What is the idea behind the concept of alibi in a legal system in which people are innocent until proven guilty?
An alibi is a particular kind of defense strategy bearing on the burden of proof: here is the alibi jury instruction for California.
The defendant contends (he/she) did not commit (this/these) crime[s] and that (he/she) was somewhere else when the crime[s] (was/were) committed. The People must prove that the defendant was present and committed the crime[s] with which (he/she) is charged. The defendant does not need to prove (he/she) was elsewhere at the time of the crime.
"I had put the gun down" is not an alibi defense, "I was not in my right mind at the time" is not an alibi defense. The alibi defense is essentially a formality that clarifies the logic of "reasonable doubt" for the jury. If the defendant could not have committed the crime because they weren't there, then that is the end of the discussion.
The prosecution will introduce various facts that suggest that the defendant may have committed the crime. The "reasonable doubt" instruction is widely interpreted by jurors to involve a defense obligation to disprove that evidence, which is a reason why a number of jurisdictions have adopted a different instruction based on "being firmly convinced". Reasons to doubt prosecution evidence can be weak and highly speculative, and the courts have struggled for years to find a good way to convey exactly what "reasonable doubt" is. An alibi goes way beyond merely detracting from the prosecution's case.
An alibi is not an affirmative defense where you have to prove the claim by a preponderance of evidence. If you can establish that you were somewhere else when the crime happened, had no opportunity to have committed the crime and could not have committed the crime by another means, you have defended yourself against the charge (assuming that the prosecution does not successfully challenge the credibility of the alibi witness). And in California, you have to give notice that you intend to use the alibi defense.
In most countries, the following happens before a conviction:
- The police believes the suspect to be guilty and passes the case to the prosecution. If the police don't believe the suspect is guilty, the suspect is not a suspect any more.
- The prosecution believes that they can win the case, based on the evidence, and files the case in court. This is not just a professional judgement of the guilt of the defendant but also of the sufficiency of the evidence. If they do not think that they can win, it is the duty of the prosecution to stop wasting taxpayer money.
- The prosecution convinces the judge or jury that the evidence shows guilt beyond reasonable doubt.
If an innocent defendant reaches step 3, there must be evidence which can be misconstrued to show guilt. At this point, just saying "I plead not guilty and there is reasonable doubt" is a terribly risky defense strategy. It gambles that the judge or jury will not make the same mistake as the prosecution.
So it makes sense to show evidence of innocence (the alibi) to rebut the evidence of the prosecution.
The phrase "innocent until proven guilty" is often misunderstood, not least because it isn't the whole phrase; it omits the the word "presumed."
An accused party is not literally held to be innocent until proven guilty but is rather presumed to be innocent until proven guilty. The primary effect of this presumption is to place the burden of proof on the prosecution.
One may indeed then reasonably ask what the role of alibi is in such a system. Why must a defendant (or a suspect) show that he or she was not in the place where the crime was committed at the time of commission? The answer is that this isn't in fact necessary unless the prosecution (or investigators) have some other evidence that tends to suggest that the defendant did commit the crime.
For example, if Alice owns a red 1969 Ford Mustang and Bob was seen to have been shot dead by the driver of a red 1969 Ford Mustang then this is evidence of Alice's guilt even though there are other possible explanations. A prosecutor can present this evidence at trial, and one response to this evidence could be for Alice to submit that she and her car were somewhere else at the time, leaving the "trier of fact" to make sense of the conflicting submissions. Alice could also simply argue that the prosecutor hadn't proven that the car from which Bob was shot was Alice's nor that Alice was in the car, much less that Alice had pulled the trigger, but you can see that this is a somewhat weaker argument. "It couldn't have been Alice" is a stronger statement than "you haven't shown that it was Alice."
In the early stages of an investigation, as we frequently see in fictional depictions, investigators use alibi to narrow the scope of their task. They may not at that point have sufficient evidence -- or any evidence at all -- to suggest that Alice killed Bob, but if they can establish quickly that Alice couldn't have killed Bob then they can save themselves a lot of work investigating Alice.
To put it another way, inspired by Sneftel's comment asking about "other evidence presented in defence," the prosecutor must present evidence of the defendant's guilt, but the defendant can present evidence to counter or "rebut" the prosecutor's evidence. Alibi is just one category of such evidence.
