The relative who took a test¶

Genetic genealogy is re-identification through family. A person who never took a DNA test, and never consented to anything, can be identified because a cousin did.

The method¶

Investigative genetic genealogy uploads a DNA profile, from a crime scene or in principle from anywhere, to a consumer genealogy database such as GEDmatch or FamilyTreeDNA, and searches for relatives by shared DNA segments. From a few matched cousins and conventional genealogy, an investigator triangulates back to a single person. GEDmatch has held around 1.2 million profiles; a 2018 study of a database of similar size, 1.28 million MyHeritage profiles, estimated that about 60% of searches for a person of European descent return a third cousin or closer; once such a database covers roughly 2% of a population, more than 90% of that population becomes reachable. Either way, the technique reaches far beyond the people who actually tested.

The breakthrough was the 2018 identification of the Golden State Killer, Joseph DeAngelo, from a relative’s data on GEDmatch. By December 2023 the method had helped solve more than 650 criminal cases. In 2023 The Intercept reported that forensic practitioners had circumvented GEDmatch users’ opt-out settings, giving law enforcement access to profiles whose owners had not agreed to it.

Identified through your relatives¶

This is re-identification with a property the others do not have: it identifies people through their relatives, so consent by the individual is beside the point. It is the sharpest instance of the consent problem, one person’s decision to test exposes a whole family tree, and it hands a law-enforcement adversary an identification method that needs no warrant against the target, only a distant cousin who once mailed off a saliva sample.

Last reviewed: 2026-07-08.