Match research
400 cM shared DNA: is this a first cousin?
Why 400 cM can fit several cousin and half-family paths, and the evidence that makes a first-cousin hypothesis stronger or weaker.
Updated 2026-09-13 · 6 min read · Genealogy & education only
Why a low first-cousin total needs context
First cousins do not inherit a fixed amount from their shared grandparents. A lower total is still possible, which is why 400 cM cannot rule first cousin out. But it places the result in a busy part of the range chart.
The most useful next question is whether the match belongs to your generation. If so, build out your grandparents’ descendants. If not, sketch removed-cousin and avuncular paths before deciding the tree is missing a person.
Use the tree to test, not to persuade
A helpful tree hypothesis predicts more than one thing. It predicts which known relatives should match, roughly how old the person may be, and which ancestors should repeat in the cluster. If those predictions fail, revise the hypothesis.
That habit is what keeps a DNA match from becoming a story with only one supporting fact.
What else could fit?
| Possibility | Why it can look similar | What to check |
|---|---|---|
| First cousin | The lower range can reach 400 cM. | Same generation and two-grandparent match cluster. |
| First cousin once removed | Its upper range overlaps lower first-cousin totals. | One-generation difference. |
| Half first cousin | A half line can sit in this territory. | One shared grandparent rather than two. |
What to do next
- Write a tree path for each of the three closest candidates.
- Compare those paths to the age and shared-match pattern.
- Use records to look for the ancestor pair that all shared matches can explain.
Questions people ask
Can first cousins share only 400 cM?
Yes. It is low, but possible.
Does 400 cM mean half cousin?
It can fit a half-cousin path, but the total alone cannot establish that.
How can I rule out first cousin?
A clear generation mismatch or a shared-match cluster on an incompatible branch can rule it down.
Published September 14, 2026 · Reviewed against DNADojo dataset v1.0.0-simulated · Sources and limitations