New players are to be banned from dressing up as women, as part of Major League Baseball’s anti-bullying policy.
Other prohibited costumes include those that are “offensive to individuals based on their race, sex, nationality, age, sexual orientation, gender identity or other characteristic”.
Superhero costumes such as Batman and Spiderman are still allowed.
The annual dress up day is a tradition for rookies in an initiation practice called “hazing”.
In September, the New York Mets posted photos and video of players wearing uniforms from the All-American Girls Professional Baseball League, as portrayed in the 1992 Hollywood movie “A League of Their Own.”
The anti-hazing and anti-bullying policy, part of the MLB’s new labour contract, was obtained by the Associated Press.
“Times have changed. There is certain conduct that we have to be conscious of,” said Dave Prouty, from the players’ union.
The league’s vice president Paul Mifsud said the new rules were “in light of social media” which he felt “unfortunately publicised” the costumes, and “those kind of things which in our view were insensitive and potentially offensive to a number of groups”.
He also said a number of players had complained about the tradition.
Other hazing acts that have been banned include making players consume alcohol, drugs or “undesirable unwanted substance (food, drink, concoction)”.
“The purpose of this policy is not to prohibit all traditions regarding rookies or players,” the new policy states, “but rather to prohibit conduct that may cause players physical anguish or harm, may be offensive to some players, club staff or fans, or are distracting to the operation of the club or MLB.”