Instapundit posts a letter that echos stuff I've read based on early blood-type testing: around 16% of people have different fathers than their mothers told them. Men in our culture are so often portrayed as the bad guys (e.g., "deadbeat dads") that I think men as a gender should get some credit for being so trusting and accepting despite these high rates of false paternity.
I am familiar with a massive ongoing multi-generational genetic study. . . . (Please don't mention either it or my name.) The participants were predominantly "greatest generation" and their kids' generation. Middle-class and white a bit more than the general population. It was looking for hereditary cancers (not too common, maybe 10% or so, last time I checked).
But, of course, in the process of all this, they discovered so-called "false paternities". (Their rules prohibited them from divulging this info to participants.) Anyway, the overall false paternity rate for this bunch from the "Leave it to Beaver demographic" was about 16%.
16%. One in six. In middle America. Not your mom, of course, nor mine, but hey, that's going to be a lot of data to discuss around the dinner table.
I haven't been able to find the study data I've read in the past that showed similar results back when blood-typing was first discovered. From what I remember, the high paternal discrepancy rate actually led early researchers to doubt the accuracy of blood types.