Here is an interesting statistic: according to a recent study in England, about 4% of all children born in the country are not fathered by the person claimed, according to DNA tests.
The authors of the study say that governments and society have not yet woken up to how DNA testing and genetic profiling are lifting the lid on a "Pandora's box" of hidden sexual behavior or how the results might affect individuals, family relationships, or public health.
Their review of estimates of so-called paternal discrepancy over more than 50 years suggests that the father was not the natural parent in between 1% and 30% of cases. The team from Liverpool John Moores University agreed that the figures, drawn from studies of men and women seeking proof of paternity, might be exaggerated because uncertainty over fatherhood is usually the reason for tests.
I'd suggest that you read all the statistics in the full story on the Guardian's web site at http://society.guardian.co.uk/children/story/0,1074,1546809,00.html before jumping to any conclusions.
It does make one wonder how accurate our genealogy databases are…