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Over the last two decades, more than a thousand amici, representing hundreds of organizations, companies, and individuals, have signed onto amicus briefs in over a hundred patent cases, many of them landmark decisions. This paper turns the spotlight on these “behind-the-scenes” actors in the patent system, combining theoretical insights with an empirical study of amicus briefs filed in patent cases over the last 20 years in an examination of patent interest groups, the positions they have advocated, and the effectiveness of their advocacy. Amicus filers appear to have been instrumental in shaping the courts’ agenda; the Supreme Court was seven times more ikely to grant cert, and the Federal Circuit eight times likely grant a petition for en banc rehearing, if urged to by an amicus. Thus, consistent with the affected groups theory, the courts seem to listen to, or at least agree with, their friends about which cases are important. It was harder to assess the influence of briefs on the merits. This study failed to find evidence that the courts were more likely to rule for the side supported by the greatest number of amici, but did not address or rule out other forms of influence that are harder to measure. Among individual amici, the briefs of the United States stood out as being exceptionally prescient. Over the 20 years studied, every single amicus brief authored by the United States in a Supreme Court patent case except one predicted the case outcome. That is to say, in almost all cases, the Court sided with the party the Government told it to, and in one case, dismissed cert as improvidently granted when the Government recommended doing so. In terms of who files briefs and their agenda, the results are somewhat surprising. Although debates about the patent system are usually cast as a fight between the pharmaceutical and hi-tech industries, patent lawyers, on their own and represented by professional associations, filed the most briefs of any single group. In addition, among companies, what seems largely to determine how amici advocate is their business model – non-practicing entities, for example, nearly always weighed in for the patentee and public companies, often against the patentee. These and other results have implications for those seeking to understand the patent system and those seeking to influence it.

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