In the debate over a plan to build an Islamic center near Ground Zero in New York, one number has been elusive: the population of Muslims in the city. Commentators have cited figures that have been all over the map—from 600,000 in the city’s five boroughs to between 600,000 and 800,000 in Manhattan alone.
That is because no one knows for sure how many Muslims, or Jews, or Christians, live in New York or anywhere else in the U.S. The Census Bureau doesn’t ask Americans to disclose their faith, and the surveys and studies that attempt to fill in the gap come up with different numbers—particularly when it comes to relatively small religious groups. That leaves the U.S. behind much of the world when it comes to understanding the religious beliefs of its inhabitants, and has sparked controversy about the number of Muslims in the country.
These are important numbers because “religion is very consequential in how people see the world, view politicians, and view social, economic and even environmental issues,” says Barry Kosmin, a sociologist at Trinity College and a principal investigator with the American Religious Identification Survey, which has surveyed Americans about their faith three times in the past decade.
National censuses in at least 70 countries, including the U.K. and Canada, ask about religion, according to United Nations data. “The census question on religious affiliation is used to measure cultural and religious diversity in Canada,” Peter Frayne, spokesman for Statistics Canada, wrote in an email. It is used, among other things, to monitor discrimination, diversity and the need for faith-based schools, Mr. Frayne says.
The U.S. Census Bureau debated including a question on religion in the late 1950s and again in 1976. But each time it decided not to. The census director in 1976, Vincent P. Barabba, decided to exclude such a question, concerned that including it would lead to a controversy over the separation of church and state that could impede the bureau’s work. Later that year, Congress passed a law prohibiting mandatory questions about religion.
“It’s a tough trade-off,” says Mr. Barabba, who is now retired. “You want to collect everything that’s of interest in society. [However,] you want to be careful about which ones you ask.”
That opens the door for controversy about true population counts. There is particularly sharp disagreement about the U.S. Muslim population. Several surveys have arrived at figures of 2.4 million or lower. But Ibrahim Hooper, spokesman for the Council on American-Islamic Relations, says those surveys have “real weaknesses,” and cites an estimate of seven million. He referred further questions to Ihsan Bagby, associate professor of Islamic studies at the University of Kentucky.
Prof. Bagby surveyed mosques in 2000 to arrive at a figure of two million active Muslims, including those who attend religious services on high holy days. That suggests to Prof. Bagby a true count of about four million or five million in 2000, including those who identify as Muslim but don’t attend mosques. He has found the number of mosques has increased by more than 50% to 1,900 since 2000, though it is unclear if that reflects population growth or construction to address existing demand.
Columbia University researchers estimating New York City’s Muslim population a decade ago used a figure of five million to seven million for the national population in arriving at their count of 600,000—later revised to 700,000—because they figured, based on studies of mosque affiliation, that one in 10 American Muslims live in New York.
But general population surveys suggest a much lower number of Muslims. The American Religious Identification Survey asks simply, “What is your religion, if any?” In 2008, the survey found 1.3 million adult Muslims in the U.S.
That survey, though, may undercount non-Christian groups, because members of some such minority groups may be reluctant to respond. Also, the survey is conducted only in English and Spanish. Surveys that focus on non-Christian groups can help produce more statistically reliable results. For instance, a Pew survey in 2007 of American Muslims also conducted interviews in Arabic, Farsi and Urdu. The survey also drew respondents more heavily from places expected to have more Muslims to produce a greater sample size and learn more demographic details about the population. Then researchers weighted the results to correct for the oversampling.
This may have helped the survey arrive at a larger number of Muslims than many other polls: 1.4 million adults and 2.35 million total Muslims. Yet even this survey had shortcomings, for instance, not dialing cellphones, though many Muslims are young immigrants who may be more likely to eschew landlines, according to the Pew report.
Similar issues have afflicted efforts to count the number of American Jews. Jewish groups funded a survey a decade ago that found the total population was 5.2 million. Brandeis University researchers reanalyzed the data and found because of groups that were underrepresented in the survey, the true population likely was between six million and 6.4 million.
Not all religion researchers are interested in more reliable population counts. “I don’t think it matters that much,” says Daniel A. Madigan, associate professor of theology at Georgetown University. How people self-identify doesn’t indicate much about their involvement in the religion, nor attitude toward it. “The actual numbers don’t translate necessarily into anything else.”