In 1910, the US Congress passed the Mann Act, making it illegal for anyone to transport across state lines, for immoral purposes, ... what ...?

Officially known as the White-Slave Traffic Act, this 1910 law was a product of the Progressive Era's moral crusades. Amid widespread public panic over forced prostitution, often fueled by sensationalized and racially charged stories, Congress made it a federal crime to transport any woman or girl across state lines for "prostitution, debauchery, or any other immoral purpose." It was a significant expansion of federal power, using the Constitution's Commerce Clause to legislate a moral issue that had previously been a state or local matter.

The act’s true power, and controversy, came from the vague wording of "immoral purpose." While ostensibly aimed at organized trafficking rings, prosecutors quickly began using the law to target individuals for consensual premarital or extramarital relationships. Its most famous application was the racially motivated prosecution of Black heavyweight boxing champion Jack Johnson for traveling with his white girlfriend. Other prominent figures, including actor Charlie Chaplin and musician Chuck (Review) Berry, also faced charges under the act, demonstrating its use as a tool against unpopular or controversial public figures.

While it may seem like a historical relic, the Mann Act is still on the books, though it has been amended. In 1986, the law was updated to be gender-neutral, making it illegal to transport "any individual" for criminal sexual activity. Its legacy continues in modern federal anti-human trafficking laws, which build upon the precedent it set for using federal authority to combat crimes that cross state borders.