¹ Although this study focuses on American organizations with the tax status of 501(c), we employ the term ‘interest group’ rather than ‘non-profit organization’, because, as we describe in this article, their tax statuses notwithstanding, ‘nonprofits’ possess many opportunities to engage in the political process, and it is this potential, we argue, that is central to understanding their role in the polity. The legal capacity of nonprofits to engage in legislative or electoral activity is often misunderstood, even by the organizations themselves; see Berry, Jeffrey and Arons, David, A Voice for Nonprofits (Washington, D.C.: The Brookings Institution, 2005)Google Scholar. As a result, we feel it is particularly important to locate groups with this tax status firmly within the context of the interest group universe as typically described in the literature on groups in American politics; see Baumgartner, Frank F. and Leech, Beth L., Basic Interests: The Importance of Interest Groups in Politics and in Political Science (Princeton, N.J.: Princeton University Press, 1998)CrossRefGoogle Scholar.

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¹¹ As explained in greater detail later in this article, we use the media market as our unit of analysis because doing so provides us with an inclusive measure that provides a useful approximation of American communities.

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¹³ All 501(c) organizations are tax-exempt. Nonprofits with 501(c) 3 status (those having religious, charitable or educational functions, for example) comprise 64 per cent of our sample; these groups are limited by their tax status in the amount of political activity they may undertake. The rest of the sample includes other types of 501(c) groups, particularly 501(c) 4s, such as social welfare organizations, veterans’ organizations, employees’ associations and fraternal societies. Organizations with 501(c) 4 status are subject to fewer IRS limitations on their political involvement – they are permitted to advocate for or against specific legislation and to campaign on behalf of particular candidates. See the Appendix for an account of the distribution of 501(c) groups in our dataset.

¹⁴ Many organizations, if not most, that visit congressional offices to educate members of Congress and their staff fall into this category. Organizations such as the Heritage Foundation, Planned Parenthood and Campaign for Tobacco-Free Kids provide members of Congress with important information about legislation – providing valuable analyses of legislation and amendments, for example – without officially lobbying for or against a bill.

¹⁵ Kollman, Ken, Outside Lobbying: Public Opinion and Interest Group Strategies (Princeton, N.J.: Princeton University Press, 1998)Google Scholar; Paxton, Pamela, ‘Social Capital and Democracy: An Interdependent Relationship’, American Sociological Review, 67 (2002), 254–277CrossRefGoogle Scholar.

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²⁸ See also Lowry, Robert C. and Potoski, Matthew, ‘Organized Interests and the Politics of Federal Discretionary Grants’, Journal of Politics, 66 (2004), 513–533CrossRefGoogle Scholar.

²⁹ Although institutions may act as patrons (churches, foundations or the federal government, for example), as Nownes and Neeley, ‘Public Interest Group Entrepreneurship’, p. 142, note, it is individuals who ‘are still the driving force behind public interest group formation’. See also Walker, ‘The Origins and Maintenance of Interest Groups in America’, and Walker, Mobilizing Interest Groups in America.

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³¹ Walker, ‘The Origins and Maintenance of Interest Groups in America’; Walker, Mobilizing Interest Groups in America; Nownes and Neeley, ‘Public Interest Group Entrepreneurship’, offered qualified support.

³² Here, we employ the more general term ‘donor’ rather than ‘member’ because, while many interest groups are membership based, many others are donor based, and an unspecified number rely on both members and individual contributors who do not wish to become official members. See discussion in Baumgartner and Leech, Basic Interests, chap. 2, and Barakso, Maryann, Governing NOW: Grassroots Activism in the National Organization for Women (Ithaca, N.Y.: Cornell University Press, 2004)Google Scholar.

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³⁶ Putnam, Bowling Alone, p. 201; see also Lindstrom, Martin, Merlo, Juan and Ostergren, Per-Olof, ‘Individual and Neighbourhood Determinants of Social Participation and Social Capital: A Multilevel Analysis of the City of Malmo, Sweden’, Social Science & Medicine, 54 (2002), 1779–1791CrossRefGoogle Scholar; and Kang, Naewon and Kwak, Nojin, ‘A Multilevel Approach to Civic Participation’, Communication Research, 30 (2003), 80–106CrossRefGoogle Scholar.

