WASHINGTON (CBS DC) – A growing trend emerged from a comprehensive 2010 study of worldwide religions: roughly one-in-six (1.1 billion, 16 percent) of the world’s population claims no religious affiliation.

This makes them the third-largest group worldwide, behind Christians who number nearly 2.2 billion (32 percent) and Muslims at 1.6 billion (23 percent). Their size is relative to those of Catholic faith.

The research is from a demographic study published on Dec. 18 of more than 230 countries and territories by the Pew Research Center’s Forum on Religion & Public Life. The study is based on analysis of more than 2,500 censuses, surveys and population registers across the globe.

And those who identify with religious practices still make up the vast majority of the world’s population.

The study shows that the religiously-affiliated make up 84 percent of earth’s population, with 5.8 billion of the world’s 6.9 billion people associating themselves with a religion in 2010.

Following Christians and Muslims in size, there are nearly 1 billion Hindus (15 percent), almost 500 million Buddhists (7 percent) and 14 million Jews (0.2 percent). Additionally, more than 400 million people (6 percent) practice various folk or traditional religions, including African traditional religions, Chinese folk religions, Native American religions and Australian aboriginal religions.

But the religiously-unaffiliated are a rapidly growing group.

The religiously unaffiliated includes atheists, agnostics and others who do not identify with any particular religion in surveys. However, many of the religiously unaffiliated have some religious beliefs. For example, belief in God or a higher power is shared by 7 percent of Chinese unaffiliated adults, 30 percent of French unaffiliated adults and 68 percent of unaffiliated U.S. adults.

The religiously unaffiliated are heavily concentrated in Asia and the Pacific, where more than three-quarters (76 percent) of the world’s unaffiliated population resides.

There are six countries where the religiously unaffiliated compose a majority of the population: the Czech Republic (76 percent are religiously unaffiliated), North Korea (71 percent), Estonia (60 percent), Japan (57 percent), Hong Kong (56 percent) and China (52 percent).

This report estimates that 16.4 percent of the total U.S. population (adults and children) was unaffiliated as of 2010.

But a 2012 survey shows that number has increased to 19.6 percent of adult Americans reporting they are non-religious. The recent study also showed that one-fifth of the U.S. public – and a third of adults under the age of 30 – are religiously unaffiliated — the highest percentages ever in Pew Research Center polling.