The Brookings Institution was once again ranked the top think tank in the world in 2013 by the Think Tanks and Civil Societies Program at the University of Pennsylvania.
Brookings, a center-left research organization in Washington, has placed first on the list every year since 2009.
Chatham House, a London organization, took the second spot. The Carnegie Endowment for International Peace and the Center for Strategic and International Studies, both Washington-based think tanks focused on international relations, placed third and fourth. Bruegel, an international economics research institution in Brussels funded by European Union governments, rounded out the top five.
In the U.S., the right-leaning Heritage Foundation (8), the libertarian Cato Institute (9), and liberal Center for American Progress (10) all made the top 10. The American Enterprise Institute, another top D.C. institution, checked in at number 11.
AEI performed better in the rankings of U.S. think tanks for domestic economics, where it placed seventh, behind Brookings, the National Bureau of Economic Research, the Adam Smith Institute in London, the Peterson Institute for International Economics, Cato, and the Rand Corporation.
Other notable think tanks in the overall rankings of U.S. institutions: The conservative Hoover Institution at Stanford University (19), the liberal Center on Budget and Policy Priorities (23), the centrist/liberal Urban Institute Institute (24), the conservative Hudson Institute (31), the right-religious Acton Institute (34), the libertarian Reason Foundation (41), and the Manhattan Institute, a conservative think tank in New York (42).
The rankings were based on voting by "think tanks, journalists, public and private donors, and policy maker groups," according to the Penn program, which is a think tank dedicated to researching think tanks.
The votes were cast on the basis of criteria such as the quality of a think tank's leadership, staff, output, media impressions and influence.