Human reproduction naturally implies sexual inequality in kin knowledge. Women naturally know for certain their biological children. Men know their biological children by ignorance, faith, or modern paternity-testing technology.
The widespread availability of contraceptives does not make paternity uncertainty insignificant. A gynocentric study found that 49% of pregnancies in the U.S. in 2001 were unintended, meaning that the pregnant women did not intend to create a pregnancy. A different study asked men about children that they believed they had fathered. These men described 33% of their perceived children as unintended. Even with widespread availability of contraceptives, unplanned parenthood (meaning unplanned motherhood) and abortion have been huge public issues for decades. The widespread availability of contraceptives similarly does not eliminate unplanned fatherhood or paternity uncertainty.
Survey data on sexual behavior suggests that more than 10% of births in the U.S. are to women with more than one concurrent heterosexual partner. Surveys from the early 1990s that identify concurrent heterosexual partners report about 12% of women ages 15-44 having such sexual relations. Among persons ages 15 to 44 in the U.S. in 2002 and reporting heterosexual activity in the prior year , 17% of women and 23% of men reported more than one sexual partner in the past year. Breaking down such figures by marital status and weighing by share of births in the corresponding category implies 14% births are to women who have had more than one sexual partner in the previous year. A similar calculation using age categories implies 20% of births are to women who have had more than one sexual partner in the previous year. The differences in these figures indicates non-independence of giving birth and having more than one sexual partner. This issue, as well as possible misreporting, complicate interpreting the figures. But a plausible inference is that the share of births attributable to women with concurrent sexual partners is greater than 10%.
Limited available information about tests of biological paternity in high-income Western countries suggests that men falsely regard as their own biological children about 4% of children. A relationship-testing accreditation organization serving mainly the U.S. reported for 2006 a total of 15,082 paternity tests that were not part of legal procedures and for which paternity exclusion (non-paternity) was tracked. A plausible sample frame for non-legal tests are men for whom paternity uncertainty is highly salient. The non-paternity share among these tests was 30.4%. A similar figure comes from meta-analysis of scholarly studies in the past two decades that analyzed human biological paternity in paternity-testing samples from high-income countries. None of these studies adequately relate the sample tested to a large, general population. Another small set of studies over the past two decades for high-income countries includes small samples of men that typically have a close physical and social relationship to the child and its mother. The median non-paternity share among these studies is 1.5%. Assume that men who seek a paternity test outside of a legal proceeding (30% non-paternity) are representative of the presumed fathers of 10% of children. Assume that 90% of children have fathers for whom the other, more general studies are representative (1.5% non-paternity). Then over-all men falsely believe to be their own biological children about 4% of children.
Studies of socially monogamous non-human animals suggest that the non-paternity rate in humans probably is higher than 4%. A comprehensive study of extra-pair paternity in birds found that, among socially monogamous bird species, extra-pair offspring averaged 11.1% of total offspring and were included on average in 18.7% of broods. Studies of paternity encompassing 17 populations (from 14 different species) of socially monogamous non-human mammals having sexual-social interaction patterns broadly similar to humans show in aggregate a non-paternity share of 23%. Four species of lizards that display strong pair-bonds were measured to have a 21% extra-pair paternity share. Social structure for socially monogamous mammals, and for reptiles more generally, significantly affects extra-pair paternity. Changes in social structure that increase the frequency of opposite-sex, extra-pair interaction, such as humans engaging in more mixed-sex work outside the home, tend to increase the share of extra-pair offspring.
A large number of children and men today have false beliefs about their biological kinship. Recent discussion of paternity has lacked adequate review of available scientific knowledge. Given the current scientific knowledge reviewed above, the share of children having false paternity beliefs is plausibly estimated at roughly 5% in high-income Western countries. For the U.S., that means about 4 million children are wrongly identifying their biological father. The number of men in the U.S. who falsely believe children to be their biological children is probably over 3 million. Whether attributed to ignorance or faith, these false fatherhood beliefs could easily be enlightened with cheap, scientific paternity-testing technology.
Enlightenment does not necessarily destroy love. Imagine a man, married to a woman. He’s required to leave the country for a year for an over-seas assignment. When he returns after a year, he finds his wife pregnant. With the wisdom known to men for millennia, the man knows that he is not the father of this child. His knowledge offers an opportunity for exercising saintly love. He could choose to continue to love his wife and to accept the child as his own. Modern paternity-testing technology would give millions of men a similar choice. Extraordinary love is worth the risk of knowledge.
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- the paternity information economy
- legal obfuscation of differences between having sex and fathering a child
- incarcerating men for consensual sex
 Finer, Lawrence B, and Stanley K Henshaw. 2006. “Disparities in Rates of Unintended Pregnancy in the United States, 1994 and 2001“. Perspectives on Sexual and Reproductive Health. 38 (2): 90 (49% of pregnancies unintended in U.S. in 2001); Martinez, Gladys. 2006. Fertility, contraception, and fatherhood: data on men and women from Cycle 6 (2002) of the National Survey of Family Growth. Hyattsville, Md: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Health Statistics, Table 8, p. 39 (men in 2002 report 8.6% of their children over the past five years unwanted, 24.8% mistimed).
