In addition to these, the U.S. introduced a third standard called clear and convincing evidence, (which is the medium level of proof).
The first attempt to quantify reasonable doubt was made by Simon in 1970. In the attempt, she presented a trial to groups of students. Half of the students decided the guilt or innocence of the defendant. The other half recorded their perceived likelihood, given as a percentage, that the defendant committed the crime. She then matched the highest likelihoods of guilt with the guilty verdicts and the lowest likelihoods of guilt with the innocent verdicts. From this, she gauged that the cutoff for reasonable doubt fell somewhere between the highest likelihood of guilt matched to an innocent verdict and the lowest likelihood of guilt matched to a guilty verdict. From these samples, Simon concluded that the standard was between 0.70 and 0.74.
A good illustration of this is the continuum of a typical police/citizen interaction:
Consensual encounter between officer and citizen (no level of suspicion required) →a stop initiated by the officer that would cause a reasonable person to feel that he or she is not free to leave (reasonable suspicion required) →arrest (probable cause required).
In the criminal context, the U.S. Supreme Court in United States v. Sokolow, , determined that probable cause requires "a fair probability that contraband or evidence of a crime will be found" in determining whether Drug Enforcement Administration agents had a reason to execute a search. Courts vary when determining what constitutes a "fair probability," some say 30%, others 40%, others 51%.
What is the burden of proof? First, we must address the meaning of the word “burden.” Most often jurors interpret this word as meaning weight. Jurors picture the state in the person of the prosecutor with a massive object on his back attempting to carry it up some incline for some distance– defense attorneys have been heard to say that the state has a “heavy burden.” The word “burden” has nothing to do with weight, mass or any other physical properties – the word simply means responsibility. It is the state’s responsibility to prove the defendant’s guilt. It has nothing to do with the degree or intensity of proof. Who has to prove the defendant’s guilt? The State does. To what degree must guilt be proven? Beyond a reasonable doubt. What does that mean? Again the problem is with words being used in an abnormal or special way. The word “beyond” normally means farther than or more than. Clearly this is not the meaning of the word in the phrase “beyond a reasonable doubt.” The state does not have to “carry its burden” beyond some point that constitutes reasonable doubt. The state certainly is not trying to prove that there is more than a reasonable doubt.If anything the state’s responsibility is to prove that there is less than a reasonable doubt. The word “beyond” in the phrase beyond a reasonable doubt means “to the exclusion of.” That is the state must exclude any and all reasonable doubt as to the defendant’s guilt. Simply put, the phrase means that if a juror has a reasonable doubt it is her duty to return a verdict of not guilty. On the other hand, if a juror does not have a reasonable doubt then the state has met its burden of proof and it is the juror’s duty to return a verdict of guilty.
“What is a reasonable doubt?” Jury instructions typically say that a reasonable doubt is a doubt based on reason and common sense and typically use phrases such as “fully satisfied” or “entirely convinced” in an effort to quantify the standard of proof. .These efforts tend to create more problems that they solve. For example, take the phrases “fully satisfied” and “entirely convinced.” A person is satisfied when she is content, pleased, happy, comfortable or at ease. The fellow leans back in his chair after a meal, pats his stomach and says, “that was one satisfying meal.” Is that what the state must do - offer sufficient proof that a juror is content, happy, pleased or comfortable with her verdict. Absolutely not. A juror is not required to be pleased with the verdict or happy with the verdict. The state is not required to produce sufficient evidence to eliminate all reasonable doubt AND to please the juror or to eliminate all reservations about whether the juror has done the right thing. “Satisfied” in the phrase “fully satisfied” simply means convinced. [See, http://www.merriam-webster.com/dictionary/satisfied]. Likewise the modifiers "entirely" and "fully" do not mean that you have to be 100 percent certain of the defendant’s guilt. The standard of proof is not absolute certainty. A juror is "fully satisfied" or "entirely convinced" when the state had eliminated all reasonable doubt.
Jury instructions often state that a reasonable doubt can arise from the "lack or insufficiency of the evidence." This phrase is rich with possibilities for concocting doubt – Where are the fingerprints? Where is the DNA evidence? Where are the other officers who assisted with the arrest? These arguments invite, actually require that the jury engage in speculation – something a jury is specifically instructed not to do. An example, a person enters a store. The clerk who is talking to her friend on the telephone sees the man. She tells her friend that the man appeared to be casing the place and asks her friend to call the police. A few minutes later the man leaves the store, walks to his car, opens the trunk, and retrieves a ski-mask and a shotgun. The man dons the mask, re-enters the store and tells the clerk to give it up. The clerk does as she is told and put the contents of the till into a bag which she hands to the man. The man then leaves the store. As he is running to his car the police arrive. The man flees from the scene with the police officers in hot pursuit. As he runs the man tosses the bag, gun and mask. He is caught shortly thereafter, returned to the store and is positively identified by the clerk as the man who cased the store and then robbed her. The bag is retrieved and the money in the bag exactly matches to the penny the amount taken from the register. At the trial, the defense attorney asks the lead investigator whether hair samples were taken from the mask and submitted to the lab for analysis. The investigator says no. During closing arguments the defense attorney conveniently ignores all the evidence of guilt and pounds away at the sloppy investigation and argues that had the hair analysis could have provided the jury with "irrefutable evidence" of the defendant's guilt or innocence. Is the absence of the hair evidence what the phrase “lack of insufficiency of the evidence” refers to. No. The phrase refers to the convincing force of the evidence presented. The presence or absence of reasonable doubt is to be determined by the evidence presented at trial not what might have been presented. There is a standard objection- Calls for speculation – that is exactly what the defense attorney is asking the jury to do, to speculate. Not simple speculation but a series of "what ifs." What if a hair sample had been found, what if the hair sample had been sent to the lab for DNA analysis, what if he DNA profile had not “matched” the defendant’s. What if + what if + what if = reasonable doubt. Remember that the state’s duty is to eliminate any reasonable doubt, any logical explanation that arises from the evidence. The defense's argument is not a proper argument. It is a “tool of logical inversion” All the evidence would compel one to say the defendant is guilty. However, the defendant wants the jurors to think, "but still there is that missing hair analysis evidence. I wonder what that would have shown?" A jury properly draw conclusion based on the evidence and inferences drawn from the evidence. The strength of the conclusions is based on the persuasive force of the evidence. With one exception, "Lack or insufficiency" refers to the convincing force of the evidence presented. The exception is the missing witness rule.
