began restricting the jury's role: no longer would the jury announce whether the defendant was "guilty" or "not guilty"; instead it would decide whether specific factual allegations were "proven" or "not proven"; and the judge would then decide whether to convict. Some historians, however, such as Ian Douglas Willock, have rejected the traditional account.
The (re)introduction of the "not guilty" verdict was part of a wider movement during the 16th and 17th century which saw a gradual increase in the power of juries, such as the trial of William Penn in 1670, in which an English jury first gained the right to pass a verdict contrary to the law (known as jury nullification).
Although jurors continued to use both "not guilty and "not proven", jurors tended to favour the "not guilty" verdict over the "not proven" and the interpretation changed.
The Scottish verdict has not been permanently adopted outside its home country, but it was sometimes used in colonial Canada, especially by some judges in southwestern Ontario. Its most famous use outside of Scottish law came when Senator Arlen Specter tried to vote "not proven" on an article of impeachment of Bill Clinton (see Lewinsky scandal), and when, at the O.J. Simpson murder case, various reformers, including Fred Goldman, Ron Goldman's father, pushed for a change to "not proven" because of what they felt was an incorrect presumption of innocence on the part of Simpson.
A recent proposal to introduce the not proven verdict into the United States is:
The "not proven" verdict has been used in popular writing (as by Carl Sagan) as a metaphor for the operation of the scientific method, in which conclusions are never certain, but the most that can be said about a theory is what the preponderance of the evidence suggests.