How much DNA do cousins share?
First to fourth cousins, half and double cousins, and DNA test ranges
Matic Broz, PhD9 min
First cousins share 12.5% of their DNA by descent on average, about 850 centimorgans (cM), the unit consumer DNA tests report. Among 3,337 first-cousin pairs submitted to the Shared cM Project, the average was 866 cM and the middle 99% of pairs fell between 396 and 1,397 cM.
Each step further out roughly halves the share. First cousins once removed and half first cousins average 6.25%, second cousins 3.125%, third cousins 0.78% and fourth cousins about 0.2%. Double first cousins, related through both parents, average 25%. From third cousins onward, some pairs share no detectable DNA at all, and those who do match often share more than the expected value.
How much DNA do cousins share?
Cousins share DNA because they descend from the same ancestors, usually a pair of grandparents or great-grandparents. Each parent passes on half of their DNA, so every generation on the path between two cousins halves what reaches them from the shared couple. The expected share is one-eighth for first cousins, who are third-degree relatives, and one thirty-second for second cousins, who are fifth-degree relatives.[1]
These percentages refer to the 22 pairs of autosomes, the non-sex chromosomes, and count only DNA inherited from the shared ancestors. "Once removed" means the two people sit one generation apart, as with a person and their parent's first cousin. Each removal halves the expected share again, so a first cousin once removed averages the same 6.25% as a half first cousin, who shares only one grandparent.[1]
DNA tests measure shared stretches in centimorgans, a unit of genetic map length rather than physical length. A segment of 100 cM is passed on with one crossover on average, and 100 million base pairs measure about 120 cM.[1] The Shared cM Project, run by genetic genealogist Blaine Bettinger, collected 59,714 totals from test takers who knew their relationship, kept 55,418 for the 48 relationships it analyzed, and trimmed 0.5% from each end of every relationship.[2]
| Relationship | Expected share by descent | Shared cM, average (range) | 23andMe range |
|---|---|---|---|
| Double first cousins | 25% | Not listed | Not listed |
| First cousins | 12.5% (850 cM) | 866 (396 to 1,397) | 8% to 22% |
| Half first cousins | 6.25% (425 cM) | 449 (156 to 979) | 3.2% to 11.5% |
| First cousins once removed | 6.25% (425 cM) | 433 (102 to 980) | 3.2% to 11.5% |
| Second cousins | 3.125% (213 cM) | 229 (41 to 592) | 1.1% to 6.3% |
| First cousins twice removed | 3.125% (213 cM) | 221 (33 to 471) | Not listed |
| Second cousins once removed | 1.56% (106 cM) | 122 (14 to 353) | 0.3% to 3.6% |
| Third cousins | 0.78% (53 cM) | 73 (0 to 234) | 0.06% to 2.2% |
| Fourth cousins | 0.195% (13 cM) | 35 (0 to 139) | 0.01% to 0.92% |
Down to second cousins, the observed averages sit within 8% of the expected values. Further out the gap widens: 15% for second cousins once removed, then 73 cM against 53 cM for third cousins and 35 cM against 13 cM for fourth cousins. The reason comes down to which pairs get counted at all.
How much DNA do first cousins share?
First cousins share a pair of grandparents and average 12.5% of their DNA by descent. The Shared cM Project lists 850 cM as the expected total and found an average of 866 cM across 3,337 pairs, with a standard deviation of 161 cM.[2] 23andMe gives a range of 8% to 22% for the group that includes first cousins, great-grandparents and great-aunts or great-uncles.[4]
The 12.5% follows from counting transmissions. DNA from one grandparent reaches each cousin through two parent-to-child steps, four steps in all, so each grandparent contributes an expected (1/2)4, or 1/16. Two shared grandparents give 1/8.
Every simulated first-cousin pair in a 2026 study by Williams shared at least one segment, and nearly all shared five or more. Even counting only segments of 40 cM or longer, just 0.29% of first cousins had none.[1] A real first cousin always shows up as a match; the total can still land hundreds of centimorgans away from 866.
How much DNA do double first cousins and half first cousins share?
