Eighteen percent of adults younger than 40 years old now label themselves "spiritual" but not "religious." That's up from 9 percent of this cohort just 11 years ago.

So says Mark Chaves, a sociologist at Duke University, parsing the data from the new General Social Survey, often considered the best national survey of American opinion. 

It's hardly surprising that younger adults are more likely to embrace the label "spiritual" while rejecting "religious." But another finding may come as a surprise to some: The growth in this category is not because the number of people calling themselves "religious" has shrunk. Rather, it's because the number of irreligious people who call themselves "spiritual" has grown.

In a recent blog post at Duke Divinity School's Call & Response Blog, Chaves offers a paragraph of analysis of the data:

What does the growth of this "spiritual but not religious" segment of the population mean for organized religion in the United States? If what people mean when they say they are spiritual but not religious is that they are generally concerned with spiritual matters (whatever that means) but they are not interested in organized religion, then this trend indicates a growing minority of the population whose spiritual inclinations do not lead them to become involved in churches, synagogues, or mosques. This kind of generic, diffuse, and unorganized "spirituality" may provide a growing market for certain kinds of religious products, such as self-help books with spiritual themes, but, even if it continues to rise, it is difficult to see it becoming a solid foundation for new kinds of religious institutions or new forms of religious collective action.

Read Chaves's entire blog post here.