Ancestry.com has a feature where it will use all available trees to find a common ancestor for a DNA match. I believe that it looks for overlaps in trees. It has done a good job for many of my matches.

There are a few common ancestors that it has not found, even though I copied many people from the match's tree. There should be plenty of overlap in our trees for Ancestry to find the common ancestor.

Does anyone know exactly what the algorithm looks for?

I know that the task runs in the background, sometimes the paths to the common ancestor won't show up for a few days. It has been a lot more than a few days for the cases that I am referring to.

Edit: more background. Here are snippets from the two trees. Yesterday, only the people in red were common to both trees. In my experience this should be enough. I just added the siblings of Helen Beg (b1899) to my tree, we will see if this makes a difference.

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The algorithm is very impressive in some other cases. For one case, it found the common ancestor when the other person had a poor tree, and my tree didn't connect. It used a third tree to connect everything!

  • Interesting question. I can only speculate but surely there must be some sort of prioritisation of routing back from your DNA match. If your match thinks that their Grandad was X but 99 other trees think that the match's GF was Y, then could Ancestry really follow both paths? I suspect not. I do have a feeling that I've seen paths back from a match leap out of the match's tree before reaching the brick wall in that tree. No answers but these are the sorts of issues that I'd ponder.
    – AdrianB38
    May 21, 2022 at 18:27
  • I'm guessing that you have some sort of paper trail to support these ancestor claims, and not just unsourced (or poorly sourced) claims in other people's trees?
    – cleaverkin
    May 24, 2022 at 19:37
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    I'm not sure the algorithm is all that good to be honest. My impression is that it is dominated by the number of trees which all say the same thing (because they all copied one optimist). I wish it used the DNA match/connection flags, or used the Shared Matches data. [For example, working today on Match A I keep being offered Common Ancestor X, when I know exactly how I am related to Shared Matches B, C where the Common Ancestor is Y - and X is on my father's side, Y on my mother's. I've worked hard at this and know exactly which tree is generating the nonsense - which others have copied.] May 28, 2022 at 14:15

1 Answer 1


Ancestry has white papers for some of their main AncestryDNA features like matches, ethnicity estimates, and communites, but they don't have one for the common ancestor hints.

Their support page for Common Ancestor Hints says:

To get common ancestor hints, link your family tree to your AncestryDNA® test and fill it out as much as possible. The more people you have in your tree, the more likely you are to share an ancestor in your tree with a DNA match. After you link your tree to your test, it may take up to 24 hours for common ancestor hints to appear.

When a potential common ancestor is identified, we compare several aspects of the person in each match's tree: their names, birthdays, locations, parents, spouses, and children. If everything lines up, a hint is created.

Because common ancestor hints depend on information from family trees, they're only as accurate as the trees themselves.

Since you're getting hints for some of your matches, I would guess that your own tree is matched to your DNA test correctly. However, it could be that some of your matches have unlinked trees, and that may make them invisible to the algorithm. If you haven't already, check your match list to see if everyone's trees are linked, and if their trees are public, that the trees extend out as far as yours.

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    Ancestry found the common ancestor. Adding the siblings to Helen Beg (b1899) made the difference.
    – Mattman944
    May 23, 2022 at 21:43

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