In the 1925 case Hammer v. United States, the Supreme Court upheld a conviction under the Mann Act, which made it illegal to transport women across state lines for "immoral purposes." The defendant, Dr. Andrew Hammer, had taken his secretary from Washington D.C. to Virginia and then Maryland where they engaged in sexual relations while he was still married to another woman. He argued that their relationship was consensual and therefore not "immoral" as defined by the act. However, Justice Oliver Wendell Holmes Jr., writing for a unanimous court, rejected this argument stating that adultery is widely regarded as immoral regardless of consent or lack thereof between parties involved in adulterous relationships. Therefore Dr.Hammer's actions were indeed covered by provisions of Mann Act leading to upholding his conviction.

In the dissenting opinion for Hammer v. United States, Justice Oliver Wendell Holmes Jr., joined by Justice Louis Brandeis, argued that the federal government had overstepped its constitutional bounds in prosecuting a man under child labor laws. The majority ruled that Benjamin Hammer's interstate sale of items produced with child labor constituted illegal trafficking and was therefore within Congress' power to regulate under the Commerce Clause. However, Holmes contended this interpretation stretched the clause too far and intruded on states' rights to regulate their own internal affairs - including labor conditions. He believed it was not Congress’ role to dictate morality or establish standards of proper conduct but rather each state’s responsibility to determine such matters according to their individual circumstances and values. Thus, he viewed this case as an example of federal overreach into areas traditionally governed by state law.