How to Calculate Cousins

by Patrick Gleeson, Ph. D., Registered Investment Adv

The whole cousin business can get pretty confusing. What's the difference, for instance, between a second cousin and a first cousin twice removed? Figuring out answers to cousin questions can make your eyes glaze over. What you need is a way of looking at cousinship that's straightforward and to the point.

Cousins Removed

You have a first cousin and that cousin has a child, often referred to as a "second cousin." However, that's wrong. The daughter of your first cousin is still your first cousin, but she's "your first cousin once removed."

To straighten out this common misunderstanding, go back to the beginning (and essential ingredient) of a cousin relationship. All cousins have a common ancestor, but only first cousins have a common grandparent.

If two persons we'll call Adam and Ada marry and have two children, Bob and Betty – siblings therefore – and Bob has a child Calvin and Betty has a child Cinda, then Calvin and Cinda are first cousins because they have common grandparents, Adam and Ada.

If Calvin has a child, Dick, and Cinda has a child, Dolores, then Dick and Dolores are still first cousins, but they're first cousins once-removed, because they're one generation removed from the original first cousins, Calvin and Cinda.

The generalization here is that descendants of first cousins are always first cousins. They're removed from the original first cousin by the number of generations between the original first cousins and all the direct descendants of any of those first cousins. They may be first cousins eight or ten generations removed and have never heard of one another, but they're still first cousins.

Second Cousins vs. Second Cousins Once Removed

Second cousins don't share common grandparents; instead, they share a set of common great-grandparents. Your second cousin is the grandchild of a great-aunt or great-uncle that you and your second cousin have in common.

The same "removed" designations applies to second cousins, third cousins (who share one set of great-great grandparents) , fourth cousins and so on, to theoretical infinity. The removal number designates how many generations separate the cousins. Your second cousin once removed, for example, is the child of your second cousin.

Cousin Calculator

Fortunately, there are a number of free online genealogy calculators, including cousin calculators, that will do this kind of genealogy work for you. When you get up into third cousin twice removed territory and beyond, you should probably use a calculator. You can find one popular cousin calculator at the Ancestor Search website.

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.

The term "removed" refers to the number of generations separating the cousins themselves. 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.

About the Author

Patrick Gleeson received a doctorate in 18th century English literature at the University of Washington. He served as a professor of English at the University of Victoria and was head of freshman English at San Francisco State University. Gleeson is the director of technical publications for McClarie Group and manages an investment fund. He is a Registered Investment Advisor.