In a system where suspects have no obligation to respond to police questioning, suspects are not obliged to present an alibi if asked, even if they are in fact innocent of the crime under investigation (for example, they may have been committing another crime at the time). Defendants similarly do not need to present alibi evidence at trial; they may use other means to rebut prosecutors' evidence, or they may even refrain from doing so and accept a conviction for a crime they did not commit.
Proof beyond a reasonable doubt does not require that a prosecutor show that there is no conceivable string of coincidences which would be be consistent with the evidence even if the accused was not guilty, but merely that any such string of coincidences be sufficiently unlikely as to be implausible.
If the defendant has a distinctive red head resembling Lucille Ball, and someone who looked like that was caught on video robbing a store, and a defendant is found in the neighboorhood fairly soon thereafter, it may be possible that the defendant had just driven into the neighborhood at the same time as someone else who looked like Lucille Ball had robbed the store, but that would generally be unlikely.
If, however, the defendant could prove that the defendant had just entered the neighborhood after performing a comedy routine in front of a large audience, and that the defendant was on stage at the moment the robbery was taking place, that would suggest that there must have been some other red head in the neighborhood, even though that would seem an unlikely coincidence.
Alternatively, a defendant might introduce evidence that there was a Lucille Ball lookalike contest in the neighborhood, and that while her hairdo might have been distinctive compared to the population at large, it would have not stood out at all from many of the other women in the contest. This wouldn't be an alibi as such, but would again serve the purpose of showing that what might have seemed an implausibly unlikely coincidence had quite possibly in fact happened.
Note that some jurisdictions may require that claimed alibis be made available to the prosecution before trial. This is done to ensure that the prosecution has a fair chance to vet any alibi witnesses, determine whether they are reliable, and ensure that the jury is made aware of any evidence that might suggest the alibi witness is not especially reliable. It would also ensure that the prosecution can identify any evidence such as video takes to determine whether they might have been falsified. A prosecutor isn't allowed to simply say in trial "We have no way of knowing for certain whether the defendant's video might have been faked", inducing speculation that it might be. Instead, the prosecutor must be given time to find evidence--if any exists--of the falsification.
"Any person charged with an offence has the right... to be presumed innocent until proven guilty according to law..." (Charter, s. 11(d)).
When it appears to the defence that the prosecution will otherwise prove the offence, there will often be a practical burden on the defendant to introduce their alibi in order to introduce reasonable doubt about their guilt.
See Justice Pigeon's explanation in R. v. Proudlock (1978),  1 S.C.R. 525.
If there is nothing in the evidence adduced by the Crown from which a reasonable doubt can arise, then the accused will necessarily have the burden of adducing evidence if he is to escape conviction.
However, the burden is always on the prosecution to prove the guilt of the defendant beyond a reasonable doubt. A typical jury instruction includes a phrase such as:
Even if you are not left in a reasonable doubt by the evidence in support of the defence of alibi, you must still go on to determine whether or not on the basis of all the evidence the accused is guilty.
(R. v. Hibbert, 2002 SCC 39 at para. 55)
A defendant is asked to provide an alibi during a trial. It means that there was already enough evidence to prosecute the defendant. In this case the alibi is needed to counter that evidence. Absence of an alibi does not mean that the defendant is immediately considered guilty, but it makes more likely a guilty verdict at the end of the trial.
A suspect is asked to provide an alibi during an interrogation by the police. In this case it is in the interest of the suspect to provide an alibi to avoid being investigated and eventually prosecuted. Providing immediately an alibi is not mandatory, but providing that alibi later without a reasonable explanation might make it appear fabricated.
People are not guilty if they are “proven guilty beyond reasonable doubt”. They are guilty if the prosecutor can convince a jury that they are guilty beyond reasonable doubt. Not the same.
You are not “presumed innocent until proven guilty”, you are “presumed innocent until the prosecutor convinces the jury that you are guilty”. Not the same thing. You can be innocent yet be convicted.
An alibi makes it very much harder for the prosecutor to show that you are “guilty beyond reasonable doubt”, or just “guilty”. You would be mad not to show that you have an alibi, because that could be the difference between an incorrect conviction where you go to jail, and bring let go.