IIRC, in Leeds, England, 20% of the putative fathers of the kids were not the actual fathers. This percentage held up in several other countries.
There was some saying about women preferring to be the second wife of (insert name of rich & famous person) than the first wife of Joe Schmoe.
One of the crooked lawyers I hired once told me this story: a guy goes to Vietnam for more than 289 days and when he came back his wife greets him on the Tarmac with a brand new baby.
The lawyer told him: "If you were a straight arrow the whole time you were in 'Nam then you have genuine gripe."