Despite a 15 percent rise in the U.S. population, a new survey shows the number of Americans who don't go to church has nearly doubled in the past 13 years, rising from 39 million to 75 million.

The report by the Barna Group, a California-based consulting firm following trends related to faith, culture and leadership in the country, says the percentage of adults that is "unchurched" has risen from 21 percent in 1991 to 34 percent today.

The group defines "unchurched" as not having attended a Christian church service – other than for holiday services like Christmas or Easter, or for special events such as a wedding or funeral – at any time in the past six months.

The annual church-attendance tracking survey is based on telephone interviews with a nationwide random sample of 1,014 adults conducted in late January and early February of this year with a margin of error of ?3 percentage points.

In a demographic breakdown, the survey revealed four dimensions:

1. Men dominate the ranks of the unchurched. Although they comprise slightly less than half of the national population, men constitute 55 percent of the unchurched, and they represent only 38 percent of the public which refers to itself as "born again."

2. The unchurched are younger than the norm. The median age of U.S. adults is 43, but it's just 38 among the unchurched.

3. Unchurched people are more likely than others to be single and to never have been married. Whereas one-quarter of American adults (26 percent) are single-never-married, nearly two-fifths of the unchurched fit that definition (37 percent).

4. The unchurched are also attracted to the coastal regions of the country. Although just four out of ten adults (42 percent) live in the Northeast or West, more than half of the unchurched (51 percent) live there. In fact, the two most populous states in the nation – California and New York – contain 18 percent of the nation's residents, but one-quarter of its unchurched adults (23 percent).

The study also found significant religious differences when comparing churchgoers to non-churchgoers.

In a typical week, unchurched people are less likely than all adults to read the Bible (19 percent compared to 44 percent) and to pray (63 percent vs. 83 percent), and they are less likely to have embraced Jesus Christ as their savior.

On a surprising note, while about half of the churched population has accepted Jesus as their savior, one of every six unchurched adults (17 percent) has done so as well.

Interestingly, says the report, if the minority of unchurched adults who are born again were connected to a church, the resulting increase would be nearly 13 million new people – more than have joined the nation's churches in the past decade combined.

Among the theological differences uncovered were that unchurched adults are less likely than others to believe the Bible is accurate, that Jesus was sinless, that Satan is real, that salvation is through the grace of God, and that God is the creator and present-day ruler of the universe.

George Barna

"The unchurched are more likely than others to be somewhat isolated from the mainstream activities of the society in which they live," explained author and researcher George Barna.

"They see themselves as outsiders and often take refuge in that status. Evidence of this arms-length approach to life, beyond their refusal to participate in church life, includes lower levels of voter registration, less money donated to non-profit organizations, fewer non-profits supported, lower levels of media usage, and less engagement in community service activities."

Additional distinguishing characteristics include the independence of the unchurched and what Barna calls their non-committal nature.

"You can see this emotional and intellectual distancing of themselves through their more moderate ideology, their more ambiguous theological perspectives, a lower likelihood of embracing terms used to describe oneself (such as 'generous,' 'friendly' and 'deeply spiritual'), a substantially lower level of self-professed commitment to their faith of choice, and their rejection of the idea of responsibility for nurturing other people's faith," he said.

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