Counting Cousins: How to Calculate Cousinhood

Counting Cousins: How to Calculate Cousinhood

How, exactly, are you related to the child of your great-great-grandmother’s sister’s son? We’ll explain the steps to calculating cousinhood.

Ever play the “Six Degrees of Kevin Bacon” game? It involves linking famous actors and actresses to Kevin Bacon via movies they’ve appeared in together. George Clooney, for example, clocks in with a Bacon number of just 2: He appeared in Hail, Caesar! with Matthew Skomo, who was in X-Men: First Class with Kevin Bacon. Turns out just about everyone in Hollywood is six or fewer movies “removed” from Kevin Bacon. (Check it out online.) And supposedly, none of us is further than six connections from any one person on earth—even Kevin Bacon.

When it comes to cousinhood, the relationship possibilities are just as endless. Your number of grandparents doubles with each generation. Count back 10 generations, and that’s 2,046 total ancestors, which means the cousin potential is exponential. You could have millions of them: fourth cousins, second cousins three times removed, tenth cousins twice removed … we could go on. And with DNA testing, Facebook, online family trees and message boards that connect you to new cousins every day, you’re bound to get curious about exactly how you’re related. Good thing we’re here with this guide on figuring out what kind of cousins you are, based on degrees of separation from shared ancestors. Who knows? Maybe you’ll even discover Kevin Bacon’s your kin.

Starting simply

What makes someone a cousin? The simple fact that you share an ancestor with that person. But to understand the intricacies of cousin relationships, you have to get this: Your ancestors are only the people in your direct line: parents, grandparents, great-grandparents and so on. Your ancestors’ siblings are aunts and uncles (no matter how many greats you add)—not ancestors.

Just about any other blood relative who isn’t your sibling, ancestor, aunt or uncle is your cousin. To determine your degree of cousinhood—first, second, third, fourth—you need to identify the ancestor you share with your cousin, and how many generations separate each of you from that ancestor.

Your first cousin (sometimes called a full cousin, but usually just a cousin) is the child of your aunt or uncle. The most recent ancestor you and your first cousin share is your grandparent. You typically share 12.5 percent of your first cousin’s DNA.

Your second cousins are the children of your parents’ first cousins. Take a look at your family tree, and you’ll see that you and your second cousins have the same great-grandparents. You typically share 3.125 percent of your second cousin’s DNA. For third cousins, great-great-grandparents are the most recent common ancestor and you share .781 percent of your DNA. You get the picture.

Time for a pop quiz: What’s the relationship between your granddaughter and your sister’s grandson? And the answer is … second cousins. The kids’ most recent common ancestor is their great-grandparent (your parent).

How far removed?

“I aced that one,” you say, “but what about a removed cousin? Or a fourth cousin three times removed? What does that mean?”

A remove happens when two cousins have different numbers of generations back to their most recent common ancestor. One generation of difference equals one remove. First, count back the number of generations from each cousin to the common ancestor. The cousin with the lower number of generations determines the degree of cousinhood—first, second, third and so on. Then subtract the lower number of generations from the higher number to find out how many times removed the cousins are.

Take my son, Leo. He and my cousin Matt (son of my mom’s sister) share my grandmother as their most recent common ancestor. My grandma is Matt’s grandmother, too, but she’s Leo’s great-grandmother. Matt is just one generation away from their common ancestor, so he and Leo are first cousins. But Leo is two generations away from the common ancestor—making Leo and Matt first cousins once removed. They share about 6.25 percent of their DNA. Of course, the further removed a cousin gets, the less DNA they share, as you can see in the chart below. You can be distantly related to long-deceased individuals through removes, too. For example, say you’re fourth cousins three times removed with Warren G. Harding. (Because of the mind-boggling number of cousins you have, there’s bound to be someone famous in your family tree.) That would mean your sixth-great-grandparents are Harding’s third-great-grandparents.

Anthropologists call the process of figuring out cousin relationships “collateral degree calculation” (don’t worry, we won’t spring that term on you again). Multiple removes and degrees of cousinhood can get complicated, but you don’t have to be a scientist to get it right. The chart below will help straighten out your cousin confusion; just follow the instructions for using it. For example, to figure out how you’re related to your great-great-grandmother’s sister’s son, first determine the ancestor you share with him: your third-great-grandmother. Find her on the chart, then count down one generation for the sister and one more to the sister’s son. He’s your first cousin three times removed.

Double the fun

You may have heard people say they’re double cousins. That’s a special cousin category for the offspring of brothers- and sisters-in-law—for example, your sister weds your husband’s brother. Instead of sharing one set of grandparents, as first cousins do, double cousins share both sets of grandparents. As you might expect, double cousins have more DNA in common than typical first cousins—about 25 percent.

Despite how it sounds, a kissing cousin isn’t a cousin you marry. Rather, it’s any distant relative you know well enough to kiss hello at family gatherings. Now we’re begging the question: How close a cousin is too close to wed? States have different laws governing consanguineous marriages (and we’ve heard all the jokes, so just stop right now). It’s best to ask a lawyer about statutes for the state in question.

And while we’re on the topic: Due to limited mobility in our ancestors’ day, most of us have instances in our family trees of cousins who married, whether knowingly or unknowingly. That means you can be related to the same person in multiple ways.

Someone you’re related to by marriage, rather than by blood, isn’t your cousin. You might be in-laws, or your relationship might not have a name other than (we hope) good friends. You can read more about collateral degree calculation — oops, we mean family relationships—in Dozens of Cousins by Lois Horowitz (Ten Speed Press) and Jackie Smith Arnold’s Kinship: It’s All Relative, 2nd edition (Genealogical Publishing Co.).

Tip: Remember that the shared DNA numbers shown in our chart are averages. Due to the random way DNA is inherited, it’s possible you don’t share any DNA with a given relative beyond about second cousins.

Click the image above to view or download a higher-res version of this chart.

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Diane Haddad is the editor of Family Tree Magazine. She has 20 first cousins and an unknown number of second cousins and beyond.

This article originally appeared in the July/August issue of Family Tree Magazine.

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