Levels of cousins, also called degrees of cousinship, are fairly tricky to figure out. Especially for people who come from a large family, deciding how to determine first, second, and third cousins, and also what once or twice removed means, can be difficult. Cousins are not based on the relationship of a person's parents to his or her siblings, and they’re not based on marriages, except on an informal basis. Instead, cousinship is based on two people’s common ancestor.
For instance, if a person's mother's sister has kids, he might conclude that these children are his first cousins. This would be correct, but the relationship is not based on the fact that these are the children of the mother's sister. Instead, the children of his aunt are his first cousins because they share a grandparent in common. This is the common ancestor, to whom both individuals claim the same relationship. Note that this doesn't apply for children of the same parents who share the same grandparent. Instead, these children's closest common ancestors are their parents, so they are siblings, not cousins.
When children share the same great grandparent, but not the same grandparent, they are considered second cousins. So, if a man has children, and his first cousin, (his aunt's child) has children, then these children will have a great grandparent in common. It gets a little more complicated when considering a cousin's children.
When the common ancestor does not have the same relationship to two people, then the issue of removals comes into. In the case of a person's cousin's children and the person, the common ancestor is their grandparent, but to these children, that person is their great-grandparent. Therefore, his first cousin's children are his first cousins once removed. Removals occur only when the relationship to the ancestor is separated by generation.
Here are some examples of levels of cousins with removals:
- Jane has a grandparent who is the great-grandparent of Joey.
Jane and Joey are first cousins once removed.
- Jane has a grandparent who is the great-great-grandparent of Jim.
They are first cousins twice removed.
- Jane has a grandparent who is the great-great-great-grandparent of John.
They are first cousins three times removed.
Cousinship without removals always mean the common ancestor has the same relationship to each cousin:
- First cousins: two people share a grandparent
- Second cousins: two people share a great-grandparent
- Third cousins: two people share a great-great-grand parent.
It gets considerably more complicated when determining second or third levels of cousins and degree of removal. When someone has a great-grandparent who is someone else's great-great-great-grandparent, they are second cousins once removed. If they have a common ancestor who is one person's great-great-great-grandparent and the other's great-great-great-great-grandparent, they are third cousins once removed. Removal essentially counts generational differences or how many generations two people are apart from the common ancestor.
There is one special cousin relationship called double cousins, which makes people doubly related to each other. This occurs when two sisters marry two brothers. The children of these marriages will share not only a common grandparent, but they will share two sets of grandparents. Therefore, they are double cousins and likely to be closely related, from a genetic standpoint, to each other.
Cousinship here is determined from European and American standards. Levels may be different in other cultures, and the term "cousin" may not even exist in certain world cultures. It can get a little murky figuring out these relationships, and some people essentially avoid the issue and just call any relatives they know of as cousins or second cousins. From a genealogical standpoint, this isn’t quite correct, but still implies a family relationship.