18 U.S. Code § 2. Principals
(a) Whoever commits an offense against the United States or aids, abets, counsels, commands, induces or procures its commission, is punishable as a principal.
(b) Whoever willfully causes an act to be done which if directly performed by him or another would be an offense against the United States, is punishable as a principal.
Bolding courtesy of Green Eagle.
Notice that there is no exception there for "I was just joking."