4th Cousin: What It Means, the DNA, and Whether to Bother
A 4th cousin is someone who shares a set of great-great-great-grandparents with you. You are both five generations below that couple, which puts you on the same rung of the family tree with four generations of separation between your lines. Fourth cousins share roughly 0.195% of their DNA on average, about 35 centimorgans, and a large share of genuine fourth cousin pairs share no detectable DNA at all.
That last sentence is the reason this relationship confuses people. Your DNA test hands you thousands of matches labeled around this level, most of them will never lead anywhere, and a substantial number of your real fourth cousins are missing from the list entirely. Both things are true at once, and understanding why is what turns a useless wall of names into something you can actually work with.
Who Your 4th Cousin Is
A fourth cousin descends from your great-great-great-grandparents, also written as your third-great-grandparents, through a different line than yours.
Walk it out concretely. Your third-great-grandparents had children. One of those children is your second-great-grandparent. A different one is your fourth cousin's second-great-grandparent. Follow both lines down through great-grandparent, grandparent, and parent, and you and your fourth cousin arrive at the bottom together.
Another way to say the same thing, using the shortcut that works at every level: your parents' third cousins' children are your fourth cousins. The cousin number goes up by exactly one with each generation, as long as nobody is skipping a rung.
Here is the ladder in full:
| Relationship | Shared ancestor | Generations up |
|---|---|---|
| Sibling | Parents | 1 |
| First cousin | Grandparents | 2 |
| Second cousin | Great-grandparents | 3 |
| Third cousin | Great-great-grandparents | 4 |
| Fourth cousin | Great-great-great-grandparents | 5 |
| Fifth cousin | 4x-great-grandparents | 6 |
In practical terms, a fourth cousin's most recent shared ancestors with you were probably born somewhere in the early to mid nineteenth century, though that shifts a lot depending on how young or old people in your lines were when they had children. Twenty-five to thirty years per generation is the usual working assumption, and five generations back from a person born today lands roughly in the 1830s to 1870s.
If you have a name and want the exact term rather than doing the arithmetic, the free relationship calculator at cousinchart.com will work it out for you.
How Much DNA Fourth Cousins Share
Fourth cousins share about 0.195% of their DNA on average, roughly 35 centimorgans. Documented pairs range from 0 cM up to around 139 cM.
The most important number in that sentence is the zero.
| Relationship | Avg. shared DNA | Avg. shared cM | Observed range (cM) | Roughly how often detected |
|---|---|---|---|---|
| Second cousin | ~3.125% | ~229 | 41–592 | Essentially always |
| Third cousin | ~0.78% | ~73 | 0–234 | Around 90% |
| Fourth cousin | ~0.195% | ~35 | 0–139 | Roughly half |
| Fifth cousin | ~0.05% | ~17 | 0–117 | A small minority |
| Sixth cousin | ~0.01% | ~4 | 0–71 | Rarely |
The observed ranges come from the Shared cM Project, Blaine Bettinger's crowdsourced dataset of tens of thousands of documented relationships. Every figure above is an average across many pairs, and any individual pair can land far from it.
On detection rates specifically, be careful with any source that gives you a single confident number. Commonly cited estimates for fourth cousins cluster around half, with published figures running from roughly 45% to around 60% depending on the testing company, its matching threshold, and how the study defined a match. Treat "roughly half" as the honest working figure.
Why the zeros happen. You inherit exactly 50% of your DNA from each parent, and that is the only fixed step in the entire chain. Everything after that is recombination, a shuffling process that runs fresh in every generation. Each ancestor's contribution gets cut and reshuffled on the way down, and by five generations back the average contribution from any one third-great-grandparent is roughly 1.5% of your genome, spread across a small number of segments. Some of those ancestors contribute nothing detectable at all. If a shared ancestor's DNA dropped out of your line, or out of your cousin's line, or both, the two of you share a documented relationship and no measurable DNA.
This is not a testing failure. It is the expected behavior of the system.
