Half-Cousins: Are they Really Just Cousins?

Recently, I participated in a LinkedIn discussion about a perennial genealogy problem:  How do you calculate degrees of cousins and “removes”?  Most people are, understandably, lost when you refer to a 3rd cousin twice removed.  I offered my two cents on how the system worked.  Then a new topic came up.  What do you call the relationship between children of half-siblings, children with only one common parent?  “Half-cousins”? Can you have “Half 3rd cousins twice removed”?  Is this even a real term?  Many people commented on this and apparently this discussion has been around a while.  So, I figured I’d chime in.
Before beginning to tackle the half-cousin question, let’s get our terminology straight.  Degrees and removes are confusing to many.  Here’s what I posted to LinkedIn on the topic How does one determine cousins?:

David Lynch, PMP, APM APMP •It's actually pretty simple. Take two people with a common ancestor. Count back from each person the number of steps to reach that common ancestor. Now, the "degree" or "cousin" is determined by the smaller number of steps. One step is sibling or aunt/uncle. Two is first cousins. Three is second cousins. etc. Now consider the person with the larger number of steps. The difference between the larger and smaller steps are the "removes". Removes are essentially saying that the two people are from different generations.

So, suppose we take Mary and John. Their common ancestor, Mark, is 4 steps from Mary (Mary's great-great grandfather) and 3 steps from John (John's great grandfather). Since John has the fewest steps (3) that sets the degree. Deduct one to determine the cousin. They are second cousins.

Now for the removes. Mary is 4 steps away, John is 3 steps away. 4-3=1 so they are once removed. 2nd cousins, 1x removed.

Now consider another one. Lets say that we want to determine the relationship between Bob and someone in his family tree who died many years ago, Agnes. Say the common ancestor is 9 generations back from Bob and only 3 generations back from Agnes. The shortest hop is from Agnes, 3 steps. So they're second cousins again. But this time they are 6x removed (9-3=6). See how easy!!

It trips most everyone up!

Now that we all understand the classification of cousins, we turn to the “half” problem. A number of people have tried to answer this question from many perspectives.  None other than the esteemed Elizabeth Shown Mills has tackled the thorny problem, writing in the Association of Professional Genealogists Quarterly, June 2005 in Navigating the Kinship Maze. I recommend this article highly. She outlines the problem from various perspectives, although she doesn’t really take a position. So, what do those with opinions say?
Geneticist:  Absolutely!
A member of the LinkedIn community posted that she consulted someone at Tech Geneticist at Stanford University who answered:

A: the children of half siblings are not full first cousins. The children of siblings share 1/8 of their DNA while the children of half siblings share 1/16.

This is based on the standard models of Autosomal DNA testing.  If you want to see all the tables, look here.
Genealogist:  Absolutely Not!
Dick Eastman wrote a blog entry in September 2010 entitled There is No Such Thing as a Half-Cousin!  In it he cites Black’s Law Dictionary as stating that second cousins are:

"Persons who are related to each other by descending from the same
great-grandfather or great-grandmother."

Using this definition, half-cousins don’t, in fact, exist.  At least according to Black’s.
I’m not as crazy about Eastman’s answer, in that it presupposes that we accept Black’s as the authority. Black’s Law Dictionary is descriptive of how kinship is handled legally in America, but it doesn’t purport to be an authority on existential issues. I’m not thrilled with Stanford University’s answer, in that it presupposes that our definition of cousins is based on similarity of DNA.  We’ve know about cousins a lot longer than we’ve known about DNA.
My answer:  It depends!
In thinking about the question, I think it’s not well-posed.  I don’t think the question is “do half cousins” exist.  I think the question is really: “Are half (first, second, third) cousins still (first, second, third) cousins, or are they something different?  That is, are all half cousins still cousins?  Are two people, grandchildren of a particular person, cousins if they had different grandmothers?
To determine this we need to understand a definitive definition of “cousin”, of whatever degree.  I consulted my copy of the Oxford English Dictionary (OED) and saw that the term “second cousin” was used at least as far back as 1660 in English.  While the term sometimes meant the relationship between a person and their first cousin’s child, most of the time it meant the same thing that we mean today, the relationship between the children of first cousins.  According to the OED:

Cousin-3:  n. First cousins, second cousins etc. Expressing the relationship of persons descended the same number of steps in distinct lines from a common ancestor.

According to the OED, it “a” common ancestor, not “common couple”.  So if a man had two wives and the children of those two marriages had children, those children would be first cousins.  No mention is made of half cousins.  All it takes is a single common ancestor.
Now, I tend to consider the OED as pretty authoritative, but an appeal to any authority isn’t really good critical thinking.  I thought about where the terms came from and what use they were.  That is, why did we need a way to quantify such distant relations and discriminate between them (first or second cousins, removes, etc.).  Two reasons came to mind.  Legally, inheritance was apportioned according to closeness of kin, so it was important to determine which of two family members was “next of kin”.  For that, it was necessary to quantify the “closeness”.  Secondly, it was important to determine the “degree of consanguinity” to determine if marriages were allowed, both by the Church and local law, particularly for the nobility.  The Catholic Church put strict bans on marriage of people who were too closely related (of course, strictness was often a matter of price).  Thinkers spent a great deal of time establishing what counted as a first, second, third cousin and all the removes.
But in all my research, I haven’t found a single reference to the Church discussing “halves” beyond siblings.  All of their writing, the Papal Encyclicals, the works of theologians, is on the degrees and removes of cousins.  It is as if the Church simply didn’t think of the need to deal with half-sibling’s progeny.
Knowing now what I know about life in the pre 20th century, it was very common for men and women to be married multiple times with children from each marriage.  In my family from the Virgin Islands, it was rare to have someone marry only once before 1800.  The nobility was just as bad.  Henry VIII had six wives, after all.  Women often died in childbirth and older husbands often died leaving young widows without support and protection.    I’d say it was more the rule than the exception. The Church would not have simply overlooked this everyday occurrence.
Since the system of degrees and removes was designed to handle property and consanguinity issues in the post-medieval world, it is highly unlikely that it was so flawed in such a fundamental way.  Since the term “half cousin” is not a part of the system used to describe a society that was rife with half siblings, and that system persisted, unchanged and unchallenged, for several centuries, I would suggest that the term simply wasn’t needed.  In this regard, Dick Eastman is right: a cousin is a cousin is a cousin. .
But…
That said, the term “half cousin” does serve a purpose.  It provides a descriptor to the term “cousin”.  It tells us something that “cousin” does not.  In the same way, we can refer to a “second son” or a “female cousin”.  While these terms provide additional information, no one is likely to claim that a “second son” is not a “son” or that a “female cousin” is not a “cousin”.  To paraphrase Eastman, a son is a son.  Of course, these people have the same DNA percentages of the common ancestor.
Does the percentage of shared DNA define the “cousinship”?  What would you do with “double cousins”?  Double cousins are what you get when two brothers marry two sisters, the children are double cousins. According to the math, double cousins are genetically similar to half siblings.  In this case, as you would expect, for family purposes, the two children are still first-cousins, the “double” is a modifier; it adds additional information.  While it is genetically meaningful, it doesn’t affect the family classification.
So my take on this is really that everyone’s right, but not for the stated reasons.
Of course, if two half brothers married two half sisters you would have two half first cousins who were also double first cousins. Then would the double cancel the half and make them just ordinary first cousins?  According to the genetic percentage, it does.