Beyond (a) reasonable doubt is a legal standard of proof required to validate a criminal conviction in most adversarial legal systems. It is a higher standard of proof than the standard of balance of probabilities (US English: preponderance of the evidence) commonly used in civil cases because the stakes are much higher in a criminal case: a person found guilty can be deprived of liberty, or in extreme cases, life, as well as suffering the collateral consequences and social stigma attached to a conviction. The prosecution is tasked with providing evidence that establishes guilt beyond a reasonable doubt in order to get a conviction; albeit prosecution may fail to complete such task, the trier-of-fact's acceptance that guilt has been proven beyond a reasonable doubt will in theory lead to conviction of the defendant. A failure for the trier-of-fact to accept that the standard of proof of guilt beyond a reasonable doubt has been met thus entitles the accused to an acquittal. This standard of proof is widely accepted in many criminal justice systems, and its origin can be traced to Blackstone's ratio, "It is better that ten guilty persons escape than that one innocent suffer."
In practice
Because a defendant is presumed to be innocent, the prosecution has the burden of proving the defendant's guilt on every element of each criminal charge beyond a reasonable doubt. To do so, the prosecution must present compelling evidence that leaves little real doubt in the mind of the trier of fact (the judge or jury) that the defendant is almost certainly guilty. For any reasonable doubt to exist, it must come from insufficient evidence, or conflicts within the evidence, that would leave an impartial factfinder less than fully convinced of the defendant's guilt. Accordingly, the standard of proof forces the factfinder to ignore unreasonable doubts—doubts that are frivolous, hypothetical, or not logically linked to the evidence—and to consider evidence favoring the accused, since reasonable doubt entitles them to an acquittal.
The definitions of the term "reasonable doubt" can be criticised for having a circular definition. Therefore, jurisdictions using this standard often rely on additional or supplemental measures, such as a judge's specific instructions to a jury, to simplify or qualify reasonable doubt. Legal systems have tended to avoid quantifying the reasonable doubt standard (for example, "over 90% probability"); legal scholars from a variety of analytical perspectives have argued in favor of quantification of the criminal standard of proof. In a 2019 YouGov survey conducted in the United Kingdom, participants were asked to quantify how accurate an evidence had to be before they could consider it to be beyond a reasonable doubt; 15% of Britons said they would accept an evidence that was 99% accurate, while 14% preferred an accuracy of no less than 100%, and 10% said it should be at least 90% or 95% accurate.
By jurisdiction
Medieval Roman law, followed by the English jurist Edward Coke, expressed a similar idea by requiring "proofs clearer than light" for criminal conviction. The formulation "beyond reasonable doubt" is characteristic of Anglophone legal systems since the eighteenth century.
United Kingdom
England and Wales
In English common law prior to the reasonable doubt standard, passing judgment in criminal trials had severe religious repercussions for jurors. According to judicial law prior to the 1780s, "the Juryman who finds any other person guilty, is liable to the Vengeance of God upon his Family and Trade, Body and Soul, in this world and that to come." It was also believed that "[i]n every case of doubt, where one's salvation is in peril, one must always take the safer way. ... A judge who is in doubt must refuse to judge." It was in reaction to these religious fears that "reasonable doubt" was introduced in the late 18th century to English common law, thereby allowing jurors to more easily convict. Therefore, the original use of the "reasonable doubt" standard was opposite to its modern use of limiting a juror's ability to convict.
Juries in criminal courts in England and Wales are no longer customarily directed to consider whether there is reasonable doubt regarding a defendant's guilt. A 2008 conviction was appealed after the judge had said to the jury, "You must be satisfied of guilt beyond all reasonable doubt." The conviction was upheld; but the Appeal Court made clear their unhappiness with the judge's remark, indicating that the judge should instead have said to the jury simply that before they can return a verdict of guilty, they "must be sure that the defendant is guilty".