What Is a Second Cousin? One Name, Two Relatives

Even with the outliers trimmed, the 4,172 pairs of second cousins in Blaine Bettinger’s Shared cM Project shared anywhere from 41 to 592 centimorgans (cM) of DNA. A grandparent’s first cousin averages 221 cM against a second cousin’s 229, so DNA alone cannot always tell which one a match is. A family tree can, as long as each relative gets the right name.

What is a second cousin?

A second cousin is a relative in your own generation who shares a set of great-grandparents with you, but no grandparents. In practice that means the child of one of your parents’ first cousins. The children of two first cousins are second cousins to each other.

Relative	Name on a family tree	Closest shared ancestors	Average shared DNA (cM)
Parent’s sibling’s child	First cousin	Your grandparents	866
First cousin’s child	First cousin once removed	Your grandparents	433
Parent’s first cousin	First cousin once removed	Your great-grandparents	433
Parent’s first cousin’s child	Second cousin	Your great-grandparents	229
Second cousin’s child	Second cousin once removed	Your great-grandparents	122
Grandparent’s first cousin	First cousin twice removed	Your great-great-grandparents	221
Parent’s second cousin	Second cousin once removed	Your great-great-grandparents	122
Parent’s second cousin’s child	Third cousin	Your great-great-grandparents	73

Averages are from version 4.0 of the Shared cM Project, March 2020. The third-cousin average counts only pairs who shared detectable DNA.

Two counts name every cousin

Count the generations from each person up to your shared ancestors. Wikipedia’s cousin article gives the rule: the cousin number is the smaller count minus one, and the removal is the difference between the two counts. You and a second cousin each sit three generations below your shared great-grandparents: three minus one makes you second cousins, and a difference of zero means no removal.

Your parent’s first cousin sits two generations below those same great-grandparents. The smaller count, two, makes the relationship first cousin, and the gap of one makes it once removed. Removal counts generations, so a parent’s first cousin can be around your age.

FamilySearch offers a shortcut from the ancestor’s end: count the greats in the shared ancestor’s title, as seen by whichever of you is closer to that ancestor, and add one.

Your cousin’s child and your parent’s cousin are first cousins once removed

The American Heritage Dictionary gives second cousin two senses: “a child of a first cousin of one’s parent” and “a child of one’s first cousin; a first cousin once removed”. Half-mast and half-staff can both be correct because one is British and the other American. The two senses of second cousin name two people on one family tree, so genealogy keeps the first and treats the second as loose usage.

The same dictionary defines removed with this very relative: “A first cousin’s child is one’s first cousin once removed.” A family tree marks a generation gap with removed, never with a higher cousin number.

The Michigan Tech archives give the standard case: “Bob is the child of Alice’s first cousin. This makes Bob and Alice first cousins once removed.” The label runs both ways: a parent’s first cousin is a first cousin once removed too, one generation up instead of down.

The mix-up may reach DNA research too. The Shared cM Project warns that some submitted relationships “were almost certainly entered incorrectly, which might be due to misunderstandings of ‘removed’ relationships in genealogy.”

A DNA test cannot always tell a second cousin from a grandparent’s first cousin

A centimorgan is a unit of genetic recombination worth roughly 1 million base pairs of human genome on average, according to the genetics glossary at genome.gov. The Shared cM Project pooled nearly 60,000 known relationships and trimmed the top and bottom half-percent of submissions for each.

Two other relationships sit almost entirely inside the second-cousin range. First cousins twice removed, such as a grandparent’s first cousin, averaged 221 cM, and half first cousins once removed averaged 224 cM. The project puts all three in one grouping and says its method “cannot definitely identify a specific relationship.”

One rung further out, among the children of two second cousins, some pairs share nothing from the ancestors that make them cousins. Simulations by Graham Coop, a UC Davis professor of evolution and ecology, found that roughly 1 in 100 pairs of third cousins share no blocks of their genome from that pair of ancestors, as do about 1 in 4 fourth cousins.

The church’s fourth degree moved from third cousins to first

Church law has counted cousins two ways. The Roman civil method adds the generations on both sides of the family, so first cousins stand in the fourth degree. The church’s own method, which the 1908 Catholic Encyclopedia says became papal law under Alexander II in the 11th century, counts one side only and puts first cousins in the second degree.

In 1215 the Fourth Lateran Council restricted the kinship bar on marriage to the fourth degree of that count, which reaches third cousins. Third cousins needed a dispensation to marry until the 1917 Code of Canon Law took effect in 1918. That Code kept the one-sided count in canon 96 but cut the bar to the third degree in canon 1076, which still reached second cousins.

The current 1983 Code of Canon Law counts the Roman way. Canon 108 finds “as many degrees as there are persons in both the lines together, not counting the common ancestor”, and canon 1091 makes marriage invalid “up to and including the fourth degree”. By that count the fourth degree now stops at first cousins: the phrase came back with a new count under it, much as the moment outlived the sundial hours that once made it 90 seconds.

“The fourth degree” drew the Catholic marriage line in 1215 and draws it again in the 1983 Code. Counted along one side of the family it reached third cousins; counted across both sides, it stops at first cousins.

Second cousins stand in the sixth degree by the Roman count, and genetics draws its line at them. A 2010 paper in PNAS by Bittles and Black measures consanguineous marriage by couples “related as second cousins or closer”, and estimates that those couples and their children make up about 10.4 percent of the world’s population.

Cousin’s Latin root named one kind of cousin

The word’s Latin ancestor could be narrower than any of this. Etymonline traces cousin to Latin consobrinus, originally “mother’s sister’s son”, one of several Latin words for particular cousins. English used it loosely from the start: through Middle English, cousin “also was used of grandchildren, godchildren, etc.”

So when someone at a family reunion introduces a cousin’s child as a second cousin, the dictionary is on their side and the family tree is not. The tree asks for two counts and a subtraction, and it answers first cousin once removed. Strictly FYI.