To convict a criminal defendant, the prosecutor must prove guilt beyond a reasonable doubt. Of course, the defendant gets an opportunity to present a defense. There are many defenses, from "I didn't do it" to "I did it, but it was self-defense" and beyond. Read more about some common defenses below.
Most often, defendants go with the claim that they simply did not commit the alleged act in question. They might argue that someone else committed the crime or that no crime was committed in the first place. A defense attorney taking the position that no offense happened might claim that witnesses' versions of events are untrue or that the client's actions didn't satisfy all the elements of the offense.
Below are a couple concepts within the "didn't do it" line of defense: the presumption of innocence and alibi.
All people accused of crime are legally presumed to be innocent until the point of conviction, whether that comes by way of trial or plea. This presumption means that the prosecution must convince the jury of the defendant's guilt, rather than the defendant having to prove innocence. A defendant may simply remain silent and not present any witnesses, then argue that the prosecution failed to prove its case. But, in practice, defense attorneys often present their own witnesses in order to counteract the government's case.
The prosecutor must convince the fact-finder of the defendant's guilt "beyond a reasonable doubt." This heavy burden of proof requires that the jury (in some cases, the judge) have a moral certainty that the defendant is guilty. With such a high burden on the opposition, defense attorneys often impress upon juries that thinking the defendant committed the crime isn't enough for a conviction.
An alibi defense consists of evidence that the defendant was somewhere other than the scene of the crime at the time of the crime. For example, assume that Jones is accused of committing a burglary on Hampton Street at midnight on September 14. Alibi evidence might consist of a friend's testimony that he and Jones were at the movie theater from 11:00 p.m. to 1:00 a.m. on the night of the 14th. Additional alibi evidence would be the movie tickets and a credit card statement showing that Jones purchased them shortly before 11:00 p.m.
Sometimes an acquittal is the result even if the prosecutor showed that that the defendant did, without a doubt, commit the alleged act. There are many kinds of "excuse" defenses; read about a few below.
Self-defense is commonly asserted by those charged with crimes of violence, such as battery, assault, or murder. The defendant admits to having used violence, but claims that it was justified by the other person's threatening or violent actions. The core issues in self-defense cases often are:
Self-defense is rooted in the belief that people should be allowed to protect themselves. In part, this means that someone doesn't necessarily have to wait to the point of actually being struck in order to act in self-defense. Whatever the circumstances are, any force that the would-be defendant uses has to be reasonable under the circumstances.
The insanity defense is generally based on the principle that punishment is justified only where defendants are capable of controlling their behavior and understanding that what they've done is wrong. The insanity defense prevents some people who can't function fully from being criminally punished.
The insanity defense is an extremely complex topic; many scholarly works are devoted entirely to explaining its nuances. Here are some major points of interest:
Defendants who commit crimes under the influence of drugs or alcohol sometimes argue that their mental functioning was so impaired that they cannot be held accountable for their actions. Generally, however, voluntary intoxication does not excuse criminal conduct. (Involuntary intoxication, where applicable, tends to excuse more behavior.) The theory is that defendants know (or should know) how alcohol and drugs affect functioning, and that they shouldn't be off the hook because they chose to get themselves intoxicated.
Some states have an exception to this general rule: A defendant accused of committing a crime that requires "specific intent" can claim having been too drunk or high to have formed that intent. (Specific intent generally describes a situation where the defendant intends the precise consequences of an act in addition to intending the act itself.) Specific intent may be only a partial defense, however. For example, it might lead to an acquittal for assault with specific intent to kill but a conviction of assault with a deadly weapon, the latter of which typically doesn't require specific intent.
Entrapment occurs when the government induces the commission of a crime that the person wouldn't have otherwise committed. However, law enforcement officers can provide opportunities for defendants to commit crimes without committing entrapment. Entrapment is often difficult to prove, and, like the defenses above, can be complicated.
If you face criminal charges, consult an experienced criminal defense attorney. A knowledgeable lawyer should be able to fully explain the applicable law—which will depend on your jurisdiction—and protect your rights.