https://www.criminaldefenselawyer.c...criminal-defense-case/affirmative-defense.htm
Sometimes a criminal defendant is entitled to acquittal even though the prosecution has proven every element of the charged offense. This happens when the defendant has successfully raised and the jury has accepted an “affirmative defense” that operates by law to exonerate him.
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The defense strategy described just above isn’t the only way a criminal defendant can obtain an acquittal. Instead of (or in addition to) trying to defeat the prosecutor’s goal of proving every element, a defendant can also introduce evidence of his own that, if believed by the jury, will defeat the charge.
The word “affirmative” in the term refers to the requirement that the defendant prove the defense, as opposed to negating the prosecution’s evidence of an element of the crime.
An affirmative defense operates to prevent conviction even when the prosecutor has proof beyond a reasonable doubt as to every element of the crime. For example, consider again the elements that the state must prove when charging murder (that a human died, killed by another human, who intended to do so). Now imagine a claim of self-defense—the defendant says that he shot the victim only after the victim attacked him. There is no doubt that a human died, killed by another human who intended to do so. But if the jury believes the defendant’s claim, he will escape conviction.
An affirmative defense operates to prevent conviction even when the prosecutor has proof beyond a reasonable doubt as to every element of the crime.
For example, consider again the elements that the state must prove when charging murder (that a human died, killed by another human, who intended to do so). Now imagine a claim of self-defense—the defendant says that he shot the victim only after the victim attacked him. There is no doubt that a human died, killed by another human who intended to do so. But if the jury believes the defendant’s claim, he will escape conviction.