The Mathematics of Family History

The mathematics of genealogy and population distribution in past centuries make people much more closely related to each other than is commonly realized.

Genealogy Fact: 80% of World’s Marriages Between Cousins

“If we could only get into God’s memory, we would find that 80% of the world’s marriages have been with at least second cousins,” says British kinship expert Robin Fox. An anthropologist, Fox is the author of Kinship and Marriage: An Anthropological Perspective, one of the world’s most widely read anthropology texts.

“In a population of between 300 and 600 people (the size of most towns throughout much of history), after six generations or so,” reports Fox, “there are only third cousins or closer to marry.” He also explains that the 19th century Englishman usually married a woman within five miles of home, the distance he could easily walk twice on a day off. The bicycle, says Fox, expanded the possibilities to a 25-mile range.

The Genealogy Phenomena of Pedigree Collapse

“Pedigree Collapse” is what trims the family tree, according to Alex Shoumatoff, author of a New Yorker article on the mathematics of family history, published more than 20 years ago.

Pedigree collapse occurs when cousins marry cousins, sometimes intentionally, but often unknowingly. “When cousins of any degree marry, their genealogical lines fold back on themselves,” he explains. Over the generations, this greatly reduces the number of ancestors, but only to an unmanageable few billion.

Family Trees Are Shaped Like Diamonds

Because of the small populations that supplied marriageable partners, Shoumatoff says, the rate of marriage between cousins increases the further back family researchers try to extend family trees. As the genealogist goes further back in time, doubling ancestors with each generation, the available people pool diminishes in size. At some point, the number of ancestors in a generation actually is, because of these factors, smaller than the generation before. Our family trees are shaped more like diamonds than ever-bearing trees.

90% of Ancestors Are Lost

A startling Shoumatoff revelation is his belief that out of all the people who ever existed, 90% are—record or identity wise—lost to us. Professional genealogists affirm that you only have to track families back a few hundred years to realize that the only lineages stretching further back are primarily those of royalty, certain religious figures and society’s most important people.

The Genealogical Society of Utah estimates that records now exist for less than ten billion people. The Mormon Church comes closest to having all those records stored in one place. Baptizing ancestors is part of their religion and has resulted in the largest genealogical collection of our time.

What Does It Mean?

What does this mathematical approach to genealogy mean? Simply, that we are all cousins. The Bible tells us so, mathematics and history confirm it and genealogical research validates it.

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