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Generally the prosecution bears the burden of proof and is required to prove their version of events to this standard. This means that the proposition being presented by the prosecution must be proven to the extent that there could be no "reasonable doubt" in the mind of a "reasonable person" that the defendant is guilty. There can still be a doubt, but only to the extent that it would not affect a reasonable person's belief regarding whether or not the defendant is guilty. "The shadow of a doubt" is sometimes used interchangeably with reasonable doubt, but this extends beyond the latter, to the extent that it may be considered an impossible standard. The term "reasonable doubt" is therefore used.
If doubt does affect a "reasonable person's" belief that the defendant is guilty, the jury is not satisfied beyond "reasonable doubt". The precise meaning of words such as "reasonable" and "doubt" are usually defined within jurisprudence of the applicable country.
England and Wales
The use of "reasonable doubt" as a standard requirement in the Western justice system originated in medieval England. 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 "In 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.
However, juries in criminal courts in England are no longer customarily directed to consider whether there is "reasonable doubt" about a defendant's guilt. Indeed, a recent 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".
The principle of 'beyond reasonable doubt' was expounded in: Woolmington v DPP  UKHL 1 
- "Juries are always told that if conviction there is to be the prosecution must prove the case beyond reasonable doubt. This statement cannot mean that in order to be acquitted the prisoner must " satisfy " the jury. This is the law as laid down in the Court of Criminal Appeal in R. v. Davies (8 C.A.R. 211) the head-note of which correctly states that where | intent is an ingredient of a crime there is no onus on the Defendant to prove that the act alleged was accidental. Throughout the web of the English Criminal Law one golden thread is always to be seen that it is the duty of the prosecution to prove the prisoner's guilt subject to what I have already said as to the defence of insanity and subject also to any statutory exception. If, at the end of and on the whole of the case, there is a reasonable doubt, created by the evidence given by either the prosecution or the prisoner, as to whether the prisoner killed the deceased with a malicious intention, the prosecution has not made out the case and the prisoner is entitled to an acquittal. No matter what the charge or where the trial, the principle that the prosecution must prove the guilt of the prisoner is part of the common law of England and no attempt to whittle it down can be entertained.
In Canada, the expression "beyond a reasonable doubt" requires clarification for the benefit of the jury.  The leading decision is R. v. Lifchus, where the Supreme Court discussed the proper elements of a charge to the jury on the concept of "reasonable doubt" and noted that "[t]he correct explanation of the requisite burden of proof is essential to ensure a fair criminal trial." While the Court did not prescribe any specific wording that a trial judge must use to explain the concept, it recommended certain elements that should be included in a jury charge, as well as pointing out comments that should be avoided.
The Supreme Court suggested that the concept of proof beyond a reasonable doubt should be explained to juries as follows:
- The standard of proof beyond a reasonable doubt is inextricably intertwined with that principle fundamental to all criminal trials, the presumption of innocence.
- The burden of proof rests on the prosecution throughout the trial and never shifts to the accused.
- A reasonable doubt is not a doubt based upon sympathy or prejudice, and instead, is based on reason and common sense.
- Reasonable doubt is logically connected to the evidence or absence of evidence.
- Proof beyond a reasonable doubt does not involve proof to an absolute certainty. It is not proof beyond any doubt, nor is it an imaginary or frivolous doubt.
- More is required than proof that the accused is probably guilty. A jury which concludes only that the accused is probably guilty must acquit.
The Court also warned trial judges that they should avoid explaining the concept in the following ways:
- By describing the term “reasonable doubt” as an ordinary expression which has no special meaning in the criminal law context.
- By inviting jurors to apply to the task before them the same standard of proof that they apply to important, or even the most important, decisions in their own lives.
- By equating proof “beyond a reasonable doubt” to proof “to a moral certainty”.
- By qualifying the word “doubt” with adjectives other than “reasonable”, such as “serious”, “substantial”, or “haunting”, which may mislead the jury.
- By instructing jurors that they may convict if they are “sure” that the accused is guilty, before providing them with a proper definition as to the meaning of the words “beyond a reasonable doubt”.
The Supreme Court of Canada has since emphasized in R. v. Starr that an effective way to explain the concept is to tell the jury that proof beyond a reasonable doubt "falls much closer to absolute certainty than to proof on a balance of probabilities." It is not enough to believe that the accused is probably guilty, or likely guilty. Proof of probable guilt, or likely guilt, is not proof beyond a reasonable doubt.
In New Zealand, jurors are typically told throughout a trial that the offence must be proved "beyond reasonable doubt", and judges usually include this in the summing-up. There is no absolute prescription as to how judges should explain reasonable doubt to juries. Judges usually tell jurors that they will be satisfied beyond reasonable doubt if they "feel sure" or "are sure" that the defendant is guilty. In line with appellate court direction, judges do little to elaborate on this or to explain what it means.
Research published in 1999 found that many jurors were uncertain what "beyond reasonable doubt" meant. "They generally thought in terms of percentages, and debated and disagreed with each other about the percentage certainty required for 'beyond reasonable doubt', variously interpreting it as 100 per cent, 95 per cent, 75 per cent and even 50 per cent. Occasionally this produced profound misunderstandings about the standard of proof."
In the United States, juries must be instructed to apply the reasonable doubt standard when determining the guilt or innocence of a criminal defendant, but there is much disagreement as to whether the jury should be given a definition of "reasonable doubt." In Victor v. Nebraska, the U.S. Supreme Court expressed disapproval of the unclear reasonable doubt instructions at issue, but stopped short of setting forth an exemplary jury instruction.
Since 1945, Japan has also operated by a "reasonable doubt" standard, including the exceedingly liberal doctrine of in dubio pro reo, which was instituted by the Supreme Court during a controversial murder trial in 1975. However, this is not considered an essential standard in Japan and lower level judges sometimes disregard it.
- Critical thinking
- Moral certainty
- Probable cause
- Reasonable suspicion
- In dubio pro reo
- What Are the Origins of Reasonable Doubt?, HNN.us, February 25, 2008.
- R v Majid  EWCA Crim 2563 (12 October 2009)
- Woolmington v DPP  UKHL 1 (23 May 1935)
- R. v. Brydon, [1995 4 SCR 253] (SCC 1995).
- R. v. Lifchus, [1997 3 SCR 320] (SCC 1997).
- R. v. Starr, [2000 2 SCR 144], 242 (SCC 2000) (“In my view, an effective way to define the reasonable doubt standard for a jury is to explain that it falls much closer to absolute certainty than to proof on a balance of probabilities.”).
- R. v. Layton, [2009 2 SCR 540] (SCC 2009).
- Young, Warren; Cameron, Neil; Tinsley, Yvette (November 1999). Juries in Criminal Trials: Part Two (PDF). Preliminary Paper 37 2. Wellington, New Zealand: Law Commission. p. 54. ISBN 1-877187-42-9. Retrieved 14 April 2012.
- Young, William (2003). "Summing-up to juries in criminal cases – what jury research says about current rules and practice". Crim LR 665: 674. Retrieved 14 April 2012.
- Diamond, H. A. (1990). "Reasonable doubt: to define, or not to define". Columbia Law Review 90 (6): 1716–1736. doi:10.2307/1122751.
- Victor v. Nebraska, 511 U.S. 1 (1994)