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. People are generally not convinced when I point out that this would mean that nearly 30% of paternal grandfathers are not paternal grandfathers. Most of my scientist acquaintances fancy up the myth by suggesting that they received this datum from research on family groups (where you have to take into account the error introduced by paternity misattribution) or organ matching for purposes of donation.
Evolutionary biologist Marlene Zuk has some informal survey data which she presents in an article in The Los Angeles Times: