The U.S. census asks for detailed, sometimes quite personal information about people’s lives, but neglects to ask about one thing that influences many: religion.

For more than 70 years, the census has not asked about religion, even before it was banned by Congress as a mandatory question in 1976. The census can still ask about religion in optional questions, but doesn’t.

“I’m just aware that in general, we don’t,” said Steve Laue, an information specialist in the U.S. Census Bureau’s Chicago office.

The census asked individuals to respond to questions about religion from the 1880s until 1902. In 1906, the Census Bureau created the census of religious bodies, which collected information from religious organizations instead of individuals.

“There was a period of time from 1906 to 1936 when there was some question on religious bodies, numbers of churches,” Laue said.

Congress discontinued funding for the religious bodies survey in the 1950s, ending the bureau’s role collecting that information. Today the bureau reports religious affiliation based on data collected by the American Religious Identification Survey conducted by Trinity College. The Trinity survey shows that about 80 percent of the U.S. population identifies with a religion.

Jamie Harris, research associate for the Association of Religious Data Archives, said that if the census asked information about affiliation, researchers could find out more about religion in America. Researchers would have better numbers about increases and decreases in religions, Harris maintained, as well as how religion influences death, birth and migration rates. They could see, for instance, whether religion still has a strong hold in the Bible Belt.

“We can tell if that still holds true, or if it’s just one of many areas. The information would only be as reliable as the census,” Harris said, noting that despite efforts to get a complete count, not everyone participates.

Census data on religion can be skewed in other ways. In the United Kingdom’s 2000 count, an email campaign urged citizens to mark “Jedi” under religion. As a result, “Jediism” made up almost 1 percent of the population’s religious beliefs, surpassing Judaism and Buddhism. The same campaign affected the counts in Australia and New Zealand.

Despite the flaws in counting, Harris insists that the census could give researchers clearer information about religion-but without this, researchers must rely on other groups to fill in the gaps.

“There are organizations that are trying to, but no organization has the kind of resources the census does,” he said.