AncestryDNA ThruLines Is Not Proof

AncestryDNA Thrulines is a great tool for finding “potential” common ancestors, but it is NOT PROOF. In Ancestry’s own words (emphasis mine):

ThruLines® shows you how you may be related to your DNA matches. ThruLines are based on information from family trees; they don’t change the information in trees. If there’s inaccurate information in your tree, you may receive inaccurate ThruLines.
AncestryDNA® ThruLines®

Said more bluntly: Due to the countless errors in the millions of trees on Ancestry, there is a high probability that Thrulines could send you down the wrong path.

Don’t confuse “shared matches” with ThruLines. The DNA shared matches reports produced by AncestryDNA (and other firms) are scientifically based and are extremely accurate. Experts disagree as to how many centiMorgans (cM) of shared DNA are required to constitute a valid “identical-by-descent” (IBD) relationship. However, most agree that 20cM or more is a valid relationship, while 8cM or less could be the result of a random coincidence. FamilyTreeDNA uses the following criteria:

A match is declared if two people share a segment of 9 cM or more, regardless of the number of total shared cM. However, if there’s not a block that’s 9 cM or greater, the minimum of 20 shared cM with a longest block of 7.69 cM applies.
Autosomal DNA match thresholds as described by the International Society of Genetic Genealogy

Error Propagation Via ThruLines

While ThruLines is a valid tool for generating hints and identifying some possibilities, I also believe it is a leading contributor to error propagation for trees on the Ancestry website. Most tree builders on Ancestry are desperate to break down their brick walls and find the next generation of their ancestors. Far too often we see people add a person to their tree simply because they had the same surname and lived in the same era and area. No proof is supplied — no sources are provided.

One might believe that all Rowland families in 1700s Virginia were related, and many trees reflect this inaccurate assumption. Meanwhile, the Rowland Y-DNA Project at FamilyTreeDNA has identified and grouped three distinctive Y-DNA lines, and at least two additional early Virginia Rowland family groups that are yet to be tested.

Rowland Genealogy Newsletter #03 (December 2020) documented an example of bad tree error propagation run amok. It described 585 separate trees on Ancestry that were wrongly connected to Robert Rowland of early Surry County Virginia (Rowland DNA Group J) instead of Robert Rowland of early Botetourt County Virginia (Rowland DNA Group F). Needless to say, these 585 incorrect trees ended up in the ThruLines report for numerous individuals. Without further investigation, the wrong Robert Rowland was often attached, and the tree-builder celebrated another generation being added (incorrectly) to their tree.

If your tree has an error, then everyone with a DNA match to you can potentially propagate your error if they see it in their ThruLines report. If the trees of others in your Thrulines report are in error (and we just showed this is a real problem), then there is a possibility you will add an incorrect person to your tree, smug under the assumption that DNA cannot lie. While it is true that DNA cannot lie, unproven relationships suggested by ThruLines often do lie. Again, I offer Ancestry’s own words:

Mistakes in family trees can cause inaccurate ThruLines. Because they’re based on trees, ThruLines don’t prove your specific connection to a DNA match.
AncestryDNA ThruLines Accuracy