Religious Pluralism Index
Also known as: Religious Diversity Index, Religious Fractionalization Index, Herfindahl Religious Concentration Index, Denominational Pluralism Index
The religious pluralism index measures how religiously diverse a population is by computing the probability that two randomly selected people belong to different religious groups. It is the religious application of the Herfindahl-Hirschman concentration measure: one minus the sum of squared denominational shares, ranging from zero (a single dominant group) toward one (many evenly sized groups). The index became central to the sociology of religion because the religious-economies paradigm predicted that greater pluralism, by signaling competition among firms, should raise participation. David Voas, Daniel Olson, and Alasdair Crockett's 2002 American Sociological Review article showed that much of the prior literature testing this claim was undermined by a mathematical artifact linking the index to participation, making careful construction and interpretation of the index a methodological topic in its own right.
Key highlights
- Provides a single, interpretable number - the probability two people differ in religion - that is comparable across units and time.
- Borrows a well-understood concentration measure (Herfindahl/Simpson) with clear mathematical properties.
- Easy to compute from standard adherence or census counts and to use as a descriptive variable or regressor.
- Anchors a major, falsifiable hypothesis in the sociology of religion about diversity and participation.
Intuition
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How it works
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When to use it
Use the religious pluralism index when you need a compact, comparable summary of how religiously diverse different geographic units are - for descriptive mapping, as a covariate, or to test hypotheses about diversity and social outcomes. It is appropriate when you have reliable counts of adherents across a coherent set of mutually exclusive groups and a defensible classification scheme. Exercise particular caution when using it to test the competition-participation hypothesis, because the index can be mechanically confounded with participation measures derived from the same data; in that setting you must use independent outcome data or corrected estimators. The index is less suitable where group boundaries are fluid or contested, where the unaffiliated dominate and distort the partition, or where the substantive question concerns the depth of belief rather than the spread of affiliation.
Strengths & limitations
- Provides a single, interpretable number - the probability two people differ in religion - that is comparable across units and time.
- Borrows a well-understood concentration measure (Herfindahl/Simpson) with clear mathematical properties.
- Easy to compute from standard adherence or census counts and to use as a descriptive variable or regressor.
- Anchors a major, falsifiable hypothesis in the sociology of religion about diversity and participation.
- Highly sensitive to how denominations are classified and aggregated, which can swing the value substantially.
- Mechanically confounded with participation when both are computed from the same counts, biasing competition tests.
- Bounded above by a value that depends on the number of groups, complicating comparisons across units with different group counts.
- Captures the spread of affiliation only, saying nothing about commitment, belief, or the intensity of religious life.
Common pitfalls
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Applications
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Frequently asked
What exactly does the pluralism index measure?
It measures the probability that two randomly selected members of a population belong to different religious groups. Computed as one minus the sum of squared denominational shares, it equals zero when a single group accounts for everyone and approaches one as membership is spread evenly across many groups. It is identical in form to the Simpson diversity index in ecology and the complement of the Herfindahl concentration index in economics, so a high value means high diversity of affiliation, not necessarily high religiosity.
Why did Voas, Olson, and Crockett say previous research was wrong?
They showed that when the pluralism index and the participation measure are both derived from the same group counts, they become mechanically related, producing systematic positive or negative correlations that have nothing to do with any real competitive effect. Because most prior studies used such overlapping data, their findings - whether supporting or rejecting the competition hypothesis - reflected this arithmetic artifact rather than the underlying theory. Their conclusion was that almost all of the 193 tests they reviewed should be abandoned and the question studied with independent data.
How is the index affected by how I classify denominations?
Strongly. Because the index is built from group shares, decisions about whether to lump related denominations together or split them apart directly change the squared-share sum and therefore the index. Splitting one large group into several smaller ones raises measured pluralism even though nothing about the population changed. This is why a coherent, consistently applied classification scheme is essential, and why comparisons that use different schemes are not comparable.
Sources
- 1.Voas, D., Olson, D. V. A., & Crockett, A. (2002). Religious Pluralism and Participation: Why Previous Research Is Wrong. American Sociological Review, 67(2), 212-230.
- 2.Stark, R., & Finke, R. (2000). Acts of Faith: Explaining the Human Side of Religion. Berkeley: University of California Press.ISBN 9780520222021
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Cite this page
ScholarGate. (2026, June 23). Religious Pluralism Index. ScholarGate. https://scholargate.app/sociology-of-religion/religious-pluralism-index