A religious study showing that a large number of Americans are religiously unaffiliated has caused pollsters to question whether the US is heading down the same religious path as that of Europe - where people who consider themselves religious do not necessarily belong to any institution.

The Pew Forum on Religion & Public Life study, based on more than 35,000 interviews, found that 16.1 per cent of the US population described themselves as unaffiliated - the group with the greatest net gain in members.

The unaffiliated category includes atheists and agnostics, but is mostly made up of respondents who said they were "nothing in particular" (12.1 per cent).

More than a third of those unaffiliated said that although they have no particular religion, they at least find religion to be "somewhat" important to "very" important in their personal life.

"Let me underscore again one of the significant findings of the unaffiliated," said Greg Smith, research fellow of the Pew Forum on Religion & Public Life. "A high percentage of them said religion is still important to them, about six per cent of the American public as a whole."

In future studies, Smith said the group will see if the US is experiencing the religious "phenomenon" that is already quite common in Europe - "believing without belonging".

"[For] many people in Europe, religion has become deinstitutionalised even though they still have certain religious beliefs," Smith said. "So one key question is whether the rank of those who believe but do not belong to religious institutions is a trend that is expanding as we go forward."

While the unaffiliated group is growing as a whole, within the group, the number of those who consider themselves religious but who are not affiliated with any religion is also growing, according to Luis Lugo, director of the Pew Forum on Religion & Public Life, who noted the "double pattern".

"I think both of those things are really quite interesting," Lugo commented.

The US Religious Landscape Survey estimates the US is still 78 per cent Christian, but is tethering on its reputation as a Protestant nation, at 51 per cent and falling.

America's Young

Among the unaffiliated, young people make up a significant portion, which is not a surprise since it has been a common trend "for quite some time", according to John Green, senior fellow at the Pew Forum on Religion & Public Life.

But what is new is the indication from this and other surveys that the number of young unaffiliated Americans is larger than in past generations.

One in four adults under the age of 30 years old claims no affiliation with a religious institution, according to the poll.

"Given the large size of young people among the unaffiliated, this could have a profound effect upon the character of American religion," Green predicted.

He said that his team will keep an eye out to see if this young unaffiliated group later returns to organised religious institutions.

Green gave two hypotheses of why young Americans are increasingly moving towards being religiously unaffiliated. He said young adults, when they leave home, are "busy figuring out" their career, family situation, where they want to live, and their religion among other issues.

"In that situation, even people who are quite religious in the level of belief stray away from organised religion," Green offered.

He added that young people go through a life cycle with periods of transitions that includes a transition in religion.

Other reasons given for the higher number of unaffiliated young Americans include highly-publicised problems within major denominations and the greater number of religious options available to young people in American society.

The Pew study released Monday is the first of three reports to be released based on data collected from interviews with more than 35,000 Americans. The phone interviews, which included 40 questions, were conducted from May 8 to August 13, 2007.