Thus, an act is illegal without extrinsic proof of any surrounding circumstances such as lack of scienter (knowledge) or other defenses.
In the United States, illegal per se often refers to categories of anti-competitive behavior in antitrust law conclusively presumed to be an "unreasonable restraint on trade" and thus anti competitive.
The United States Supreme Court has, in the past, determined activities such as price fixing, geographic market division, and group boycott to be illegal per se regardless of the reasonableness of such actions.
Traditionally, illegal per se anti-trust acts describe horizontal market arrangements among competitors.
The illegal per se category can trace its origins in the 1898 Supreme Court case Addyston Pipe & Steel Co. v. U.S., 175 U.S. 211 (1898).