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⁴⁵ Verba, Schlozman and Brady, Voice and Equality; Putnam, Bowling Alone.

⁴⁶ Lowry, ‘Explaining the Variation in Organized Civil Society Across States and Time’.

⁴⁷ Baumgartner and Leech, Basic Interests.

⁴⁸ Berry and Arons, A Voice for Nonprofits. ‘501(c)3’ refers to a tax-exempt status given to nonprofit organizations in the United States.

⁴⁹ For the distinction between institutions and interest groups, see Salisbury, Robert H., ‘Interest Representation: The Dominance of Institutions’, American Political Science Review, 78 (1984), 64–76CrossRefGoogle Scholar.

⁵⁰ The institutions comprise fewer than 10 per cent of all organizations included in the NCCS dataset. When we estimated an identical model with these groups included, our results were not substantively different from those presented here.

⁵¹ Powell, Walter W. and Steinberg, Richard, The Non-profit Sector: A Research Handbook (New Haven, Conn.: Yale University Press, 2006)Google Scholar; Lowry and Potoski, ‘Organized Interests and the Politics of Federal Discretionary Grants’.

⁵² The NCCS data only include an observation for a group if it is a separate tax entity. Therefore, if a parent organization maintains and supports several regional branches, only the group's headquarters will appear in the data. However, for many organizations, state and local chapters exist as separate tax entities and each appears in the data. Lowry, ‘Explaining the Variation in Organized Civil Society across States and Time’, uses the NCCS data to construct a measure of organizations at the state level, and Barakso, Maryann and Schaffner, Brian F., ‘Exit, Voice, and Interest Group Governance’, American Politics Research, 36 (2008), 186–209CrossRefGoogle Scholar, drew a sample from the NCCS data and did not discover any cases where an organization was located in a different metropolitan area than their NCCS address indicated.

⁵³ Media market (or Designated Market Area) boundaries are defined by the Nielsen Corporation to denote areas that receive the same local television broadcasts. They vary significantly in terms of population, from under 40,000 in the North Platte, Nebraska, market to over 20 million citizens in the New York City market area.

⁵⁴ The Census does not provide data at the market level; however, media markets are defined so that they almost never divide counties. Therefore, we are able to use county-level data to build up to the market level since markets do not divide county boundaries.

⁵⁵ These measures of income are correlated at 0.73, introducing the possibility of multicollinearity. However, we conducted analyses using each of these variables in our model by themselves and the substantive results were not altered by the model specification changes.

⁵⁶ Lowry, ‘Explaining the Variation in Organized Civil Society across States and Time’.

⁵⁷ Gray, Virginia and Lowery, David, ‘Interest Representation and Democratic Gridlock’, Legislative Studies Quarterly, 20 (1995), 531–552CrossRefGoogle Scholar; Boehmke, Frederick J., ‘The Effect of Direct Democracy on the Size and Diversity of State Interest Group Populations’, Journal of Politics, 64 (2002), 827–844CrossRefGoogle Scholar.

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⁵⁹ Though some scholars note that interest group organizations may be more likely to locate in areas where there are more churches; see Corbin, John J., ‘A Study of Factors Influencing the Growth of Nonprofits in Social Services’, Non-profit and Voluntary Sector Quarterly, 28 (1999), 296–314CrossRefGoogle Scholar.

⁶⁰ The percentage of church members in the market is the percentage of the market's population that belongs to a church based on the ‘Religious Congregations & Membership’ study available from the Glenmary Research Centre.

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⁶⁷ Theoretically, we would expect markets with very low and very high values of non-Hispanic whites to have the most groups since these would be the most homogeneous markets. However, only one market is less than 50 per cent non-Hispanic white (Greenville, SC, is 37.3 per cent white), and therefore the lowest values in our sample represent the most heterogeneous markets and the highest values are the least diverse. Therefore, we expect a linear relationship in our model.

⁶⁸ Boehmke, ‘The Effect of Direct Democracy on the Size and Diversity of State Interest Group Populations’.

⁶⁹ Lowry, ‘Explaining the Variation in Organized Civil Society across States and Time’.

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⁷³ Gray and Lowery, ‘Interest Representation and Democratic Gridlock’, p. 4.

⁷⁴ Lowry and Potoski, ‘Organized Interests and the Politics of Federal Discretionary Grants’.