Why 4th Cousins Flood Your Match List
You have vastly more fourth cousins than close relatives, because the number of relatives at each degree grows explosively as you move outward. That is why most of a typical match list sits at this level.
The arithmetic is stark. You have four grandparents, eight great-grandparents, sixteen great-great-grandparents, and thirty-two third-great-grandparents, which is sixteen ancestral couples. Every one of those couples has descendants spreading down five generations through however many children they had. Multiply sixteen couples by five generations of branching and the count of living fourth cousins runs into the hundreds and frequently into the thousands.
Then subtract, because only about half of them are detectable, and only a fraction of those have tested. What is left is still enough to dominate your results.
A few things this explains:
- The "fourth cousin or closer" bucket. Testing companies group distant matches into broad categories, and most of your list will land in the widest of them. That label is a sorting bin, not a conclusion about any individual person.
- The estimate is a range, not an identification. A match reported as fourth cousin could genuinely be a third cousin twice removed, a fifth cousin, a half fourth cousin, or a relationship through a line you have never documented. The centimorgan value alone cannot tell you.
- Most of them will not respond. Distant matches typically have less invested in genealogy and are far less likely to reply to a message. Most researchers find only a minority of outreach attempts get answered, and that rate drops as the relationship gets more distant.
The Small Segment Problem
Below about 20 centimorgans, a reported match is unreliable enough that most experienced genealogists will not build conclusions on it.
This matters enormously for fourth cousins, whose average sharing of 35 cM sits uncomfortably close to that line and whose real values frequently fall below it.
Small segments go wrong in two distinct ways:
False positives. Consumer tests read your DNA at many individual positions but cannot always tell which parent each reading came from. That ambiguity lets the matching algorithm stitch together a segment that looks continuous but is actually pieced from both of your parents' contributions. The segment does not exist as an inherited unit. Genealogists call these pseudosegments, and they are more common at small sizes.
True but ancient. Some small shared segments are genuinely inherited, just not from the ancestor you are hoping for. They may descend from a common ancestor many generations further back than any tree you will ever build, in which case the match is real and completely unusable for identifying a connection.
The practical guidance most researchers converge on:
- Treat matches under about 20 cM as background noise unless something else corroborates them.
- Give more weight to a single larger segment than to several small ones adding up to the same total.
- Never build a tree conclusion on one small match. Look for several independent descendants of the same ancestral couple pointing the same direction.
Endogamy Changes All of These Numbers
If your ancestry traces to a population where people married within a relatively closed community over many generations, every distance estimate in your results will read closer than it truly is.
This affects a lot of people. Communities where this pattern is well documented include Ashkenazi Jewish, Acadian and Cajun, Amish and Mennonite, some Polynesian and Pacific Islander populations, colonial New England lines, and various island and isolated rural communities worldwide.
The mechanism is simple. When the same families intermarry across generations, two people are related through many different paths at once rather than one. All those small contributions add up, so a pair whose closest documented relationship is fourth cousin may share DNA in the range a second cousin normally would.
What this means in practice:
- Relationship predictions from the testing company will systematically overstate closeness.
- Shared cM values cannot be converted to a relationship using the standard tables.
- Match lists are much longer, sometimes many times longer, than for someone without endogamy.
- Small segments are even less trustworthy than usual, because there are so many more of them.
If this describes your ancestry, focus on the largest matches, lean much harder on documentary research, and consider tools that let you look at specific shared segments rather than totals.
Are 4th Cousin Matches Worth Contacting?
Individually, usually not. As a group filtered for specific signals, absolutely yes.
The mistake most people make is working down the match list one name at a time in whatever order the site presents them. That approach burns hours and produces very little, because the great majority of distant matches carry no information you can use.