Double first cousins are related through both parents, for example when two brothers marry two sisters. They share all four grandparents instead of two, and their expected share doubles to 25%, the same as half-siblings or an aunt and nephew.[3]
At about 1 in 16 positions in the genome, double first cousins match on both chromosome copies, which ordinary first cousins never do.[3] Centimorgan totals count such a region once, as the figures for full siblings show, so a double first cousin usually reads below twice the first-cousin average. The Shared cM Project collects double cousins separately and leaves them out of its main tables.[2]
Half first cousins share only one grandparent, because their parents are half-siblings. Their expected share is 6.25%, and the Shared cM Project found an average of 449 cM across 691 pairs, close to the 433 cM of first cousins once removed.[2] Totals cannot separate these two relationships. Half relatives share fewer but longer segments than full relatives of the same degree, though, and relationship predictions weigh segment patterns, ages and shared matches alongside the total.[1][4]
How much DNA do second, third and fourth cousins share?
Second cousins share a pair of great-grandparents and average 3.125%, or about 213 cM. The Shared cM Project observed 229 cM across 4,172 pairs, with a range of 41 to 592 cM, and 23andMe gives 3.4% with a range of 1.1% to 6.3%.[2][4] A first cousin twice removed, such as a grandparent's first cousin, sits at the same genetic distance and averages 221 cM.[1][2]
Third cousins share great-great-grandparents and are expected to share 0.78%, about 53 cM. Fourth cousins, one generation further back, are expected to share 0.195%, about 13 cM. The observed averages are 73 cM for third cousins and 35 cM for fourth cousins, and both relationships include pairs with no shared DNA.[2]
The ranges overlap heavily. A total of 400 cM fits a typical first cousin once removed or half first cousin as well as a high-sharing second cousin, and 100 cM fits anything from a second cousin once removed to a fourth cousin. A centimorgan total narrows the possibilities; family trees, ages and matches shared with other relatives decide between them.[4]
Can cousins share no DNA?
Close cousins always share some DNA, but from third cousins onward a pair can share none from their common ancestors. Williams simulated 100,000 pairs for each relationship, modeling crossover interference and the different genetic maps of eggs and sperm. Every first cousin, first cousin once removed and second cousin shared at least one segment, but 0.07% of second cousins once removed and 1.5% of third cousins shared none, and only 32.7% of fifth cousins shared any.[1]
A DNA test misses more than that, because companies ignore short segments to keep false matches down.[2] With a 7 cM minimum, Williams found that only 48.5% of fourth cousins would carry a qualifying segment.[1] A known fourth cousin who does not appear as a match is an ordinary result, not evidence against the family tree.
The same filter explains why distant cousins in Table 1 share more than expected: the Shared cM Project averages count only pairs with shared DNA.[2] Williams found that simulated fourth cousins once removed share 6.5 cM on average when pairs with no segments count as zero, but 12.7 cM among pairs that share any.[1] Endogamy and pedigree collapse, where the same ancestors appear more than once in a family tree, push real totals higher still.[2]
Why do cousins share different amounts of DNA?
The spread comes from how chromosomes are passed on. Before an egg or sperm forms, each chromosome pair exchanges segments in a step called crossing over, so each reproductive cell carries a different mix of a parent's DNA.[5] Crossovers are rare enough that DNA travels in long blocks, about 83 million base pairs on average, and each block is either passed on or not.[1] With so few blocks, chance does not average out, and two first cousins can differ by hundreds of centimorgans.
Eggs also carry about 1.6 times as many crossovers as sperm, so DNA inherited through a mother reaches the next generation in more, shorter pieces than DNA inherited through a father.[1] The sex of each ancestor along the path therefore changes how the shared DNA is cut up, even when the expected total stays the same.
The X chromosome follows stricter rules. A son receives his only X from his mother, so fathers cannot pass X-linked traits to their sons.[6] Two cousins whose fathers are brothers can therefore share X DNA from that side only if both are daughters, since each girl's paternal X came through her father from their shared grandmother. If either cousin is a son, the pair shares no X DNA from those grandparents at all.
Expected sharing halves at each step, real pairs scatter widely around it, and the scatter grows relative to the average as cousins become more distant, until many share nothing. These percentages measure something narrower than overall similarity. Any two people are about 99.9% identical in DNA sequence,[7] the kind of base-by-base comparison a sequence aligner such as MUMmer4 makes, while cousin matching looks only for long segments inherited from the same recent ancestors.
