Castration anxiety is a fear posited by Sigmund Freud in his writings on the Oedipus complex at the genital stage of sexual development. It asserts that boys seeing a girl's genitalia will falsely assume that the girl must have had her penis removed, probably as punishment for some misbehavior, and will be anxious lest the same happen to him.
It is worth noting that in some cultures, notably 19th century Europe, it was not unheard of for parents to threaten their children with castration, or to otherwise threaten their genitals, a phenomenon Freud documents several times. This may help to explain Freud's reasoning regarding castration anxiety's role in human development.
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