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First, the presumption is not a true presumption at all.[1] An objective observer in the position of the juror would reasonably conclude that the defendant probably committed the crime with which he is charged.[2] The observable facts clearly support such an inference—the defendant has been charged with a crime, is present in court, represented by an attorney and all the participants in a criminal trial are also present and ready to proceed.[3]

The presumption of innocence is in fact a legal instrument created by the law to favor the accused based on the legal inference that most people are not criminals.[4] It is literally considered favorable evidence for the accused that automatically attaches at trial.[5] It requires that the trier of fact, be it a juror or judge, begin with the presumption that the state is unable to support its assertion.[4]To ensure this legal protection is maintained a set of three related rules govern the procedure of criminal trials. The presumption means:[1]

With respect to the critical facts of the case—whether the crime charged was committed and whether the defendant was the person who committed the crime—the state has the entire burden of proof.
With respect to the critical facts of the case, the defendant does not have any burden of proof whatsoever. The defendant does not have to testify, call witnesses or present any other evidence and if the defendant elects not to testify or present evidence this decision cannot be used against him.
The jury or judge is not to draw any inferences against the defendant from the fact that he has been charged with a crime and is present in court and represented by an attorney. They must decide the case solely on the evidence presented during the trial.
The phrase that a person is innocent until proven guilty refers to legal as opposed to factual guilt. In every case, the defendant either committed the offense or he did not; a fact that will remain true regardless of whether the jury acquits or convicts the defendant. The phrase means simply that a person is not legally guilty until a jury returns a verdict of guilty—which is little more than a tautology.

This duty on the prosecution was famously referred to as the “golden thread” in the criminal law by Lord Sankey LC in Woolmington v DPP [1935] AC 462:

“ 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... ”

[edit] The fundamental right
This right is so important in modern democracies that many have explicitly included it in their legal codes and constitutions:

The Convention for the Protection of Human Rights and Fundamental Freedoms of the Council of Europe says (art. 6.2): “Everyone charged with a criminal offence shall be presumed innocent until proved guilty according to law”. This convention has been adopted by treaty and is binding on all Council of Europe members. Currently (and in any foreseeable expansion of the EU) every country member of the European Union is also member to the Council of Europe, so this stands for EU members as a matter of course. Nevertheless, this assertion is iterated verbatim in Article 48 of the Charter of Fundamental Rights of the European Union.
In Canada, section 11(d) of the Canadian Charter of Rights and Freedoms states: “Any person charged with an offence has the right to be presumed innocent until proven guilty according to law in a fair and public hearing by an independent and impartial tribunal”.
In France, article 9 of the Declaration of the Rights of Man and of the Citizen, of constitutional value, says “Everyone is supposed innocent until having been declared guilty.” and the preliminary article of the code of criminal procedure says “any suspected or prosecuted person is presumed to be innocent until their guilt has been established”. The jurors’ oath reiterates this assertion.
Although the Constitution of the United States does not cite it explicitly, presumption of innocence is widely held to follow from the 5th, 6th and 14th amendments. See also Coffin v. United States
In the 1988 Brazilian constitution, article 5, section LVII states that “no one shall be considered guilty before the issuing of a final and unappealable penal sentence”.
The Universal Declaration of Human Rights, article 11, states: Everyone charged with a penal offence has the right to be presumed innocent until proved guilty according to law in a public trial at which they have had all the guarantees necessary for their defence.

[edit] The presumption of innocence in practice

Article 48 of the Charter of Fundamental Rights of the European Union affirms the right to the presumption of innocenceFew legal systems have employed, de jure, a presumption of guilt. Accusations of presumption of guilt generally do not imply an actual legal presumption of guilt, but rather denounce some failures to ensuring that suspects are treated well and are offered good defense conditions. Typical infringements include:

In some systems, suspects may be detained for long periods while inquiries proceed. Such long imprisonment constitutes, in practice, a hardship and a punishment for the suspect, even though he or she has not yet been sentenced. (See speedy trial)
Courts may prefer the testimonies of persons of certain class, status, ethnicity, gender, or political standing over those of others, regardless of actual circumstances.
In Europe and the Americas, prior to the French Revolution, it was common for the justice system to have suspects tortured to extract confessions from them. Even though the suspects had not yet been found legally guilty, they were exposed to considerable pain, often with lasting physical consequences.
Many public institutions such as universities punish members accused of felonies after they are indicted, even if they have not been convicted. An example is the 2006 Duke University lacrosse team scandal, in which the accused were suspended even though they had not been convicted.
Guaranteeing the presumption of innocence extends beyond the judicial system. For instance, in many countries journalistic codes of ethics state that journalists should refrain from referring to suspects as though their guilt is certain. For example, they use “suspect” or “defendant” when referring to the suspect, and use “alleged” when referring to the criminal activity that the suspect is accused of.

More subtly, publishing of the prosecution’s case without proper defense argumentation may in practice constitute presumption of guilt. Publishing a roster of arrested suspects may constitute undeserved punishment as well, since in practice it damages the reputation of innocent suspects. Private groups fighting certain abuses may also apply similar tactics, such as publishing the real name, address, and phone number of suspects, or even contacting the suspects’ employer, friends and neighbors (as an example, Perverted-Justice.com does so in order to shame suspected child molesters).

Modern practices aimed at curing social ills may run against presumption of innocence. Some civil rights activists feel that pre-employment drug testing, while legal, violates this principle, as potential employees are presumed to be users of illegal drugs, and must prove themselves innocent via the test. Similarly, critics argue that some dispositions of laws against sexual harassment or racial discrimination show a presumption of guilt. These dispositions were meant to ease the burden of proof on the victim, since in practice harassment or discrimination practices are hard to prove.

Civil rights activists note that the well-meaning practices so adopted may have a deleterious effect on justice being served. An example is the use in some sexual assault cases of a screen, which is set up to prevent the complainant from being distressed at the sight of the accused. Where a victim was in fact victimized by the accused, this may be argued to serve the principles of therapeutic justice [1] [2]. However, where an accused is in fact innocent, this may send a message to the jury that the court has already accepted that in fact a crime was committed, the burden of proof of which has traditionally been on the prosecution, and which furthermore is a matter of fact that is not for the court to judge, but rather, for the jury. Not only this but also even more importantly, such a shield may also send a message that the complainant is upset by the sight of the accused, once again because guilt is seen to have been assumed by the court in so shielding the complainant. The psychological effects of such a screen have not yet been well researched, but the tension between the two views is a problem for therapeutic justice, which must weigh protection of genuine victims from genuine offenders against the potential for an unjust conviction that such protection may create.[3]

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