It is axiomatic that an entity cannot “conspire” with itself. For example, the Supreme Court has held that a parent corporation and its subsidiary are not capable of an illegal conspiracy under the Sherman Antitrust Act.
Of course, as is true with most legal principles, what looks simple at 30,000 feet altitude becomes more complicated the closer one gets to the ground, and the courts have struggled with the definition of a “single entity” in a variety of contexts.
Dean Williamson of the DOJ Antitrust Division has written an interesting and in-depth paper analyzing the law and economics of this issue. The paper, titled Organization, Control and the Single Entity Defense in Antitrust, is published here.