The Hidden Clues in Small DNA Matches: When to Pay Attention, When to Ignore
When you first open your DNA match list, it can feel like you’ve just hit a jackpot. All of these cousin matches must mean that you can figure out the relationships at hand. You’re tempted to click on a 3rd cousin’s profile and send them the message that most DNA testers have received at least once: “Hi, how are we related?”
But not all DNA matches are equally useful. Small DNA matches, usually under 20 centimorgans (cM), can be tricky. Some are false positives. Others connect through such distant ancestors that they don’t move your research forward. But sometimes, the smallest match in your list is the one that cracks a brick wall.
This guide will help you understand when to take small matches seriously, and when to set them aside for later.
If working with DNA matches is new to you, check out my Understanding Your DNA Matches blog series.
What Do We Mean by “Small DNA Match”?
A DNA match is simply another tester who shares segments of DNA with you. The amount of DNA you share is measured in centimorgans (cM). The higher the cM, the closer your likely relationship.
Close family: hundreds to thousands of cM
3rd cousins: 50-125 cM
Small matches: usually under 20 cM
The challenge is that while a 300 cM match is basically guaranteed to be real, a 10 cM match might be a coincidence for one of several reasons. For example, DNA testing companies disagree on small match thresholds:
AncestryDNA only shows matches 6 cM and above, and will only show shared matches over 20cM.
MyHeritage shows matches down to 8 cM, but requires at least one longer segment for reliability.
Why Small Matches Can Be Misleading
Chasing small matches without context is one of the fastest ways to burn out in DNA research. They’re tricky for a few reasons:
False positives: Some DNA segments are identical by state (IBS), which is a coincidence of shared population DNA (i.e. very small amounts of DNA seen across most people from a certain population,) not a real inherited segment (DNA inherited from a common ancestor.)
Endogamy: In populations like French Canadian, Acadian, Mennonite, and Ashkenazi Jewish, intermarriage creates an inflated number of shared small segments with distant cousins.
Distant ancestors: Even if a match is valid, it may trace back 6+ generations — not very helpful if your focus is more recent brick walls.
When to Pay Attention to Small Matches
That doesn’t mean you should ignore them completely. Sometimes, the smallest matches are the missing piece in your research puzzle.
Clustered with bigger matches: If a 12 cM match also shares DNA with your 50 cM matches on your maternal grandfather’s side, that’s likely indicative of a legitimate match
Surname or location overlap: If the match’s family tree shows ancestors in the same county or parish as yours, it’s worth noting but proceed with caution if the region is a pile-up area (e.g. rural Quebec)
How A Small Match Broke a Brick Wall in My Family Tree
I had traced my mother’s paternal line back to my 3rd-great-grandfather, Robert Clarke, born in Jamaica in the 1850s. I already knew from family stories that Robert was given his mother’s last name, and allegedly his father’s last name started with L. Other research proved he was born to a woman who had been enslaved on a plantation, and I suspected one of the men from the family which owned the plantation had been the father. But the paper trail seemed to stop there.
One day a small match at 10cM showed up, also matching to the cluster of cousins I had already identified as descending from Robert Clarke. But she didn’t descend from any Clarkes—she descended from the Lindo family, the family that owned the plantation. Bingo.
Her tree was well researched and I could confirm that she and I both descend from Louis Lindo. My line is through his illegitimate son Robert, whereas her line is through his legitimate son, Ernest Lindo, making our ancestors half-brothers.
This Ancestry ThruLines chart maps out the relationship. The other 10 DNA matches who descend from Robert Clarke helped me to verify the relationship through triangulation.
When to Ignore (At Least for Now)
Your time is valuable, and chasing every 6–10 cM match is honestly a waste of time. Here are the cases where it’s safe to set them aside:
Matches under 10 cM with no tree and no shared matches.
Matches that don’t triangulate (line up with your other known matches).
Matches from populations with heavy endogamy. If you have a large number of shared matches with someone (50+) you’ve possibly found a pile-up group rather than a cousin cluster.
Practical Tips for Handling Small Matches
Instead of totally ignoring them or endlessly chasing them, try a system:
Set thresholds: Start with matches 20 cM and above. Only dip lower when you’re stuck on a specific research problem.
Use shared matches: If a small match clusters with larger, known matches, flag it and try to build a sourced tree to place the match.
Color-code and tag: On AncestryDNA and MyHeritage, use the tagging tools to group small matches by family line (check out my blog post How to Understand Your AncestryDNA Matches – Part Two: Sorting, Relationships, and Tree Comparisons to learn how to do this).
Keep notes: Even a quick “possible connection to Smith family in Ontario” can save you time later.
Final Thoughts
Small DNA matches are the noisy background of your match list. Most won’t help you, at least without context. But with the right strategy, you’ll recognize when one deserves a second look.
The key is balance. Don’t let tiny matches distract you, but don’t dismiss them entirely either. One of them might hold the clue that breaks down a tricky brick wall.
If you’re overwhelmed by your DNA match list and don’t know where to start, my DNA Match Clarity Session can help. I’ll analyze your top matches, create a visual relationship map, and give you clear next steps.
There are lots of practical ways I’ve used genetic genealogy to learn more about my ancestors and solve family mysteries. Check out my Ancestor Spotlights to see the power of DNA in action.