A better approach is triage. Contact a fourth cousin match when it clears a filter:
Green flags, worth reaching out:
- They share matches with several relatives you have already identified on one specific line
- They have a public tree going back five or more generations
- Their tree shows a surname or a place you have been stuck on
- They cluster with a group of matches you already suspect belongs to a particular ancestral couple
- The shared segment is unusually large for the relationship, well above 40 or 50 cM
Red flags, skip them:
- No tree, no shared matches you recognize, no location listed
- The total sharing is under about 20 cM with no corroboration
- You already have that line documented thoroughly and are not trying to prove anything
- Your ancestry is endogamous and the match shows nothing beyond a typical background level
When you do reach out, brevity wins. Say who you are, name the specific ancestral couple or surname you think connects you, mention one concrete detail from their tree that prompted the message, and ask one clear question. Long messages explaining your entire research history get ignored. So do messages that just say "we're a match, how are we related?"
The real payoff at this distance is not any single cousin. It is confirmation through numbers. If six independent people who each descend from a different child of the same third-great-grandparent couple all match you, that is genuinely strong evidence for that couple being your ancestors, far stronger than any one match could ever be. Fourth cousins are most valuable in aggregate, as a cluster that either supports or undermines a hypothesis you already have.
For the level just above this, where matches are more reliable and detection is far more likely, see our 3rd cousin guide. If you are still sorting out how the numbering and "removed" system works in the first place, the cousin definition piece covers the fundamentals, and the 1st cousin 1x removed guide walks through a single relationship in depth.
Fourth Cousins Once and Twice Removed
A fourth cousin once removed is one generation off from your fourth cousin. That is either your fourth cousin's child, or your parent's fourth cousin.
Expected sharing drops with each removal, and at this distance it drops below what most tests can reliably see:
| Relationship | Avg. shared cM | Observed range (cM) |
|---|---|---|
| Fourth cousin | ~35 | 0–139 |
| Fourth cousin once removed | ~28 | 0–126 |
| Fourth cousin twice removed | ~22 | 0–109 |
| Fifth cousin | ~17 | 0–117 |
Practically, everything in that table lives in the zone where a match may or may not appear, and where any match that does appear is hard to distinguish from noise. Genealogists working at this level rely mostly on documents and use DNA as a supporting check rather than a primary tool.
Frequently Asked Questions
How much DNA do fourth cousins share?
About 0.195% on average, roughly 35 centimorgans. Documented pairs in the Shared cM Project range from 0 cM to around 139 cM, which is a very wide spread. That variability is expected, because DNA inheritance is randomized at every step past parent to child.
Can fourth cousins share no DNA at all?
Yes, and it is common. Roughly half of genuine fourth cousin pairs share no detectable DNA on an autosomal test, with commonly cited estimates running from about 45% to 60% depending on the company and its matching threshold. A missing match at this distance says nothing at all about whether your paper trail is correct.
How many 4th cousins does a person have?
Hundreds to a few thousand living fourth cousins is the usual range, though it varies enormously with family size and how recently your ancestors had large families. You descend from sixteen third-great-grandparent couples, and each of those couples has five generations of descendants, which is where the large number comes from.
Is a fourth cousin actually related to you?
Yes, genealogically. You share a documented set of great-great-great-grandparents, which is a real relationship. Genetically the connection is faint, under a quarter of one percent on average, and often undetectable. So the relationship is genuine and the biological link is close to negligible.
Why does my DNA test show so many fourth cousin matches?
Because you have far more relatives at that distance than at any closer one, and because the label covers a broad centimorgan band that several different real relationships fall into. It is a sorting category rather than an identification. Shared matches and documented trees are what narrow it down.
Should I message my fourth cousin matches?
Only the ones that clear a filter. Reach out when they share matches with relatives you have already identified, when they have a tree going back five or more generations, or when they cluster around a line where your research is stuck. Working through the list one name at a time is not a good use of your time, and most distant matches never reply.
Can 4th cousins marry?
Yes. Cousin marriage restrictions in US law are aimed at first cousins and closer relationships, so fourth cousins are not affected anywhere. At under a quarter of one percent shared DNA on average, the genetic considerations are indistinguishable from those for an unrelated couple.