Non-paternity event is a term in genetic genealogy and clinical genetics to describe instances in which the biological father of a child is someone other than who it is presumed to be. The presumption may be either on the part of the presumed father or by the physician. Non-paternity may result from a number of different scenarios: it may arise from sperm donation or when the mother had sexual intercourse with a man other than the presumed father. Other than the situation of gestational surrogacy, the identity of a child's mother is seldom in doubt. Non-paternity (and non-maternity) may also result from hidden adoptions: that is, when a child is never told he or she was adopted. Where there is uncertainty, then the only definitive diagnosis of non-paternity is from DNA testing.
The discovery of previously unsuspected or undisclosed non-paternity may have both social and medical consequences. Non-paternity that is due to a previously undisclosed extra-marital relationship often has serious consequences for a marital relationship. Non-paternity is medically relevant when interpreting the results and utility of genetic screening for hereditary illnesses such as cystic fibrosis.
In genetic genealogy the term NPE is often used in a wider context to indicate a break in the link between the Y-chromosome and the surname. Such a breakage may occur because of formal or informal adoption, illegitimacy inside ("extramarital event"/infidelity or rape) or outside of marriage, child known by other surname (mother's maiden name, stepfather's name), the use of an alias or a deliberate change of surname. 
Testing for non-paternity
The only definitive test for paternity is DNA testing. Requirements for consent and counselling for DNA testing vary by country. Sons can be tested by Y-DNA and atDNA, daughters by atDNA and X-DNA.
Rates of non-paternity
However, a 2005 scientific review of international published studies of paternal discrepancy found a range in incidence from 0.8% to 30% (median 3.7%), suggesting that the widely quoted figure of 10% of non-paternal events is an overestimate. In situations where disputed parentage was the reason for the paternity testing, there were higher levels; an incidence of 17% to 33% (median of 26.9%). Most at risk of parental discrepancy were those born to younger parents, to unmarried couples and those of lower socio-economic status, or from certain cultural groups.
A 2006 study examined non-paternity rates from 67 published studies. Non-paternity rates for men who were judged to have high paternity confidence ranged from 1.9% in the U.S. and Canada, 1.6% in Europe, and 2.9% elsewhere. In contrast, men in studies of disputed paternity, considered to have low paternity confidence, the rates of non-paternity were higher – 29% in the U.S. and Canada, 29% in Europe, and 30% elsewhere.
The rates value varies according to the population studied:
- United Kingdom:
- Mexico: 9.8% to 13.8% in a sample of 396 children
- Switzerland: 0.3 to 1.3% in a sample of 1,607 children
- United States:
- France: 2.8% in a sample of 362 children
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