AURORA, CO - JULY 22: Parishioners pray during morning mass remembering victims of the theater shooting at the Queen of Peace Catholic Church on July 22, 2012 in Aurora, Colorado. Suspect James Holmes, 24, allegedly went on a shooting spree and killed 12 people and injured 59 during an early morning screening of 'The Dark Knight Rises.' (Photo by Kevork Djansezian/Getty Images) Religious affiliation is on swift decline among American adults. (Photo by Kevork Djansezian/Getty Images)

WASHINGTON (CBS WASHINGTON) — One-fifth of American adults have no religious affiliation, and this number is increasing rapidly.

The number of Americans who do not identify with any religion continues to grow at a fast pace. One-fifth of the U.S. public – and a third of adults under 30 – are religiously unaffiliated today, the highest percentages ever in Pew Research Center polling.

In the last five years alone, the unaffiliated have increased from just over 15 percent to just under 20 percent of all U.S. adults. Their ranks now include more than 13 million self-described atheists and agnostics (nearly 6 percent of the U.S. public), as well as nearly 33 million people who say they have no particular religious affiliation (14 percent).

This large and growing group of Americans is less religious than the public at large on many conventional measures, including frequency of attendance at religious services and the degree of importance they attach to religion in their lives.

But the survey may be affected by a differing view of the words “religion” and “spiritual.”

A new survey by the Pew Research Center’s Forum on Religion & Public Life, conducted jointly with the PBS television program Religion & Ethics NewsWeekly, finds that many of the country’s 46 million unaffiliated adults are religious or spiritual in some way.

Two-thirds of them say they believe in God (68 percent). More than half say they often feel a deep connection with nature and the earth (58 percent), while more than a third classify themselves as “spiritual” but not “religious” (37 percent), and one-in-five (21 percent) say they pray every day.

With few exceptions, though, the unaffiliated say they are not looking for a religion that would be right for them. Overwhelmingly, they think that religious organizations are too concerned with money and power, too focused on rules and too involved in politics.

The lower the age group, the less likely people are to be affiliated.

The growth in the number of religiously unaffiliated Americans – sometimes called the rise of the “nones” – is largely driven by generational replacement, the gradual supplanting of older generations by newer ones.

A third of adults under 30 have no religious affiliation (32 percent), compared with just one-in-ten who are 65 and older (9 percent). And young adults today are much more likely to be unaffiliated than previous generations were at a similar stage in their lives.

In addition, this report contains capsule summaries of some leading theories put forward by scholars in an attempt to explain the root causes of the rise of the “nones.” These theories run the gamut from a backlash against the entanglement of religion and politics to a global relationship between economic development and secularization.