Razib Khan of Discover Blogs says

An urban myth, often asserted with a wink & a nod in some circles, is that a very high proportion of children in Western countries are not raised by their biological father, and in fact are not aware that their putative biological father is not their real biological father. The numbers I see and hear vary, but 10% is a low bound.

Read the quote in context here.

Khan quotes another biologist, Marlene Zuk, on how enormous people imagine cuckoldry to be

When asked to estimate the frequency of misassigned paternity in the general population, most people hazard a guess of 10%, 20% or even 30%, with the last number coming from a class of biology undergraduates in a South Carolina university that I polled last year. I pointed out that this would mean that nearly 20 people in the class of 60-some students had lived their lives calling the wrong man Dad, at least biologically. They just nodded cynically, undaunted.

Khan continues (emphasis mine)

What are the real numbers? Zuck asserts that they’re more in the 1-5% range, with 3.7% being a high-bound figure for one study. This varies by culture and socioeconomic group, and the segment of the population being surveyed. Studies which rely on a data set consisting of men who have requested paternity tests are strongly sample biased toward those who have a reason to have suspicions. ... And yet even in the cases of men who have suspicions only a minority have misattributed paternity.

Got that? The high figures you hear cited, generally instigated by bitter divorced men and other “men’s rights” activists, and abetted by tabloid cable shows, are heavily inflated because paternity tests are sought mainly by men who are already suspicious. And yet, as Khan points out, even when men imagine cause for suspicion the suspicions are usually unfounded.