Whoever knowingly transports any individual in interstate or foreign commerce, or in any Territory or Possession of the United States, with intent that such individual engage in prostitution, or in any sexual activity for which any person can be charged with a criminal offense, shall be fined under this title or imprisoned not more than five years, or both.
A 1986 amendment to the Mann Act substituted the current phrase "engage in prostitution, or in any sexual activity for which any person can be charged with a criminal offense" for earlier language, which prohibited transportation of any "woman or girl" in interstate or foreign commerce for the "purpose of prostitution or debauchery, or for any other immoral purpose".
Historical events leading to the enactment of the original statute are worth noting. From 1907 to 1914, there was wave of hysteria about "white slavery," the alleged practice of actually kidnapping young women and forcing them into brothels. The statute, initially passed to implement US adherence to the 1904 International Convention for the Suppression of the White Slave Traffic aimed at the trade in white women in forced sex-for-hire, was a legislative response to an early 20th century “white slavery panic”. Also, prostitution, being an established fact of life in big cities and the subject of intense concern among reformers, was viewed as a threat to public decency and public health due to the sexually transmitted diseases associated with it. Puritans, social reformers, and hygienists all agreed that prostitution was a behavioral problem that needed to be controlled and changed through legislation.
In a series of cases from Hoke v U.S., 227 U.S. 308 (1913) to Caminetti v U.S., 242 U.S. 470 (1917), the US Supreme Court upheld the constitutionality of the original language of the Mann Act under a view of federalism that expanded federal regulatory power. Since then, the Mann Act has been used to prosecute a number of controversial political and public figures. Among them were the famous black boxer Jack Johnson for his affair with prostitute Lucille Cameron whom he later married, Charlie Chaplin for his involvement with actress Joan Barry and singer Chuck Berry after he invited a 14-year-old Apache waitress whom he met in Mexico to work as a hat check girl at his club.
Whether the Mann Act is a legitimate exercise of federal power or a statute that has been misused by overreaching federal prosecutors is a debate worth having. Useful reading on the topic is David Langum's Crossing Over the Line: Legislating Morality and the Mann Act, Call # KF9449 .L36 1994 in the BLS Library.
See also Sex, Corruption, Federalism, & the Mann Act posted in yesterday’s PrawfsBlawg.