The word "capital" in "capital punishment" refers to a person's head. In the past, people were often executed by severing their head from their body. Today, in the U.S., most prisoners are executed by lethal injection.
The United States is one of the very few industrialized countries in the world which continues to execute criminals. Further, it is one of a handful of countries in the world which executes mentally ill persons, persons with very low IQ, and child murderers (i.e. persons who were under 18 at the time of their crime).
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