Defining a first cousin is easy enough: They are the children of your parents’ siblings and share grandparents. Even second cousins—the children of your parents’ cousins who share great-grandparents—can be simply identified. Beyond that, though, the definitions get more complicated. So, what is a third cousin? We’ll explain.
Third cousin definition, and are they blood related
Let’s start with a third cousin definition. Third cousins are from the same generation in the family tree and share great-great-grandparents, according to Ancestry.com. Two third cousins’ parents are second cousins with each other, while their grandparents are first cousins with one another. The family tree goes back four generations.
The genealogy site 23andMe.com lists a 90% probability that third cousins share DNA, though it’s a low figure. First cousins share, on average, roughly 12.5% of their DNA, said CeCe Moore, the chief genetic genealogist for Parabon Nanolabs. First cousins, once removed, share 6% on average, while second cousins share a shade more than 3%. Third cousins, Moore said, typically match less than 1% of DNA. The deeper into the family tree a relationship goes, the genetics two people might share match less and less.
If third cousins are related through great-great-grandparents, that means numerous new DNA sources enter the mix before reaching that fourth generation of relations. Great-grandparents add a whole new side of a family’s DNA to the equation. Grandparents bring in another family’s DNA. Parents do the same. By the fourth generation, scores of different genetic histories have worked together to distance third cousins quite a bit in DNA.
“(The cousin definition) really affects the amount of DNA you share,” Moore said. “Even though it’s simple, it’s very important if someone is going to do genetic ancestry.”
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What is a first cousin?
This is straightforward: First cousins share grandparents. One cousin’s parent is a sibling of the other cousin’s parents. In other words, any child of your aunt or uncle is your first cousin.
“If their full siblings, that’s a full first cousins,” Moore said. “If they’re half-siblings, then they’re half-first cousins. People are really confused about that.”
This is where it’s important to note the correct label of cousins matters mostly for genealogical purposes. Culturally and socially, the “cousin” label could mean anything from an actual blood first cousin to a third cousin to a family friend and everywhere in between. “Cousin” can also be commonly used to describe relatives similar in age, even if one is actually the other’s aunt or uncle.
“People don’t use the terminology when they’re in family mode and obviously that’s perfectly fine,” Moore said, “but when you try to apply it to genealogy, it gets really important to make the distinction. It’s absolutely critical when you’re trying to understand the differences when you’re using the relationship for researching your family history.”
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What is a second cousin?
Second cousins share great-grandparents. Their parents are first cousins, and their grandparents are siblings. Some call their grandparents’ siblings “great” aunts and uncles; others call them “grand” aunts and uncles. The definition remains the same: Your great aunt or uncle’s grandchildren—or your parent’s first cousin’s children—are your second cousins.
Counting by generations, second cousins go back three generations to find their primary relations. FamilySearch, an ancestry DNA site, suggests determining levels of cousin by following an equation: Count how many greats you need to find your common ancestor, then add one. If you share great-grandparents and add one, you get second cousins.
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What is a cousin “once removed”?
This is where counting cousins gets complicated. Straightforward cousins share generations and grandparents, great-grandparents, great-great grandparents and so on. Cousins “once removed” don’t share the same generation. The tag might, for instance, come into play when your first cousin has a child. Your grandparent is your cousin’s child’s great-grandparent.
Use this fictional example to explain the rarer “once removed” tag: John and Mark are first cousins, once removed. Their genetic relationship starts with the same two people, but John calls them his great-grandparents and Mark calls them his grandparents. This is because John’s grandfather and Mark’s mother are siblings. In this scenario, John’s grandfather is likely a much older sibling who started a family while Mark’s mother was still young. By the time Mark’s mother started her own family, John’s grandfather’s children—John’s parents—had started theirs, too. John’s parents and Mark are first cousins, and when John’s parents had him, he became first cousins, once removed, with Mark.
“It’s generational-based, not age-based, and that’s important,” Moore said. “You see that a lot, where the generation is off even when the age is the same. You can’t always go by ages because some families have huge age differences in siblings.”