A castrato (Italian, plural: castrati) is a male singer who underwent castration before puberty in order to retain singing voice equivalent to that of a soprano, ...
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mezzo-soprano, or contralto. The voice can also occur in one who, due to an endocrinological condition, never reaches sexual maturity.
Castration before puberty (or in its early stages) prevents the larynx from being transformed by the normal physiological events of puberty. As a result, the vocal range of prepubescence (shared by both sexes) is largely retained, and the voice develops into adulthood in a unique way. Prepubescent castration for this purpose diminished greatly in the late 18th century.