On the Pew U.S. Religious Landscape Survey, 2
One of the big findings of the Pew survey is that the "religiously unaffiliated" are now 16% of the adult population - making them the fourth largest "religious family." However, Pew also added a nuance to their questions that most surveys miss. They had the usual categories of atheists and agnostics, and then a catch-all category of "nothing in particular." The nuance comes in the next step: they let people within the nothing in particular category further defined themselves as "secular unaffiliated" and "religious unaffiliated."
So how do the religiously unaffiliated break down? The 16% of the total falls out:
Atheist 1.6% (of the total adult population)
Secular unaffiliated 6.3
Thus, the truly not religious population of the U.S. at the moment is 10.3% of the total. Another 5.8% of the total are not affiliated with a religious institution, but nonetheless think of themselves as religious.
Moreover, the unaffiliated are a changeable group. While 16% are unaffiliated now, only 7% were raised that way. And they are young - 31% are under 30, vs. only 20% of the whole population. And they are less likely to be married - 46% vs 54% of the total. They are more likely to be men - 59 vs. 48. Interestingly, the education and income profiles of the unaffiliated are the same as the population as a whole.
What this suggests to me is that the core of the religiously unaffiliated are unmarried young men. If we check in with them later in the life course, a significant portion of them will have become affiliated.