I’ve recently noted a trend among genealogists to discount unexpected (or unwanted?) DNA test results in order to make the results fit an existing hypothesis, instead of properly re-evaluating the hypothesis in light of the new DNA evidence. (This is NOT made in reference to any specific person, post, or question; it is rather something I’ve been mulling over for some time).
Let’s take third cousins as an example. According to Family Tree DNA’s FAQ, you will share detectable DNA with approximately 90% of your third cousins under FTDNA’s threshold. According to AncestryDNA’s help page (see “Should other family members get tested?”), you will share detectable DNA with 98% of your third cousins under AncestryDNA’s threshold. In other words, if you have 100 third cousins and they all get tested (how’d you do that?), you will share DNA with 98 of them.
So what does this mean? Very simply, it means that if predicted third cousins do NOT share DNA, then there is a very strong presumption that they are NOT third cousins. It is easy to believe that the third cousins could be in the 2-10% of third cousins that don’t share DNA, but this is statistically unlikely; it is far, far more likely that they are not actually third cousins.
The presumption is rebuttable, of course, but it would take a great deal of very strong evidence to convince me that the third cousins are in the 2-10%. And I’m not sure I could be convinced without some type of DNA evidence.
So while there aren’t many absolutes in genetic genealogy, I do believe that there are a few:
- Second cousins and closer (nuclear family, grandparents, aunt/uncles, first cousins, great-grandparents, second cousins, etc.) – if you don’t share DNA with them, then you aren’t second cousins or closer. The amount of DNA you share will shed light on the relationship, but there is no relationship at this level if you don’t share DNA detectable by a testing company. Period.
- Third cousins – Since 90 to 98% of third cousins share detectable DNA (see above), predicted third cousins are almost certainly NOT third cousins if they don’t share DNA above the company threshold. A lot of very good evidence will be needed to rebut this presumption (and may not be possible without some type of DNA evidence).
- Fourth cousins – Since 50% (FTDNA) to 71% (AncestryDNA) of fourth cousins share detectable DNA, predicted fourth cousins are probably not fourth cousins if they don’t share DNA. This presumption is much easier to rebut. However, discerning genealogists should always begin with the assumption that if predicted fourth cousins don’t share DNA, it is statistically more likely than not that they are not fourth cousins.
- Fifth cousins and further – Since fewer than 32% to 10% of relationships at the fifth cousin and further level share detectable levels of DNA, it is entirely likely that actual cousins at this level will not share DNA. Lack of sharing at the fifth cousin level or beyond is unlikely to support or refute a hypothesis (with a tiny handful of very rare exceptions).
Of course, there’s a lot missing here (because they aren’t absolutes). For example, deciphering between a half-sibling and a full-sibling relies on the amount and/or pattern of shared DNA. And how does a third cousin once removed fit into this? Or a second cousin twice removed? And so on.
What do you think? Are these fundamental principles that we can use for genealogical research? How would you change them? Without getting into the amount of shared DNA, are there any that you would add?