Is Marrying Your Cousin Bad, Genetically?

How does marrying a cousin impact your genetics?

By Andy Lee


In various modern cultures, marrying your cousin has some negative connotations. But at the same time, cousins have been marrying cousins for all of human history. And the humans seem to be going strong with more than 7 billion living individuals. So is marrying your cousin really that bad?

Most of us have a family tree that branches out, growing in size with each generation, and our DNA tree does as well to a certain extent. However, many people find in their heritage that in some cases those branches are the same branch.
For most of human history, people lived in relatively isolated groups of only a few hundred people. Because of this parents weren’t always unrelated. Usually, sibling relationship pairing was forbidden, but 1st cousins and beyond held no taboo in a lot of cases.
Rudy Guiliani, Albert Einstein, HG Wells, and Charles Darwin were all married to 1st or 2nd cousins. Even US president Martin Van Buren and Vice President John C. Calhoun married their 1st cousins. In fact, it is estimated that 80% of all marriages throughout history were between 1st or 2nd cousins.
So what happens to your DNA when your tree doesn’t branch because your family kept on marrying within the same group? 
From an evolutionary standpoint, marrying someone not related to you introduces genetic diversity. For the majority of our genes, there is not much difference whether we marry a cousin or a complete stranger. Where problems can occur is with certain detrimental recessive genes.


Recessive genes are not expressed unless you get two copies of the same gene. A simple example I have talked about before is red hair. If you just have one red hair gene, like my mother, you are a carrier, but you don’t have red hair. But if two carriers have children, then those children have the potential to have red hair.

There has been a lot of studies on marrying cousins that show an increased rate of birth defects. From 2% to 4%. However, there are plenty of other factors that also increase that risk, such as smoking, drinking alcohol, or even pollution. Thus, 96% of the children born to cousins have had no birth defects.

As with most things with DNA, think of it as a game of statistics. You share approximately 12.5% of your DNA with your cousin that you are going to marry. That 12.5% of shared DNA came through one of your parents who was the sibling of one of your cousin’s parents. Then we get into an if game. 

Because of recombination, your children may get 0 recessive genes, 1 recessive gene, or 2 recessive genes. The probability of getting 2 recessive genes is between 0 and 12.5% because both of you would need to have a recessive gene on that shared portion of your DNA.

For comparison, siblings having children would have a much higher chance of passing on 2 recessive genes, between 0% and 50%.

In other words:
  • Don’t marry your sibling. 
  • If you want to marry your cousin, bad things probably won’t happen. 
  • If you are looking to marry a 2nd, 3rd, 4th, 5th cousin or more distantly removed - there really isn’t anything genetically to even worry about.  
  • Avoid drinking, smoking, and pollution.

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