The biggest issue seems to be defining what an "immoral purpose" is. Note that the first case deals with the general question of whether the powers claimed by the act falls within constitutional parameters.
These cases led to that clarification:
- Hoke v. United States, (1913). The Court held that Congress could not regulate prostitution per se, as that was strictly the province of the states. Congress could, however, regulate interstate travel for purposes of prostitution or "immoral purposes".
- Athanasaw v. United States, (1913). The Court decided that the law was not limited strictly to prostitution, but to "debauchery" as well.
- Caminetti v. United States, (1917). The Court decided that the Mann Act applied not strictly to purposes of prostitution, but to other noncommercial consensual sexual liaisons. Thus consensual extramarital sex falls within the genre of "immoral sex".
- Gebardi v. United States, (1932). The Court held that the statutory intent was not to punish a woman's acquiescence; therefore, consent by the woman does not expose her to liability.
- Cleveland v. United States, (1946). The Court decided that a person can be prosecuted under the Mann Act even when married to the woman if the marriage is polygamous. Thus polygamous marriage was determined to be an "immoral purpose".
- Bell v. United States, (1955). The Supreme Court decided that simultaneous transportation of two women across state lines constituted only one violation of the Mann Act, not two violations.
