A third cousin shares one set of great-great-grandparents with you. A fourth cousin shares one set of great-great-great-grandparents. The family tree connecting third cousins has to be built back to great-great-grandparents on both sides and then traced forward through all the descendants. For fourth cousins, the tree goes back one more generation.
Building those trees for the matches that GEDmatch returned was the core genealogical work. Rae-Venter and the investigative team used publicly available records — census data, Social Security death index, birth and death certificates, marriage records, newspaper archives, historical directories — to trace the family structures of the individuals who appeared as matches.
The census records of the twentieth century were particularly important. The U.S. Census Bureau releases individual census records after seventy-two years, meaning that records through 1950 were available for research at the time of the investigation. These records list household members by name and relationship, providing a systematic way to identify siblings, parents, and children of individuals in the family trees.
The work was time-consuming. Building a family tree for a third-cousin match requires identifying the common great-great-grandparent couple, identifying all their descendants, tracing those descendants through the generations, and eventually reaching a generation of individuals who would have been adults in California between 1974 and 1986.
For a fourth-cousin match, the process requires going back one additional generation and produces a substantially larger tree. The number of potential descendants in a family that has reproduced at average historical rates increases dramatically with each generation: a couple’s children might number two to six; their grandchildren, four to fifteen; their great-grandchildren, potentially dozens; their great-great-grandchildren, potentially hundreds.