First Cousins Once Removed

First Cousins Once Removed

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Happy New Year all, and welcome to 2010, or, as my friend Professor Orbifold prefers it, MMX. I hope your festive holiday season was as festive and enjoyable as mine.

The extended Lippert family continues to grow; this year at the annual Boxing Day party we needed two overflow tables for dinner instead of the usual one. The older cousins are now all married, some of them are bringing new babies, and some of the younger cousins are starting to bring boyfriends and girlfriends. I pointed out that the youngest person at the table was my brand-new “first cousin once removed”, which touched off a long discussion of how exactly one computes degree of cousin-hood and removed-ness. As a public service, here’s how it works.

Recursive explanation for computer programmers and mathematicians:

Base case: if X and Y are zeroth cousins no times removed then X and Y are siblings.

Recursive case 1: if n > 0 and X and Y are nth cousins no times removed then X.Parent and Y.Parent are (n-1)th cousins no times removed.

Recursive case 2: if m > 0 and X and Y are nth cousins m times removed then WOLOG assume that X is of a later generation than Y. In this case, X.Parent and Y are nth cousins (m-1) times removed.

Explanation for normal people:

Take two different people who have a common ancestor but who are not related by direct succession (that is, neither is the mother, father, grandmother, and so on, of the other):

Degree of cousin-hood is the minimum of the numbers of generations back that you have to go to find the nearest common ancestor, minus one.

Removed-ness is the absolute difference between the numbers of generations back you have to go to find the nearest common ancestor.

For example, consider this fragment of a family tree:

       /     \
   Laura     Bob
     |        |
   John     Helen
     |        |
  Xerxes   Melinda

Take Helen and Xerxes for example. Their nearest common ancestor is Mary. Helen has to go back two generations to get to Mary. Xerxes has to go back three. The minimum of two and three, minus one, is one. The absolute difference of two and three is one. So Helen and Xerxes are first cousins, once removed. So are John and Melinda.

“Zeroth cousins” are not called cousins, but have special names for the relationship. “Zeroth cousins no times removed” are of course brothers and sisters. “Zeroth cousins once removed” are uncles, aunts, nieces and nephews. “Zeroth cousins twice removed” are great-uncles, great-aunts, great-nieces and great-nephews. (Or sometimes grand-uncles, and so on.)

Summing up this family tree:

Mary is Laura and Bob’s mother, John and Helen’s grandmother, and Xerxes and Melinda’s great-grandmother.

Laura is Mary’s daughter, Bob’s sister, John’s mother, Helen’s aunt,  Xerxes’ grandmother, and Melinda’s great-aunt.

Bob is Mary’s son, Laura’s brother, John’s uncle, Helen’s father, Xerxes’ great-uncle, and Melinda’s grandfather.

John is Mary’s grandson, Laura’s son, Bob’s nephew, Helen’s first cousin (no times removed), Xerxes’ father, and Melinda’s first cousin once removed.

Helen is Mary’s granddaughter, Laura’s niece, Bob’s daughter, John’s first cousin (no times removed), Xerxes’ first cousin once removed, and Melinda’s mother.

Xerxes is Mary’s great-grandson, Laura’s grandson, Bob’s great-nephew, John’s son, Helen’s first cousin once removed, and Melinda’s second cousin (no times removed).

Melinda is Mary’s great-granddaughter, Laura’s great-niece, Bob’s granddaughter, John’s first cousin once removed, Helen’s daughter, and Xerxes’ second cousin (no times removed).

Of course, this is all perfectly straightforward. If, say, your mother dies and your father marries her sister and has more children, then working out who is whose cousin can get rather trickier. And heaven only knows what happens if you are your own grandfather.

Incidentally, it is illegal in the state of Washington for a man to marry his widow’s sister. Anyone care to guess why?


  • "John would be Melinda's second uncle, and Melinda would be John's second niece."

    But if they're the same age, it's kind of silly to call them that anyway, isn't it?

  • So, dead people can't marry in Washington. Is this called animism?

  • >it is illegal in the state of Washington for a man to marry his widow’s sister

    Is it truly illegal, or merely undefined.  :)

  • >It is believed that the dominating physical characteristics in a boy are mostly inherited from father while the dominating characteristics for a girl are inherited from mother.

    Believed by Indians, or believed by geneticists? I'm not an expert, but I don't think it works that way.

