What is a second cousin?

If you can't keep your second cousins and your first cousins twice removed straight, you are not alone. But there's a simple way to figure out the relationships between relations.

Family reunions are often filled with confused people scratching their heads, ticking off fingers and mumbling, "If my mother's aunt was her father's grandmother, then what does that make us?" The more steps there are connecting two relatives, the harder it can be to decipher.

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To understand the terms of an extended family tree, it is useful to begin with the basics. The term "ancestor" refers to people who share a direct line, according to the journal PLOS Genetics. Examples of these are your parents, grandparents, great-grandparents etc. Ancestors don't include the siblings of these people. This means that while uncles and aunts (the siblings of a parent) are blood relatives, they don't count as ancestors.

Other than aunts and uncles, any other blood relative that isn't an ancestor is a cousin of some sort. The most common use for the term "cousin" is to refer to a first cousin. This is the child of your uncle or aunt. 

What is a second cousin?

First cousins share a grandparent, second cousins share a great-grandparent, third cousins share a great-great-grandparent, and so on. The degree of cousinhood ("first," "second," etc.) denotes the number of generations between two cousins’ parents and their nearest common ancestor. 

Click on the arrows in the image below to see where a second cousin is positioned on the family tree:

When does a cousin become "removed"?

The term "removed" refers to the number of generations separating the cousins themselves, according to ancestry.com. So your first cousin once removed is the child (or parent) of your first cousin. Your second cousin once removed is the child (or parent) of your second cousin. And your first cousin twice removed is the grandchild (or grandparent) of your first cousin.

Distant relatives

It was discovered that Barack Obama (left) and Dick Cheney (right) are distantly related. (Image credit: Getty Images)

Clearly, it doesn't take many generations before your family tree is a bit unwieldy. Case in point: In 2007, it was revealed that vice president Dick Cheney and presidential hopeful Barack Obama are eighth cousins. Cheney's wife, Lynn Cheney, discovered this tidbit while researching her husband's genealogy for a memoir she was writing, the BBC reported. 

Parallel vs cross

If these distinctions aren't confusing enough, first cousins can be further parsed into parallel and cross cousins. Parallel cousins are the children of same-sex siblings, according to the journal American Anthropologist — for example, the children of your mother's sister are your parallel cousins. Cross cousins are the offspring of opposite sex siblings, such as your mother's brother's children, or your father's sister's children.

And in case you were wondering, the two relatives at the family reunion (A's mother's aunt is B's father's grandmother) are second cousins once removed.

Additional resources

Find out about the evolution of families and marriages in this book by SAGE Publications. Want to explore your own family tree? Websites such as ancestry.com allow you to trace your lineage.

Bibliography

"What is ancestry?". PLOS Genetics (2020). https://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1008624

"Complementarity and the Structures of Parallel-Cousin Marriage". American Anthropologist (1986). https://www.jstor.org/stable/679082