 For data and calculations, see multiple sexual partners worksheet (Excel version). Primary sources: Adimora, Adaora A, Victor J Schoenbach, Dana M Bonas, Francis E A Martinson, Kathryn H Donaldson, and Tonya R Stancil. 2002. “Concurrent Sexual Partnerships Among Women in the United States“. Epidemiology. 13 (3): 320. Mosher, William D., Anjani Chandra, and Jo Jones. 2005. Sexual behavior and selected health measures: men and women 15-44 years of age, United States, 2002. DHHS publication, no. (PHS) 2005-1250. [Hyattsville, MD]: U.S. Dept. of Health and Human Services, Centers for Disease Control and Prevention, National Center for Health Statistics, Tables 1-2, pp. 19-20. In a random survey of Seattle residents ages 18 to 39 in 1995, 18% of women and 27% of men reported having concurrent sexual partners. See Manhart, Lisa E, Sevgi O Aral, King K Holmes, and Betsy Foxman. 2002. “Sex Partner Concurrency: Measurement, Prevalence, and Correlates Among Urban 18-39-Year-Olds“. Sexually Transmitted Diseases. 29 (3): 133.
 Women and men differ greatly in reporting sexual partners over their lifetime, but not over the past year. See Brown, Norman E, and Robert C Sinclair. 1999. “Estimating Number of Lifetime Sexual Partners: Men and Women Do It Differently“. The Journal of Sex Research. 36 (3): 292. The extent to which women and men misreport the number of sexual partners over the past year isn’t known.
 The data on the 15,082 paternity lab cases not part of a legal proceeding (non-legal cases) are reported in the AABB Relationship Testing Annual Report, 2006. The scholarly studies are the relevant subset extracted from Anderson, Kermyt G. 2006. “Reports – How Well Does Paternity Confidence Match Actual Paternity? Evidence from Worldwide Nonpaternity Rates“. Current Anthropology. 47 (3): 513. Data from both sources, and relevant analysis, are available in an online extra-pair-paternity workbook (Excel version). Note that the non-paternity share of children is different from the share of men falsely believing their biological paternity. Anderson (2006) fails to recognize this distinction. It describes shares of fathers, but reports shares of exclusions. Shares of exclusions are most plausibly interpreted as a child-based measure. For further details, see the extra-pair-paternity workbook (Excel version).
 This figure is low relative to a recent study of subjective perceptions of non-paternity in Austria. Women estimated the share of children with falsely attributed biological paternity to be 14%, and men estimated the share to be 9% (sample size, 795 women, 763 men). See Voracek, Mart, Maryanne Fisher, and Todd K. Shackelford. 2009. “Sex Differences in Subjective Estimates of Non-Paternity Rates in Austria“. Archives of Sexual Behavior. 38 (5): 652-656.
 Griffith, Simon C., Ian P. F. Owens, and Katherine A. Thuman. 2002. “Extra pair paternity in birds: a review of interspecific variation and adaptive function“. Molecular Ecology. 11 (11): 2195-2212. Cohas, Aurelie, and Dominique Allaine. 2009. “Social structure influences extra-pair paternity in socially monogamous mammals“. Biology Letters. 5 (3): 313. I eliminated from the Cohas-Allaine dataset populations that were socially monogamous, but solitary and populations with pair-bonding with continuous pair interaction, and then calculated an over-all non-paternity share. For details, see the socially monogamous mammals worksheet in the extra-pair paternity workbook (Excel version). Uller, Tobias, and Mats Olsson. 2008. “Invited Review: Multiple paternity in reptiles: patterns and processes“. Molecular Ecology. 17 (11): 2566-2580. The lizards were four species of skinks: Egernia whitii, Egernia stokesii, Egernia cunninghami, Tiliqua rugosa, and Oligosoma grande. The cited statistics is from id. p. 2568. Id. emphasizes the importance of cost of mating and mate-encounter rates on extra-pair paternity in reptiles. Cohas and Allaine (2009) emphasizes the importance of more elaborate mammalian social structure on the extent of extra-pair paternity in socially monogamous mammals.
 U.S. Census population estimates for 2008 indicate 82.6 million persons ages 19 or younger. To the extent that some men have false paternity beliefs about more than one child, the number of men with false paternity beliefs is less than the number of extra-pair offspring. On the other hand, the number of men who falsely believe that they are not the biological father of a child is probably a large fraction of the number of men who falsely believe that they are the biological father of a child. Large investments of men’s time, attention, and financial resources in children typically distinguishes the latter false belief from the former.
Update: Wolf et. al (2012) examines nonpaternity (false identification of biological father) for 971 focal persons who were bone marrow recipients at a German university hospital from 1993 to 2008. For each focal person, the sample had biological material from the mother and the man that the focal person believed to be her or his biological father. Genetic testing found that the putative biological father was not the actual biological father for 9 focal persons (0.93% of the sample).
This result probably has large, downward non-sampling bias. While Wolf et. al (2012) referred to the focal persons as “children,” that was only a relational specification. The authors had no information on the ages of the focal persons. The median age of bone marrow donor recipients in the U.S. in the mid-2000s was 40. That’s a reasonable rough estimate for the median age in the German sample. Selecting a sample of persons of roughly median age 40, who also are able to obtain a genetic sample from the persons who they believe to be their biological fathers, is strong selection on family structure. In particular, a man not biologically related to a woman’s child is likely to have a lower probably of remaining accessible to the woman over decades while remaining falsely identified as the biological father. So I think tWolf et al (2012) could easily be consistent with roughly 5% of children in Germany today having false beliefs about who are their biological fathers.
Wolf, Michael, Jochen Musch, Juergen Enczmann, and Johannes Fischer. 2012. “Estimating the prevalence of nonpaternity in Germany.” Human Nature. 23 (2): 208-217.