The missing witness rule, is:
"The failure to call a witness raises a presumption of inference that the testimony of such person would be unfavorable to the party failing to call him, but there is no such presumption or inference where the witness is not available, or where his testimony is unimportant or cumulative, or where he is equally available to both sides.
"The reasonable-doubt standard plays a vital role in the American scheme of criminal procedure. It is a prime instrument for reducing the risk of convictions resting on factual error. The standard provides concrete substance for the presumption of innocence that bedrock "axiomatic and elementary" principle whose "enforcement lies at the foundation of the administration of our criminal law. . Proof beyond a reasonable doubt did not become the accepted standard in criminal cases until the middle of the nineteenth century. Proof beyond a reasonable doubt was not the standard by which guilt was determined when the Bill of Rights was drafted in 1789. This may explain the absence of the phrase in the constitution. Nor was it an element of due process.
Attempts to quantify the burden of proof are exercises in futility. It is more a qualitative than quantitative concept. As Rembar notes, "Proof beyond a reasonable doubt is a quantum without a number.
For example, if the defendant (D) is charged with murder, the prosecutor (P) bears the burden of proof to show the jury that D did murder someone.
In other countries, criminal law reverses the burden of proof, and there is a presumption of guilt.
However, in England and Wales, the Magistrates' Courts Act 1980, s.101 stipulates that where a defendant relies on some "exception, exemption, proviso, excuse or qualification" in his defence, the legal burden of proof as to that exception falls on the defendant, though only on the balance of probabilities. For example, a person charged with being drunk in charge of a motor vehicle can raise the defence that there was no likelihood of his driving while drunk. The prosecution have the legal burden of proof beyond reasonable doubt that the defendant exceeded the legal limit of alcohol and was in control of a motor vehicle. Possession of the keys is usually sufficient to prove control, even if the defendant is not in the vehicle and is perhaps in a nearby bar. That being proved, the defendant has the legal burden of proof on the balance of probabilities that he was not likely to drive.
Similar rules exist in trial on indictment. Some defences impose an evidential burden on the defendant which, if met, imposes a legal burden on the prosecution. For example, if a person charged with murder pleads the right of self-defense, the defendant must satisfy the evidential burden that there are some facts suggesting self-defence. The legal burden will then fall on the prosecution to prove beyond reasonable doubt that the defendant was not acting in self-defence.
In 2002, such practice in England and Wales was challenged as contrary to the European Convention on Human Rights (ECHR), art.6(2) guaranteeing right to a fair trial. The House of Lords held that such burdens were not contrary to the ECHR:
The burden of proof must be distinguished from the "burden of going forward," which simply refers to the sequence of proof, as between the plaintiff and defendant. The two concepts are often confused.
In Keyes v. Sch. Dist. No. 1, , the United States Supreme Court stated: “There are no hard-and-fast standards governing the allocation of the burden of proof in every situation. The issue, rather, ‘is merely a question of policy and fairness based on experience in the different situations.’” For support, the Court cited 9 John H. Wigmore, Evidence § 2486, at 275 (3d ed. 1940). In Keyes, the Supreme Court held that if “school authorities have been found to have practiced purposeful segregation in part of a school system,” the burden of persuasion shifts to the school to prove that it did not engaged in such discrimination in other segregated schools in the same system.
In Director, Office of Workers’ Compensation Programs v. Greenwich Collieries, , the Supreme Court explained that burden of proof is ambiguous because it has historically referred to two distinct burdens: the burden of persuasion, and the burden of production.
The Supreme Court discussed how courts should allocate the burden of proof (i.e., the burden of persuasion) in Schaffer ex rel. Schaffer v. Weast, . The Supreme Court explained that if a statute is silent about the burden of persuasion, the court will “begin with the ordinary default rule that plaintiffs bear the risk of failing to prove their claims.” In support of this proposition, the Court cited 2 J. Strong, McCormick on Evidence § 337, 412 (5th ed. 1999), which states:
At the same time, the Supreme Court also recognized “The ordinary default rule, of course, admits of exceptions.” “For example, the burden of persuasion as to certain elements of a plaintiff's claim may be shifted to defendants, when such elements can fairly be characterized as affirmative defenses or exemptions. See, e.g., FTC v. Morton Salt Co., 334 U.S. 37, 44-45 (1948). Under some circumstances this Court has even placed the burden of persuasion over an entire claim on the defendant. See Alaska Dept. of Environmental Conservation v. EPA, .” Nonetheless, “[a]bsent some reason to believe that Congress intended otherwise, therefore, [the Supreme Court] will conclude that the burden of persuasion lies where it usually falls, upon the party seeking relief.”
Taken more generally, the standard of proof demanded to establish any particular conclusion varies with the subject under discussion. Just as there is a difference between the standard required for a criminal conviction and in a civil case, so there are different standards of proof applied in many other areas of life.
The less reasonable a statement seems, the more proof it requires. The scientific consensus on cold fusion is a good example. The majority believes this can not really work, because believing that it would do so would force the alteration of a great many other tested and generally accepted theories about nuclear physics.