A new mathematical model demonstrates that many African Americans of the same age likely share family connections dating back 185 to 410 years, challenging casual uses of terms like “cousin” among friends.
Modeling Family Connections Through Slavery
Researchers developed this model, published in The American Statistician, to explore African American heritage amid the U.S. history of slavery. Poor record-keeping during that era limits ancestry tracing before the 1870 census for many.
For two randomly selected African Americans born between 1960 and 1965, the probability of sharing at least one African ancestor forcibly brought across the Atlantic ranges from 19% to 31%. This figure climbs above 50% for those born between 1985 and 1990 to two African American parents—equivalent to a coin flip.
“We used a mathematical model of genealogies to see what might be found in families where the precise story is not known,” stated Noah Rosenberg, professor of biology. “We found that there is a surprisingly high probability that two people share an ancestor who arrived as an enslaved person during the Transatlantic Slave Trade. That’s interesting both in terms of the relatedness of the population and the shape of American demographic history.”
Drawing on the Birthday Problem
The model adapts the classic “birthday problem,” a probability puzzle showing that in a group of 23 people, the chance exceeds 50% that at least two share a birthday. In larger groups of over 50, that probability nears 97%.
Researchers applied similar logic to ancestors who endured the same historical trauma, calculating overlaps in genealogical lines.
Insights into Ancestral Overlaps
African Americans born in the early 1960s each trace roughly 300 ancestors among the 400,000 to 500,000 Africans transported to North America from the early 1600s to 1860. The model assesses the likelihood of overlap between any two individuals’ ancestor sets.
One generation later, those born in the 1980s carry even more ancestors, boosting shared ancestry probabilities.
Addressing Gaps from Slavery’s Legacy
Genetic studies have estimated African and European ancestors in family trees, but this mathematical approach offers clues about collective heritage without linking specific individuals.
“In most cases, the question of whether two specific people have a shared transported ancestor cannot be directly answered because many aspects of the history of slavery led to profound loss of genealogical information,” Rosenberg noted. “A mathematical model therefore has potential to make a meaningful contribution.”
