From wikipedia (where I get all my legal advice):
Quote:
Use in jury proceedings[edit]
In criminal jury trials, the court is permitted (but not required) to instruct jurors that they can find the defendant guilty of the most serious crime charged, or of a lesser included offense of that crime (in English law, this is termed an alternative verdict).
In murder cases, however, where a convicted defendant may face capital punishment, the United States Supreme Court has held that the court must instruct the jury that they may find the defendant guilty of a lesser included offense such as voluntary manslaughter.[1] The reasoning for this ruling is that jurors, given the options of convicting a less culpable killer or letting him go free, might opt to convict of a more serious crime than the facts warrant. Therefore, they must have at least one option that falls in between these extremes.
In the case in which the jury has the option of convicting a defendant accused of a violation of law where there is a lesser included offense, if the jury acquits the defendant of the more serious offense but is otherwise unable to reach a verdict (i.e., is hung) on the lesser included offense, the defendant may be retried if the prosecutor chooses, but only for the lesser included offense. If the jury finds the defendant not guilty of the lesser included offense, there would be no need to make a determination on the more serious offense, as acquittal of a lesser included offense automatically constitutes acquittal of the more serious offense.
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http://en.wikipedia.org/wiki/Lesser_included_offense
Sounds like it has always been there actually.
Edited in: The Supreme Court Case they mention is from 1980 so that is when the SC said it has to be used in murder cases.
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