What percentage of children are not biological children of their parents?

The title question is a matter of curiosity and controversy, as European society has traditionally been concerned with ensuring that within a marriage, the male parents are the biological fathers of their children, who also receive the family name through paternal inheritance. This concern has been justified by sociology and evolutionary biology with the argument that parents invest a great deal of personal and emotional effort and many economic resources—such as inheritances—in their children, and would like to ensure that the recipients are their genetic descendants.

In the past, people relied on physical resemblance to establish paternity and kinship, but in the 1990s, paternity studies based on genetic analysis became widespread. The first data, from very limited samples from paternity testing companies, They claimed that 10% of legal children are not biological children of their parents.. This 10% figure was a popular choice and has been mentioned on multiple occasions, but the problem is that it is not statistically proven and is also biased because the parents who want to perform a paternity test most likely had some prior reason to assume this possible discrepancy, so they are not representative of society as a whole.

Furthermore, genetic paternity tests give very different results depending on the social class of the people studied, as well as depending on the religion and customs of the populations. Not all societies share the social vision of a monogamous marriage, in which the children are biological children of the couple, and in some places the percentage of children with Parenthood that differs from the established couple can reach 48%, as among the Himba population of Namibia, in which all parents feel like parents and caregivers to their shared offspring.

The Story of the Y Chromosome

Current genetic analyses reflect today's socioeconomic reality, but they do not tell us what the percentage of discrepant paternity has been throughout history. In a recent interview in the magazine Science, Belgian forensic geneticist Maarten Larmuseau explains his studies on cases of paternity outside the couple during the modern history of Europe. In his search, Larmuseau first investigated the genealogical and genetic past of Dutch and Belgian families to study if there was discrepant paternity at any time, and in what percentage.

To carry out this type of studies for genealogical research, not only the transmission of the surname through the paternal line is considered, but an exhaustive investigation is also carried out in parish records to determine marriages and descendants, and these results are correlated with the genetic analysis of the Y chromosome, given that the mas and the this allows to establish strict patrilineal lineages to determine kinship through the paternal line, many generations back. When there is no match of the Y chromosome in a male family branch, it is inferred that there has been some case of paternity outside the couple.

In more extensive studies, Larmuseau has determined that Historically, the percentage of discrepant paternity in the European population is around 1%., much lower than the 10% initially proposed. In our society, adoptions aside, discrepant paternity is usually associated with infidelity by the woman in an established couple. It is more difficult for this to happen in a rural community, where everyone knows each other, and it is easier for a woman to have a changing partner in an urban context. This type of genealogical and genetic analysis throughout history in Europe shows that discrepant paternity is more likely in times of major social change, such as during wartime or during the Industrial Revolution, with the emergence of the working class in cities, where discrepant parenting increased to 6%.

Other scientists, such as Francesc Calafell, a professor at Pompeu Fabra University, comment that this increase is probably not due to adultery, but that the explanation may be more complex: discrepant paternity in urban areas with women who worked in factories, outside the domestic environment, could be due to cases of sexual violence and social discrimination.

No Beethoven?

There are curious historical cases in which there is discrepant paternity and the surname does not match the expected Y chromosome. Larmuseau has shown that the current Van Beethovens are all genetically related.but despite being on the same family tree as the famous German composer, they do not share the same Y chromosome, probably because there was a case of paternity outside the couple in the musician's paternal ancestors. A disappointment for those who thought they shared nothing with the composer!

On the other hand, the Identification of the remains of the English monarch Richard III (known for the Shakespearean phrase "My kingdom for a horse") showed in 2014 that the Y chromosome of the Plantagenet dynasty has not been preserved, and there is at least one discrepant paternity in current descendants (surnamed Somerset), separated by 25 generations via the patri. Even assuming a 1% or 2% discrepant paternity on average, the probability that in royal dynasties all male descendants over several generations share the Y chromosome is... small.

Gemma Marfany is a professor of genetics at the University of Barcelona and head of the CIBERER unit.