First cousin once removed vs second cousin: the difference, with a chart
A first cousin once removed is your parent's first cousin or your first cousin's child. A second cousin shares your great-grandparents. Chart, DNA, degrees.
Elspeth Gilchrist · · 8 min read
A first cousin once removed is one generation away from you: your parent's first cousin, or your first cousin's child. A second cousin belongs to your own generation: the child of your parent's first cousin, someone who shares your great-grandparents. On average a first cousin once removed shares twice as much DNA with you as a second cousin does, about 6.25 percent against about 3.125 percent.
So the two sit next to each other in the family tree, one step apart, which is exactly why they get mixed up. Your parent's first cousin and that cousin's child turn up at the same weddings, and the English names give no picture of who hangs from which branch. Below, the tree is drawn out, the numbers are set side by side, and each name is built from the counts that produce it.
Kin-type letters help keep the positions straight: F father, M mother, B brother, Z sister, S son, D daughter, read outward from you, so MMBS is your mother's mother's brother's son. The full set is in our notation guide.
Side by side
| First cousin once removed | Second cousin | |
|---|---|---|
| In plain words | your parent's first cousin, or your first cousin's child | your parent's first cousin's child |
| Example kin type | MMBS (older) or MBSS (younger) | MMBSD |
| Ancestors you share | your great-grandparents, who are their grandparents, or the reverse | your great-grandparents, who are theirs too |
| Generations below the shared couple | 3 and 2, or 2 and 3 | 3 and 3 |
| Same generation as you? | no, one apart | yes |
| Coefficient of relationship r | 1/16 | 1/32 |
| Expected shared DNA | 6.25% | 3.125% |
| Expected shared cM | about 438 | about 219 |
| Shared cM Project average (range) | 433 (102 to 980) | 229 (41 to 592) |
| Degree, civil count | 5 | 6 |
| Degree, traditional canon count | 3 | 3 |
Every row except the Shared cM Project figures comes straight from our cousin calculator: enter 2 and 3 for the first column, 3 and 3 for the second. The observed figures come from a volunteer data project discussed further down.
One hypothetical family
Picture a hypothetical couple, Harold and Edith, with two children, Alice and Ben. Their descendants run down three generations:
| Generations below Harold and Edith | Alice's line | Ben's line |
|---|---|---|
| 1 | Alice | Ben |
| 2 | Carol, Alice's daughter | Dan, Ben's son |
| 3 | you, Carol's child | Fay, Dan's daughter |
Carol and Dan are first cousins, both grandchildren of Harold and Edith. You and Fay are second cousins, both great-grandchildren. Dan, your mother's first cousin, is your first cousin once removed, and Carol is Fay's first cousin once removed in exactly the same way.
The table also shows the link people tend to miss. Your second cousin Fay is the daughter of your first cousin once removed. Step up one generation from a second cousin and you reach a first cousin once removed; step down one generation from a first cousin and you reach one too.
Two counts make the name
English cousin names are built from two numbers: how many generations each person sits below the nearest ancestors they share. With those counts in hand:
- The cousin number is the smaller count minus one.
- The number of removals is the difference between the counts.
You are 3 generations below Harold and Edith, and Dan is 2. The smaller count is 2, so 2 - 1 = 1: first cousins. The difference is 3 - 2 = 1: once removed. You and Fay are both 3 down, so 3 - 1 = 2 gives second cousins, and a difference of 0 means nothing is removed.
The removal never changes the cousin number, which is fixed by whichever person is closer to the shared ancestors. That is why your first cousin's child is a first cousin once removed and not a second cousin: you are still only 2 generations below the grandparents you share with that family.
A quicker test needs no counting at all. If the relative belongs to your generation, the same number of steps down from the shared couple, the name has no "removed" in it. If they belong to your parents' generation or your children's, it does. Generation is not the same thing as age, though. If Ben was born twenty years after Alice, Dan could easily be younger than you and still be your first cousin once removed.
