Here you go little envious person:
Macintyre, S. & Sooman, A. (1992) Non-paternity and prenatal genetic screening. Lancet, 338, 839.", according to the wikipedia page for the Sperm Wars book:
Sperm Wars - Wikipedia, the free encyclopedia
Worldwide, it has been calculated from studies of blood groups that about 10 percent of children are in fact not sired by the men who they think are their fathers. This is also the average level found in Western industrial societies [...] There is a real need for an extensive study using modern techniques such as DNA finger-printing. So far, the nearest thing to such a study comes from paternity tests cariried out by child-support agencies. They are responding to absent "fathers" who demand such tests in an attempt to avoid or delay the financial support of an ex-partner. Internationally, child support agencies are reporting a nonpaternity rate of about 15 percent - higher than the average 10 percent - suggesting that men who appeal have more grounds than most for doubting their partners' fidelity.
All figures for nonpaternity are the proportion of children actually born. The nonpaternity level for children conceived is even higher. This is because a woman is more likely to abort a child conceived via a man other than her long-term partner. Almost certainly this happens primarily when her partner either knows or has a good chance of finding out that he is not the real father. The abortion is an attempt by the woman to avoid the costs of infidelity discussed in scenes 9 and 11.
Although properly controlled DNA fingerprinting studies have not been carried out on humans, they have been carried out on a wide range of appearently monogamous birds. The results for different species suggest a range from 0 percent to over 50 percent incidence of males raising other males' offspring. These levels are comparable with, but on average are probably slightly higher than, the levels found in different groups of humans [...] So it would seem that the average male bird has even less reason than the average man to be reassured by a passing resemblance to its offspring.