What is the evidence that it exists, and what does the evidence say about its scale?
A good indication that it exists is the disquiet about paternity tests! Those who criticise the availability of these tests do so because they have no doubt that a proportion of tests will bear bad tidings. Here are items from various sources, in various years, and in various countries. This material suggests that about 1 in 10 children have surprising paternity.
From Sussman L N and Schatkin S B (1957) "Blood-grouping Tests in Undisputed Paternity Proceedings", Journal of the American Medical Association: "Blood-grouping tests
in 67 cases of uncontested paternity indicate that in 6 cases, or 9%, the men admitting paternity were not the fathers of the children they accepted. Since only 50% of wrongfully accused men can be excluded by present methods of blood testing, it follows that not 6 but actually 12 men in this small series who admitted paternity were probably not the fathers of the children in question".
From Barbara Katz Rothman (1989) Recreating Motherhood: Ideology and Technology in a Patriarchal Society: "A fair percentage of us, it turns out, are not genetically related to the men we grew up with as fathers anyway. Some physicians doing tissue typing for organ donations estimate that maybe 20 percent of people are not genetically related to the men who claim fatherhood; others say it is less, perhaps as low as 5 percent".
From Child Support Agency Annual Report & Accounts 1997-98: "Discounted DNA Paternity Testing was introduced during 1995/96, giving alleged non-resident parents the opportunity to resolve a paternity dispute without the need to go to court. There has been a steep increase in the use of this method and, in 1996/97, nearly 90 per cent of tests proved positive".
From Hansard: letter from Mrs. Faith Boardman to Mr. Archy Kirkwood 1998-02-18: "In 1996/97, paternity was established in 89% of cases referred for DNA testing. In the current year, to the end of January, the figure is 87%".
From the Guardian, 1998-07-14: "More than 25 years ago the consultant obstetrician E E Phillipp reported to a symposium on embryo transfer that blood tests on between 200 and 300 women in a town in the south-east of England revealed that 30 per cent of their children could not have been fathered by the men whose blood groups had also been sampled".
From the Durban Sunday Times 1998-10-11: "The Sunday Telegraph recently reported that Professor John Burns, a geneticist at Newcastle University, argued that the figure was closer to 10 percent".
From the Dallas Morning News 1999-10-31: "DNA Diagnostics Center
an industry leader, says 30 percent of the men it tests prove to be misidentified. Similar numbers come from the Texas attorney general's office, which enforces child support: About a quarter of the men who disputed paternity in the last year turned out to be right. In Florida, the proportion was one-third".
From the Sunday Times 2000-01-23: "David Hartshorne, spokesman for Cellmark, said that in about one case in seven, the presumed father turns out to be the wrong man".
From the Santa Barbara News-Press 2000-02-27: "For the population as a whole, "The generic number used by us is 10 percent," said Dr. Bradley Popovich, vice president of the American College of Medical Genetics. [15 to 25 % has been determined from blood tests of parents and offspring in Canada and the US.]"
From The Age 2000-03-26: "About 3000 paternity tests are carried out a year in Australia. In about 20 per cent of cases the purported father is found to be unrelated to the child. This figure is estimated to be 10 per cent in the general community".
From The REPORT Newsmagazine 2000-04-24: "The rate of wrongful paternity in "stable monogamous marriages," according to the Max Planck Institute in Munich, Germany, ranges from one in 10 with the first child to one in four with the fourth".
From the Independent 2000-05-12: "... biologists Robin Baker and Mark Bellis ... review of paternity studies also suggested frequent infidelity, with extra-pair paternity running between 1.4 per cent and 30 per cent in different communities".
From The Globe and Mail 2000-05-20: "Anecdotal evidence suggests these numbers bear out in Canada as well
. Maxxam Analytics in Guelph, Ont., performs approximately two paternity tests a day. And according to Dr. Wayne Murray, head of the human DNA department, one out of four men who come in pointing a finger at their spouse is not the biological father of the child in question".
From the Sunday Times 2000-06-11: "More than 250,000 tests a year are now conducted in America, and about 15,000 in Britain.... roughly 30% of men taking the tests discover that they are not the fathers of the children they regarded as their own. In the wider community, social scientists say up to 1 in 20 children are not the offspring of the man who believes himself to be their father".
From the Observer 2000-09-03: "One study followed couples waiting for NHS fertility treatment, where the men were 'azoospermic', meaning they produced no sperm and were totally infertile. The researchers found that 25 per cent of the women became pregnant before fertility treatment started".
From the American Association of Blood Banks - 2001-02-26: "The overall exclusion rate for 1999 was 28.2% for accredited labs. Exclusion rates for non-accredited US and foreign labs were slightly less at 22.7% and 20.6% respectively".
Several of the above sources show not just the scale of the problem, but also the degree to which many men are concerned to discover the truth.