  • @Joren - yes, but as a theory it certainly explains genitals. :)

  • Deja vu:

  • >it is illegal in the state of Washington for a man to marry his widow’s sister

    Is she still a widow if there is more than one husband and only one dies (not the man under consideration)?

  • This is all just of general interest to most of us, but the folks who really take an interest in this sort of thing are probate lawyers.

    And I suppose it bears mentioning that in some number of languages, the relations on the maternal and paternal side of the have different terms. And that in English we actually have a relatively poor vocabulary for the relationships among members of an extended family.

  • OK so growing up my family mostly didn't do remove when talking about cousins.  It was more a deal of adding how far up from your side and how far down on the other.  So, in the example John and Melinda are both 3rd cousins.  I really hated this because there was multiple ways to nth cousins if n>1.  While your explanation is better it still doesn't work for me.  I personally don't think of John being related to Melinda in the same way that Melinda is related to John (you basically loss which one is of the "older" generation).  So I've always done a count up from one person to get the nth part and down the other side for the removed-ness, which means that John is Melinda's 2nd cousin not removed and Melinda is John's first cousin once removed.  The flaw in your system is that the relationship from one person to another doesn't keep the generation in tack and you even point this out by saying that a 0 cousin once removed is either a aunt/uncle or a niece/nephew.  It just makes since to me that most people are not related to you in the same way you are related to them unless you are of the same generation (siblings, nth cousins).

  • It's all about what proportion of your genes are in common. The whole reason for needing to describe this kind of relationship is to determine which group of people are closely enough related to make inter-marriage problematic in terms of the likelihood of genetically-caused diseases.

    This brings up an interesting point about social mores. I have a friend who has a known genetic disorder; people with his disorder have a considerably higher chance of heart problems later in life than the rest of the population. This trait is dominant and completely heritable; since his wife does not also have this syndrome, their children are each at a 50% risk of inheriting it.

    Here's my point. No law prevented them from getting married and making the informed choice for themselves whether or not to have children by that union. Nor is it considered socially unacceptable for someone in that position to marry and have children. But in the United States, in many places first cousins are prohibited from marrying by law, and in many places it is considered "gross" to marry your first cousin. And yet the vast majority of first cousin marriages with offspring of the union would present a far, far lower than 50% chance of reinforcing a genetic trait with serious negative health consequences. I find this inconsistency in social mores quite strange. -- Eric

    The term "degree of kindred" describes this symmetric relationship.

    A "once-removed" relationship halves the degree of kindred. In other words, a first cousin once removed has the same degree of kindred to you as a second cousin.

    In this scheme of things, half brother would be the same degree of kindred as first cousin. Of course, you can only share kinship via descent, and not via marriage. So the half brother of your half sister is no kin at all unless he's your brother. (I think... at this point my head starts to hurt.)

    There is no special terminology for discussing degree of kindred, other than to say nth-cousin. In this sense, Justin's family are also correct. The degree of kindred between first cousins once removed, is second cousin. This simply means that the "removed" terminology expresses more than simply degree of kindred.  

  • I went to High School with a girl I knew to be the granddaughter of my grandfather's first cousin (I knew this from my grandfather, of course). That made her my third cousin (none removed). She was entirely uninterested in this fact, though, and was apparently annoyed whenever I brought it up, but I digress.

    Anyway, in doing geneoligical research on my family, I discovered that my great-grandfather and his brother, the great-grandfather of my third cousin, married two women who were themselves first cousins. So this girl (well woman now) is not only my third cousin, but also my fourth cousin. Our children are both fourth and fifth cousins.

    On your comment about mores, also through my geneological research, I have discovered that among the pioneering families, as the midwest was settled in the 18th and 19th centuries, it was *extremely* common for first cousins to marry. This stands to reason, given the sparse population. I seem to be no worse for the wear (though I'm sure some my argue this point...).

  • See FAQ page at for a simple explanation of cousin relationships.

    Kate Middleton'­s Other Famous Relatives


    and her Ancestors-­at-glance fan chart at the FamilyFore­ website at:


  • Very interesting.  I have a simple question than everyone brushes off.    Why are the brothers or sisters of your grandparents not called grand-uncles or grand -aunts?  I know I have heard the terms grand-neices and grand-nephews.

    The only answer I have ever got was that grand- uncle and grand-aunt are correct but for some reason "great" is applied to all of the siblings of your direct ancestors.   My opinion: Perhaps people wanted to keep a uniform term for collateral relatives   Those answers are not good enough for me.  Do you have a better one?

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