Antitrust authorities all over the world are keen on the presence of a particularly aggressive competitor, a “maverick”. Yet there is a lack of theoretical justification. One plausible determinant of acting as a maverick is behavioral: the maverick derives utility from acting competitively. We test this conjecture in the lab. In a pretest, we classify participants by their social value orientation. Individuals who are rivalistic in an allocation task indeed bid more aggressively in a laboratory oligopoly market. This disciplines incumbents. We conclude that the existence of rivalistic attitudes may justify antitrust policies that protect mavericks.