Constructing your family tree is simple enough. You simply put your name in the box at the bottom and then enter each preceding generation—parents, grandparents, great-grandparents. great-great-grandparents, etc.—into the preceding boxes creating an ever expanding upside down pyramid.
Using this formula, if we calculated the progression of our ancestors back 30 generations using 30 years per generation, back to around the Middle Ages, we would expect to have roughly a billion ancestors. But that’s impossible…there weren’t that many people in the entire world at that time. So, how can that be?
The Diamond Theory
The truth is that our family trees are actually rather tangled. It doesn't take too many generations to find an ancestor who appears more than once in your family tree, first as a child of one ancestor and then as the spouse of another. This has prompted some to theorize that the pyramid actually begins to narrow beyond the 10th generation. People were less mobile then so there was likely more intermarriage. The exact number of ancestors for each generation is unknown, so it is represented here with an x.(3)
SELF
2 PARENTS
4 GRANDPARENTS
8 GREAT-GRANDPARENTS
16 GREAT-GREAT-GRANDPARENTS
32 GREAT-GREAT-GREAT-GRANDPARENTS
64 GREAT-GREAT-GREAT-GREAT-GRANDPARENTS
128 GREAT-GREAT-GREAT-GREAT-GREAT-GRANDPARENTS
256 GREAT-GREAT-GREAT-GREAT-GREAT-GREAT-GRANDPARENT
512 GREAT-GREAT-GREAT-GREAT-GREAT-GREAT-GREAT-GRANDPARENTS
1024 GREAT-GREAT-GREAT-GREAT-GREAT-GREAT-GREAT-GREAT-GRANDPARENTS
x G-G-GREAT-GREAT-GREAT-GREAT-GREAT-GREAT-GREAT-GRANDPARENTS
x G-G-G-G-G-GREAT-GREAT-GREAT-GREAT-GREAT-GRANDPARENTS
x G-G-G-G-G-G-G-G-GREAT-GREAT-GREAT-GRANDPARENTS
x-G-G-G-G-G-G-G-G-G-G-G-GREAT-GRANDPARENTS
Pedigree Collapse
While we might consider it taboo today, cousins used to get hitched all the time. Franklin Delano Roosevelt’s wife, Eleanor, was his fifth cousin once removed; she didn’t even have to change her name. Albert Einstein and Charles Darwin each married their cousins. More recently, Rudy Giuliani married his second cousin and Kevin Bacon married Kyra Sedgewick only to later learn that they were 9th cousins, once removed. And who can forget the European royals who have notoriously intermarried for generations.
The royal’s intermarried to forge land bonds, settle disputes, and acquire and keep great fortunes. Some have married their distant cousins due to religious rules which required them to marry within their faith, e.g. Quakers, Ashkenazi Jews, etc. And, it was inevitable it to happen for others because their families had lived for generations within a small geographic area.
There are two known instances of cousins marrying cousins in my Creger/Blayden family tree. My 4th great-grandparents, William Brickey Slinker and his wife Nancy Ann Brickey, were first cousins. William Brickey Slinker is the son of Mary Brickey and John Charles Slinker. Nancy Ann Brickey is the daughter of Mary’s brother Peter Brickey Sr. and his wife Elizabeth Brickey. Elizabeth Brickey was the first cousin of her husband Peter Brickey. Got that? I have to confess that I didn’t really grasp it until I drew this diagram.
Now it’s much easier to see the relationship between the Brickey cousins. The outcome of these Brickey intermarriages is that I have two fewer 5th great-grandparents and six fewer 6th great-grandparents in my family tree. This reduction in the number of ancestors extends back through each and every historical generation.
Robert C. Gunderson in his paper “Connecting Your Pedigree into Royal, Noble and Medieval Families” coined the phrase ‘pedigree collapse’.(2) What Gunderson and others before him recognized was that instead of consisting of all different individuals, a family tree may have multiple places occupied by a single individual.
According to a recent paper coauthored by Yaniv Erlich in the journal Science, from 1650 to 1850 a given person was, on average, fourth cousins with their spouse. Due to societal changes, by 1950 married couples were, on average, more like seventh cousins.(4)
Because they were all Brickey's, it was easy to suspect there might be cousin marriages in this line. But what if someone married a 2nd, 3rd, 4th or 5th cousin who has a different surname?
Unfortunately, there is no easy way to identify those distant cousin marriages. Perhaps some day someone will develop a software program that will scour a family tree looking for people who appear twice, once as a child and once as a spouse of another person in your tree. In the meantime, the only way to find them is to trace every descendant of every brother and sister of our direct ancestors. I generally research the brothers and sisters of my direct ancestors. I don’t generally research every one of their children to today. As a result, it's entirely possible that there may be more cousin marriages in my family tree that have yet to be identified.
So, how many ancestors do we have?
The question of how many ancestors we have is actually two fold. First there are the number of ancestors we might have and then there’s the number of ancestors that have actually been found/identified.
The first chart shows the status of my research on my Creger/Blayden lines from my Mom, Helen Pauline Creger. The number of possible ancestors has been reduced to reflect the Brickey cousin marriages. As a result, my Mom could be as many as 1009 direct ancestors going back to her 7th great grandparents. Thus far, only 177 direct ancestors have been found/identified so there’s clearly more work to be done.
The second chart shows the status of my research on the Nielsen/Westlund lines for my Dad, John Robert Nielsen. To date, no cousin marriages have been identified so it's conceivable that he might have as many as 1023 ancestors. His mother immigrated from Sweden and his father from Denmark, so as of now I've only identified 49 direct ancestors. I found the Swedish records rather easy to research. I struggle to understand/read the Danish records and so I don't spend as much time on this line as perhaps I should.
The third chart shows the status of my research on my children’s Wood/Perdue lines which they inherit from their father, Roger DeWayne Wood. I’m relatively new at researching these lines so there’s clearly quite a lot that still needs to be done.
At the top of this blog, you'll now see that I’ve added tabs which are linked to the trees I manage on ancestry.com. The first tab is linked to my Nielsen & Creger Families tree which includes both my maternal Creger/Blayden ancestors and my paternal Nielsen/Westlund ancestors. The second tab is linked to my children's paternal Wood & Perdue Families tree. If you don’t currently have an account with Ancestry and you click on the tab, you will be asked to sign up for a free one in order to see the family trees. If you have an account with Ancestry, you'll be taken directly to the tree.
I hope you've enjoyed learning more about the number of ancestors we each potentially might have, how cousin marriages can impact that number and how many ancestors I've actually identified.
Sources
- International Society of Genetic Genealogy Wiki, “How long is a generation? Science provides an answer”, by Donn Devine, CG, FNGS, https://isogg.org/wiki/How_long_is_a_generation%3F_Science_provides_an_answer
- “Connecting Your Pedigree Into Royal, Noble, and Medieval Families”, Robert C. Gunderson, Church of Jesus Christ of Latter-Day Saints, 1980.
- “HOW MANY ANCESTORS DO WE HAVE????” by Lorine McGinnis Schulze of The Olive Tree Genealogy at http://olivetreegenealogy.com/index.shtml.
- “Quantitative analysis of population-scale family trees with millions of relatives”, Science, 01 Mar 2018.
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