Endogamy makes DNA relationship predictions wildly wrong for Ashkenazi Jews, rendering standard genealogy tools useless

Ashkenazi Jews descend from a founding population of roughly 350 people who lived in the German Rhineland 600-800 years ago. Because marriages within the community were nearly universal until the 20th century, any two unrelated Ashkenazi Jews today share far more DNA than two unrelated people from an outbred population. The result: DNA testing companies predict that you have thousands of 3rd and 4th cousins when in reality these are 8th or 10th cousins whose shared DNA is an artifact of population-wide endogamy, not a recent common ancestor. This breaks the fundamental workflow of genetic genealogy. The standard approach is: see a match predicted as a 3rd cousin, look at their tree, find the common ancestor 3-4 generations back. For Ashkenazi researchers, this approach produces an avalanche of false leads. You might see 7,900 matches predicted as 4th cousins or closer, and the vast majority share DNA not because of a traceable genealogical relationship but because everyone in the population carries overlapping segments from the same medieval founders. You cannot use Ancestry's ThruLines, you cannot meaningfully cluster shared matches, and the relationship predictions that non-endogamous users rely on are essentially random noise. The problem persists because DNA testing companies built their algorithms on reference populations dominated by outbred European groups. Adjusting predictions for endogamy requires fundamentally different statistical models — not just tweaking a threshold — and the companies have been slow to implement them. The workaround experts recommend (calculating cM-to-segment ratios, only pursuing matches over 200-250 cM) is esoteric knowledge buried in blog posts, not surfaced in the product UI. This means the population most motivated to use genetic genealogy — Ashkenazi Jews, many of whom lost entire family branches in the Holocaust — gets the least accurate results from every major platform.