Since 1990, the proportion of Americans who report having no religious affiliation has more than tripled, from 6 percent to 19 percent, according to Pew Research Center data reported by USA Today. They include atheists, agnostics and people who report believing in nothing in particular.

The rise of 13 percentage points results in a number that accounts for almost one-fifth of Americans. It is the highest proportion of individuals in the U.S. who do not have strong spiritual beliefs ever documented, USA Today reported.

In 2008, the proportion of Americans who reported being unbelievers was 15 percent, according to the American Religious Identification Survey. In 2010, the proportion was 18 percent, according to the General Social Survey.

The most recently released figures on this issue are based on aggregated data from multiple surveys of a total of 19,377 people conducted by the Pew Research Center last year.

There is speculation the 19 percent number could be disproportionately high should the survey respondents have been disproportionately young, single, and highly educated.

Mark Chaves, a Duke University professor of sociology, religion, and divinity, told USA Today that fluctuating immigration rates from countries that tend to be more religious could also have an effect on the proportion of Americans who report being unbelievers.

Immigrants from Mexico are one demographic group that could affect the numbers, according to Chaves. He also said the trend isn't surprising based on the recent results.

Americans famously say they believe in some variation of God. Over 90 percent do, Chaves said. But it used to be 99 percent decades ago. The change is slow, but we can see it coming.

The co-author of multiple American Religious Identification Surveys, Barry Kosmin, would agree. Young people are resistant to the authority of institutional religion, older people are turned off by the politicization of religion, and people are simply less into theology than ever before, Kosmin told USA Today.

At the same time, the Roman Catholic Church, the Southern Baptists Convention, the Methodists, and the Lutherans all have reported membership figures that are either flat or falling, according to USA Today's reading of the data in the 2012 Yearbook of American & Canadian Churches.