Gene Expression

ScienceDaily highlights an angle on the paper I blogged a few days ago, Chromosome And Surname Study Challenges Infidelity ‘Myth’:

“People often quote a figure of one in ten for the number of people born illegitimately,” says Professor Jobling. “Our study shows that this is likely to be an exaggeration. The real figure is more likely to be less that one in twenty-five.”

I didn’t comment on this because there is research which contradicts the 1 in 10 number. There’s a lot of variation worldwide, but among mainstream Western populations the misattributed paternity rates are on the order of 2%, not 10. Here are the rates from the paper:

This approach yields the following rates: Attenborough 1.29-3.39%; Haythornthwaite 2.07-4.54%; Herrick 1.00-2.47%; Stribling 1.00-2.87%; Swindlehurst 1.04-2.76%. However, it should be noted that if, in fact, these surnames had multiple founders, but only one founding lineage had survived to yield a sampled descent cluster…then the true nonpaternity rates would be lower than our estimates.

The logic here is rather straightforward. Remember that Y chromosomes are passed only from father to son. Take a theoretical paterlineage, and connect individuals vertically from father to son and iterate. Assume a 10% rate of misattributed paternity. This means that with no other information you are 90% certain that a man’s putative father is their genetic father, and that his putative son is his genetic son. This implies that the putative relationship between a random grandfather and grandson is 81% likely to be in alignment with genetic realities. Now assume that a man has two sons, who each have two sons. If the probability of each grandson sharing the Y lineage with their paternal grandfather is 81%, then there is a 43% probability that all four grandsons are actually genetically related to their putative grandfather.

Assumptions about independence are likely not valid; men who are cuckolded once are certainly more likely to be cuckolded later for whatever reason. But even with the simplifying assumptions you can see that a 10% misattributed paternity rate would result in a great deal of non-relatedness across a lineage. When looking at phylogenies such a high rate would stand out immediately. 10 generations is ~250 years. Again with the same assumptions the chances of a patrilineal descendant of a man being his genetic descendant is on the order of 40%. In other words, 250 years into the future the correspondence between theory and reality in terms of descent should begin to uncouple to the point where enormous numbers of men within the extended lineage are unrelated on the Y chromosome. The researchers report that this is not what you see, that in fact for many rare surnames there are high frequency lineages whose most recent common ancestor go back 500-1,000 years. This is not so for common surnames like Smith, but combined with contemporary cross-cultural non-paternity rates, it seems it is likely that the lack of relatedness has less to do with infidelity than the fact that a name like Smith was adopted by many unrelated individuals within that profession.

Of course there are cross-cultural variations. That is why the French bill which required genetic tests to ascertain relatedness among immigrants drew such a sharp response from Africans. Some of this might be a different conception of family, but there’s going to be rather obvious differences in paternity rates among Saudi women, whose men hold them chattel, and West African women who have long been independent economic actors. For example, DNA Tests Offer Immigrants Hope or Despair:

So when he became an American citizen and officials suggested taking a DNA test to prove his relationship to his four sons, he embraced the notion. Imagine, he marveled as a lab technician rubbed the inside of his cheek, a tiny swab of cotton would reunite his family.

But modern-day science often unearths secrets long buried. When the DNA results landed on Isaac Owusu’s dinner table here last year, they showed that only one of the four boys — the oldest — was his biological child.

Why has the 1 in 10 number become etched into public lore? Because it’s a nice round number. Just like the “1 in 10 homosexual” number was long accepted.


  1. #1 Edward
    February 13, 2009

    One problem with your assumptions above: False paternity does not always reduce the expected matching of y-chromosomes. For example, if the actual father is the brother of the father of record.

    That being said, I can believe that the 1 in 10 is overstated, given what I’ve seen myself in human genetic data.

  2. #2 razib
    February 13, 2009

    For example, if the actual father is the brother of the father of record.

    yes. i was going to mention that. my guess would that that changes the numbers on the order of percentages (e.g., 10-20%) as opposed to multiplicative (2 or 3 times). but i don’t know the data on how common this is. it seems likely that in most parts of the world patrilocality was a stronger assumption than it is today, so intuitions derived from contemporary patterns might not be strong. though even brothers have different de novo mutations, so once the data sets get big enough perhaps this could clarified purely genetically.

  3. #3 David B
    February 14, 2009

    I commented on previous data a few years ago: see here
    and the previous post linked there.

  4. #4 Eddy
    February 14, 2009

    Infidelity? They could have used a condom ;-)

  5. #5 pconroy
    February 14, 2009

    Just saw this somewhat related article on notorious Irish bigamist and conman, Oliver Killeen, who allegedly married between 14 and 22 women, over a 25 year span, and had over 50 children. There is even a documentary film called, The Conman With 14 Wives, made about him.

  6. #6 Emilia
    March 8, 2009

    I think many groups latched onto the 1 in 10 figure because it helped them push their agenda. Old guard male chauvinists used it to illustrate the perfidy of the female soul. On the other hand some feminists liked it because it went against the in some ways equally chauvinistic view that women didn’t like sex. And your ordinary man (or woman) on the street latched onto the 1 in 10 figure because he (or she) found it exciting to believe (though one could argue that if something is common it isn’t so exciting after all -I mean would documentaries on the Hensel twins or Aceves family garner millions of viewers if one in every ten people were a conjoined twin or had hypertrichosis?). But I long suspected that the 1 in 10 figure was bunk.