In his book Changing Faith, sociologist Darren E. Sherkhat contends that Americans shift their religious identifications more often than any others in the Western world.

As a researcher focused on religion and politics, I’ve always wanted to explore religious switching in more depth, but never really had the data to do it—until now.

The Cooperative Congressional Election Study (CCES) includes a panel survey—where the same people are asked questions over an extended period of time—for 2010, 2012, and 2014. Because the CCES panel started with 9,500 respondents, it provides enough data for us to compare subgroups and track overall religious migration in the United States over the past several years.

Looking as broadly as possible, the entire dataset shows how much change in American religious identity occurred between 2010, 2012, and 2014.

The three vertical bars show the distribution of religious identities in each survey year. This visualization also contains an alluvial diagram, which represents the flow of individuals from one tradition to another (the thicker the band, the greater number of individuals migrated).

While there is a tremendous amount of religious stability amongst some religious traditions, as shown by the steady blocks of each color flowing from one year to another, we also see a lot of movement in a short period of time.

Overall, 18.9 percent of this sample reported a different religious affiliation in 2014 than they did in 2010. This defection rate means that nearly 1 in 5 Americans changed their faith identity over a four-year period.

When broken down by religious tradition, certain faiths experienced far more defection than others. On the lower end, around 10 percent of Protestants, Catholics, and Jews switched—making them half as likely as Americans overall to defect during this period. (The Protestant defection rate, however, only includes those who left the Protestant tradition outright, not those who switched denominations, say, from Southern Baptist to United Methodist.)

For atheists, the defection rate is about 18 percent. Even more striking, about 48 percent of agnostics defected, as did 42 percent of those who described their faith as “nothing in particular.”

That’s a staggering amount of flux: About half of the agnostics in the US in 2010 were no longer agnostics by 2014.

The “nothing in particular” category—along with atheists and agnostics, a group of religiously unaffiliated Americans nicknamed the “nones”—is a particularly interesting demographic to follow during this period. The chart below tracks where the Americans who had no particular faith identity in 2010 ended up in 2012 and 2014. (I excluded some smaller religious traditions, therefore some percentages adjust slightly.)

About 6 in 10 people from the “nothing in particular” group stayed that way over the years, while the rest made a switch. About half of the defectors moved away from traditional faith to atheism and agnosticism (20%), while almost as many moved in the other direction and returned to the church (17.3%). Of the 2010 nones, 13.3 percent became Protestant, and 4 percent became Catholic.

Fewer agnostics (55%) than those with no particular faith (62%) stayed that way over the course of the surveys. However, the agnostics who defected moved in one clear direction: They shifted to become fellow nones. Among agnostics, 22.5 percent went on to identify as atheists in 2014, while another 18.9 percent identified as nothing in particular.

Unlike those that began with no faith in particular, barely any agnostics moved toward the church. Just 4.5 percent of agnostics became Protestant or Catholic by 2014.

While Americans in the “nothing in particular” faith group were about as likely to become nones as to become Christians, the ratio for agnostics was drastically different. For every agnostic who became Christian, 10 agnostics moved within the nones.

It’s clear from these results that agnostics are much more likely to move away from organized religion, while Americans with no particular faith stand as a bridge between some faith and no faith.

There’s a lot to consider from just this small piece of analysis. It seems that the line of demarcation between having a religion or not is the “nothing in particular” category, even though it tends to be considered part of the “nones.”

Turns out, individuals who religiously identify as nothing in particular are just as likely to tilt toward returning to a Christian tradition as they are to become even more entrenched in having no religion. On the other hand, once an individual affirmatively denies a belief in a specific deity (i.e. describing themselves as agnostic or atheist), it’s rare for them to go back.

Religious nones are the second-largest religious tradition in the United States. According to the CCES, 18.6 percent of respondents—55 million American adults—claim their faith to be nothing in particular.

To extrapolate from the data, about 12 million Americans who fall into this category will drift to atheism and agnosticism, while another 12 million or so will become Protestant or Catholic.

That’s a tremendous amount of volatility. Further research could reveal what’s behind these switching decisions—whether demographic factors like age or social or political concerns.

Ryan P. Burge is an instructor of political science at Eastern Illinois University. His research appears on the site Religion in Public.