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I've come across double third cousins, who are third cousins through two different routes as the same two families had two separate marriages between them that leads to this situation.

For DNA comparisons, I see from this table (found here: http://isogg.org/wiki/Autosomal_DNA_statistics#mediaviewer/File:Shared_cM_Project_v2_updated.png) enter image description here

That third cousins would share an average of 79 cM and vary between 0–198 cM. In this case, I found 194.3 cM which seems to support the double third cousins situation, but wonder if we can be more specific as to what to expect for average and range in such a situation.

On Wikitree.com 3rd cousins or closer is the limit to qualify for direct autosomal DNA comfirmation without requiring DNA triangulation.

Would double 3rd cousins move one into needing triangulation or away from it to confirm the connection?

I would think it would move the relationship closer and thus be considered closer than 3rd cousins, but with DNA it might be more complicated than that.

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    Thanks for the links to the Wikitree DNA confirmation rules. I had never seen those before.
    – lkessler
    Feb 16, 2017 at 5:21

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Double third cousins would indeed have double the expected cM of single third cousins.

The 3C in your table shows an average of 79 cM and a range of 0 to 298 cM, so double third cousins should average 178 cM with a range that could be from 0 to 596 cM.

So double third cousins would move one away from needing triangulation to confirm the connection.

Yes it does make the relationship closer than single 3rd cousins.

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  • I suspect that it is not quite an exact doubling due to duplication along the two paths and thus ends up being a bit less than double.
    – WilliamKF
    Feb 16, 2017 at 19:28
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    @WilliamKF - You are correct statistically if you are using, say 90% confidence intervals. But Blaine Bettinger used sample maximums, and if one can be 298 cM, then a second can be 298cM as well with little or no overlap.
    – lkessler
    Feb 17, 2017 at 22:11

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