A few years ago I attended a seminar on science and law. One of the topics was the legal, moral, and ethical implications of genetic research. The professor conducting the seminar read a quote by a well-known Irish expert in genetics who said something to the effect that we need not worry that knowledge about genetics would eventually lead to routine genetic profiling of all individuals. One main reason he offered was that it would be too disruptive of society because about 20 percent of us would discover that we were mistaken about who our genetic fathers were. (I believe this statistic might have been limited to modern, industrialized societies, such as Europe and North America, but I don’t remember.)
One in five? Can this be true? Are so many of our fathers cuckolds? Are so many of our mothers mendacious slatterns?
The trouble with that old thread is that although several people seem to remember having seen a study, no one seems to be able to give an actual cite.
This was discussed some years ago on alt.folklore.urban and thanks to the wonders of Google I was able to find it here.
The gist of the thread seems to be that most people in the field of genetics accept a figure of between 2% and 5% illegitimate births, but that this all seems to stem from a study done some years ago which may not reflect the true state of affairs.
Here is another cite to a post found in alt.folklore.urban. This one concerns a study done on Indian populations in Mexico.
This study found that 14% of the children were incompatible with at least one parent. But the fact that they found 6% of the children incompatible with both parents suggests that there is a lot more going on here than just fathers being deceived.
The last stats I heard put the figure here at 15% - based on DNA profiling IIRC, but it seems high. The other stat I recall recently was that almost 50% are now born out of conventional wedlock.
Love to see some actual research on the subject of the OP, though.