Identifying with a religion may become "extinct" in certain countries in the future, suggests a recent study submitted for review.
Researchers applied a mathematical model to census data -- some dating back 160 years-- from nine countries. The United States was not included because the country's census does not ask citizens about religious affiliations.
In the Netherlands, where nearly 50 percent of the population claims no religious affiliation, the model projects that number to increase to 70 percent by 2050.
Further down the road, non-affiliation might approach 100 percent, the study predicts.
The model is based on two theories about human behavior, researchers say. When competing groups are vying for members, the unit with more people is considered more attractive to prospective joiners. In addition, groups that boast a higher social, political or economic utility are viewed as more appealing, too.
"In some countries -- apostasy [renunciation of a religion] is considered a crime," Daniel Abrams, a co-author of the study and assistant professor of engineering sciences and mathematics at Northwestern University, told Discovery News. "Here, the utility of identifying with a religion is extremely high."
In all census sources, Abrams says people were given the opportunity to check off a listed religion, "other" if one's religion was not included or "none," indicating no affiliation with a religion. For the study, researchers counted "none" as non-affiliated and all others as affiliated with a religion.
Also, he points out these trends don't measure spirituality or an individual's religious feelings, but rather how people identify themselves.
"Being affiliated with a denomination is not the same as believing in God or believing in any aspect of a religious ideology," Abrams said. "And being unaffiliated with a religion doesn't mean you don't believe in God, but simply that you don't want to be a part of a denomination."
Another expert thinks these distinctions are important to highlight.
"The loss or even extinction of religious affiliation does not necessarily mean the end of 'religion,'" said Anthony Petro, an assistant professor of religious studies at New York University, who was not involved in the study. "Trends in American religion since the 1960s have actually moved away from denominational modes of self-identification and affiliation and toward a rise in spirituality."
Petro adds that predictions about the fate of religions aren't new. Claims of religion's extinction in the 1950s were put to rest by a large resurgence and growth of Christianity in the United States and abroad in the 1980s.
Definitions play a role as well, Petro says. Religious affiliation, attendance, practice (both public and private) and influence all help create a working idea of religion.
Addressing one of the model's pillars, Petro thinks assuming that religious groups compete for members may not be true for all faiths.
"Most evangelical Protestants are not competing for new members, per se, as if they were operating under a market model and trying to collect more shares," he said. "Rather, they are fighting for souls; they are seeking to win as many souls for Christ as possible, not in order to gain numerical prominence, but to save fellow humans from eternal damnation."
Extinction or not -- data show that religious affiliations are declining worldwide. And Petro concedes the research is still exciting.
"The fact that scholars of engineering, mathematics and physics have taken such keen interest in religion only underscores how important religion is in public consciousness today," Petro said, "even as they predict its demise."
The study was submitted for review at the American Physical Society's journal, Physical Review Letters.