No one need have worried so much about who the father of these kids were.  Image courtesy of Wikimedia Commons.
No one need have worried so much about who the father of these kids were. Image courtesy of Wikimedia Commons.

It is a fact of life that some men are unknowingly raising kids that are not biologically related to them. One common way for this to happen is if their partner has an affair and gets pregnant. This predicament is common (or worrisome) enough that there are even magazine articles that come up with mostly wrong ways to tell if a child is really yours.

Until recently, though, no one had a really good idea about how many men were in this situation.  The 3-5 percent number commonly bandied about was extrapolated from a pool of men that had done some DNA paternity testing.  This obviously skewed sample meant that these numbers were probably too high.

Now that DNA testing is becoming cheaper and easier, we can get better numbers. A couple of recent studies from Western Europe suggest that somewhere between 0.6 and 0.9 percent of men are unknowingly raising another man’s child. While this is still a lot of men, it is much less of a problem than many people thought.  And according to another new study, it turns out that the problem wasn’t much worse a few hundred years ago (at least in one region of Western Europe).  This last result was definitely unexpected.

Most people who thought about this issue had assumed that the problem must have been worse in the past.  They assumed that the rate of women having an affair held steady at around 15-50% but since these women did not have access to either modern birth control or safe abortions, they must have had more kids that weren’t related to their husbands.  Turns out this may not have been the case.

Researchers found that the numbers of men unknowingly raising another man’s child held constant at around 1-2 percent per generation over the last four hundred years in Flanders in Western Europe.  This is in contrast to previous estimates that had put the numbers as high as 8-30 percent.

The new study was different from previous ones in that it used DNA analysis of the Y chromosome along with extensive genealogical records.  The Y chromosome is particularly useful to look at here because it hardly changes when it is passed from father to son.  This means it is easy to trace back through a family tree and see where the last name and the Y don’t match up.

Imagine we are looking at this simple family tree:


We are interested in learning whether at some point, a man somewhere on this tree was raising a child that wasn’t his.  We can learn this even though only the two circled men are alive to test.

To show why this is, let’s reduce these folks down to their X and Y chromosomes.  Here is what the chromosomes would look like if everyone is raising their own kids:


The men are XY since they have an X and a Y chromosome and the women are XX because they have two X chromosomes.  Since we aren’t interested in the X’s, they have been grayed out.  We’ll only pay attention to the Y’s.

The first thing to note is that fathers pass their Y chromosomes to their sons and not their daughters.  Although not obvious from this image, the other important fact is that the Y passes from generation to generation virtually unchanged.  This means we can trace back Y’s into our distant past.  In fact, scientists have traced it back hundreds of thousands of years to a Y-Adam.

By testing the two circled men, we can see that since they share the same Y, they must share a common great grandfather. If we kept going up, we may be able to trace their shared history back even further. The blue Y passed from generation to generation in this family tree in the right way so that everyone was raising who they thought they were.

Now imagine we get the result that these two men had different Y chromosomes. Here is one possible way that something like that might have happened:


One of the brothers from two generations back was actually raising a child that wasn’t his. This child inherited the yellow Y from his biological dad and grew up to father the living man who was tested. Of course, the change could have happened one generation forward as well, but it couldn’t go back much further in this family tree. If it did, both of the tested men would share the same Y as they would have the same great grandfather!

This is the sort of analysis the researchers did for 1071 Flemish men who could trace their family history back for at least 200 years.  When they applied two different statistical models to study the data, the researchers came to the same conclusion—around 1-2% of men in each generation were unknowingly raising someone else’s kids.

Scientists are unsure why more women weren’t getting pregnant with other men’s babies.  It could be that the rate of adultery was much lower in the past or that older methods of either birth control and/or pregnancy termination were more effective than previously thought. Or there could be some other reason.

Whatever the cause, men may have perhaps been worrying a bit too much for too long about this particular problem. And if these numbers hold up, then the unintended consequence of people finding out that their dad isn’t biologically related to them won’t be as big a problem once everyone has their DNA sequenced.  That is probably a good thing.

An important note is that these analyses were all done in Western Europe.  It could very well be that other cultures have a higher rate of men raising children who aren’t their own.  Click here for one such example.

New DNA Studies Debunk Misconceptions About Paternal Relationships 25 November,2013Dr. Barry Starr

  • S. Andrea Fronzaglia

    Perhaps the men were just displacing their own guilt regarding infidelity onto their wives.

    • Uncontainable Spirit

      Perhaps the research itself is flawed.

      • gringott

        Since the tests were limited to men who could trace their family tree back 200 years, it seems like a pretty exclusive group. That, itself, seems to me to be a fatal flaw in the study.

  • tom

    I would love to see the research on how much the data changed from the avant of paternal testing. Ray Charles had over 12 children with nine different women and babe Ruth was rumored to have a kid in every state (48).Granted these are celebrities but storys like these make it appear that men raising other children kids was far more common then it is today.

  • håvard norback

    This is old number,new Are around 5-10


Dr. Barry Starr

Dr. Barry Starr (@geneticsboy) is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA and runs their Stanford at The Tech program. The program is part of an ongoing collaboration between the Stanford Department of Genetics and The Tech Museum of Innovation. Together these two partners created the Genetics: Technology with a Twist exhibition.

You can also see additional posts by Barry at KQED Science, and read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

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