Why the parent's cousin shares more DNA
Sewall Wright's coefficient of relationship, r, published in 1922, adds up the paths that join two people through the ancestors they share. Each path is worth one half for every parent and child link on it. From you to Dan the path runs you → Carol → Alice → Harold → Ben → Dan: five links. A second path of the same length runs through Edith. So:
r = 2 × (1/2)^5 = 2/32 = 1/16
From you to Fay the path has one more link, down from Dan to his daughter:
r = 2 × (1/2)^6 = 2/64 = 1/32
Each extra link halves the expectation. For relatives who are not inbred, the expected percentage of autosomal DNA in common equals r: 6.25 percent and 3.125 percent. In centimorgans (cM), the unit testing companies report, the calculator multiplies 2r by 3,500 cM, its rounded length for one copy of the genome: 2 × 1/16 × 3,500 = 437.5, about 438 cM, and 2 × 1/32 × 3,500 = 218.75, about 219 cM. Our worked examples of the coefficient of relationship apply the same path rule to siblings, uncles and double cousins.
What real matches look like
Expected values are averages. Chromosomes pass from parent to child in long blocks, so two relatives can share a good deal more or less than r suggests. The Shared cM Project, organized by the genetic genealogist Blaine Bettinger, collects shared totals from people who know how they are related. Its version 4.0 report (March 2020) analyzed 48 relationships, after trimming the top and bottom half percent of submissions for each:
- 3,700 pairs of first cousins once removed shared between 102 and 980 cM, with an average of 433.
- 4,172 pairs of second cousins shared between 41 and 592 cM, with an average of 229.
The averages land close to the calculator's 438 and 219. The ranges, though, overlap from 102 to 592 cM. A match who shares 300 cM could be either relationship, and the total alone cannot say which.
It gets harder, because other relationships carry the same expectation. Each row below has one value of r, and the project's averages for the relationships in it sit close together:
| Expected (calculator) | Relationships with that expectation | Shared cM Project averages |
|---|---|---|
| r = 1/16, 6.25%, about 438 cM | first cousin once removed, half first cousin, great-great-aunt or uncle, half great-aunt or uncle | 433, 449, 420, 431 |
| r = 1/32, 3.125%, about 219 cM | second cousin, first cousin twice removed, half first cousin once removed, half great-great-aunt or uncle | 229, 221, 224, 208 |
A match who looks like a second cousin may be a first cousin twice removed, or a half first cousin once removed. Ages, the paper trail and the other relatives you both match have to settle it.
Where the names go wrong
- "My first cousin's child is my second cousin." No: that child is your first cousin once removed. Your children and your first cousin's children are second cousins to each other.
- "Removed means related by marriage." It does not. Removed counts generations of difference between blood relatives; in-laws are a separate matter.
- "Only the younger person is removed." The name works both ways. Dan is your first cousin once removed, and you are his.
- Careful genealogists slip too. The Shared cM Project report lists confusion over "removed" among the likely reasons some submissions carried the wrong relationship.
Drawing the tree before naming anyone, as in how to draw a kinship diagram, makes the generations hard to miscount.
Questions
Is a first cousin once removed a closer relative than a second cousin?
By descent, yes. The coefficient of relationship is twice as large, 1/16 against 1/32, and the civil count of degrees puts a first cousin once removed in the fifth degree and a second cousin in the sixth. The traditional canon-law count, which uses only the longer of the two lines, puts both in the third degree, since in both cases the longer line is three generations long.
What is my parent's second cousin to me?
Your second cousin once removed. Your parent and their second cousin are both 3 generations below a shared couple, and you are 4. The smaller count gives "second" and the difference gives "once removed".
Are my second cousin's parents my first cousins?
Neither is your first cousin. The parent through whom your second cousin descends from your shared great-grandparents is your parent's first cousin, so your first cousin once removed. The other parent, who married into the family, is not related to you by descent at all, unless the tree loops back on itself somewhere.
What do my children call my first cousin?
Their first cousin once removed, the same name your first cousin uses for them. Your first cousin's children and your own children are second cousins.
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Sources and further reading
- Bettinger, Blaine T. 2020. The Shared cM Project, Version 4.0 (March 2020). The Genetic Genealogist. https://thegeneticgenealogist.com/wp-content/uploads/2020/03/Shared-cM-Project-Version-4.pdf
- Perl, Jonny, and Blaine T. Bettinger. 2020. Shared cM Project tool, version 4 data. DNA Painter. https://dnapainter.com/tools/sharedcmv4
- Wright, Sewall. 1922. "Coefficients of inbreeding and relationship." The American Naturalist 56 (645): 330-338. https://doi.org/10.1086/279872
- Bettinger, Blaine T. 2019. The Family Tree Guide to DNA Testing and Genetic Genealogy. Family Tree Books.