DOI: 10.7910/dvn/ghoa6t
title: Replication Data for: Mapping the Amicus Ecosystem: Ideology, Influence, and Reform of Amicus Briefs at the U.S. Supreme Court

description: The modern Supreme Court sits at the center of an amicus-industrial complex. Nearly 2,000 amicus briefs now flood the Court each Term, appearing in virtually every argued case and in triple-digit flotillas in disputes like Dobbs, Obergefell, SFFA, and NetChoice. Yet basic questions about amicus curiae remain unanswered: Who are the friends of the Court? How often do they agree and oppose one another? Which cases do they target? And what does their coordinated behavior mean for constitutional fact-finding and the Court’s legitimacy?

This Article offers the most comprehensive empirical portrait of Supreme Court amicus practice ever assembled. A newly constructed dataset of 53,447 amicus briefs spanning the 1899-2024 Terms illustrates that the amicus ecosystem has been not merely enlarged but compositionally transformed. Government-based entities’ total share of amicus filings fell from 29.6% in 1980 to 9.7% in 2024, even as their absolute volume stayed roughly constant. Libertarian and free-market organizations expanded from 2.9% to 11.0% of all briefs—the largest categorical increase—whilst progressive organizations declined from 18.7% at the Warren Court’s civil-rights peak to 2.5%, and labor unions from 3.0% to 0.2%. Religious-liberty, social-conservative, and partisan/political organizations, mostly absent in the 1980s, now account for about 13% of amicus submissions before the Court.

Network analysis of agreement and opposition patterns among repeat-player amici exposes a durable oligopoly of advocacy blocs. The business-conservative coalition (Chamber, Cato, WLF, PLF, NFIB) files on the same side in 92-100% of shared cases. State and local government associations move in perfect lockstep across hundreds of disputes. Criminal-justice adversaries like CJLF and NACDL exhibit -100% opposition; they have never filed on the same side so far. But the ecosystem is not binary: libertarian organizations like Cato bridge ideological divides, achieving perfect alignment with NACDL on criminal justice (80-0) and 60% alignment with ACLU on civil liberties, even while maintaining near-total coordination with business-conservative allies on economic regulation. A four-factor account (constitutional salience, breadth of impact, mobilization capacity, and doctrinal opportunity) explains why a narrow band of cases attracts massive amicus flotillas regardless of whether the Court ultimately divides 6-3 or rules 9-0: repeat-players target perceived doctrinal stakes, not predicted controversy.

These structural findings unsettle the premises justifying the Court’s permissive amicus regime. Instead of independent voices supplying diverse expertise, the Court often hears from coordinated blocs extending party conflicts by other means. Instead of adversarial fact-checking, resource-rich coalitions sometimes inject contested causal claims (e.g., about abortion’s effects, affirmative action’s “mismatch,” regulatory costs) that bypass Rule 702 or Daubert-style safeguards, then harden into “factual precedents” binding lower federal courts. Building on foundational work on the amicus machine and amicus facts, this Article then links empirical mapping to more targeted reforms: short Empirical Annexes for briefs advancing causal claims, limited party replies focused on those empirics, lead-brief-plus-joinder incentives, and modest coordination disclosures. The overall objective is calibration, not closure, preserving broad access while addressing a specific, documented vulnerability: embedding of untested or unverified casual claims about social scientific issues into the constitutional canon.