Your Family Past, Present and Future

Diamond Theory of Ancestors

I've talked before about the Pyramid vs Diamond Theory of ancestors. If you don't know what I'm talking about, please read What's Your Number? Don't Be Too Shocked if It's Below 30%! and How Many Ancestors Do We Have????

Basically the Diamond Theory explains that we can't keep going back through the generations and doubling the number of ancestors in each generation. (That's the inverted pyramid theory). Why not? Because eventually the world's population will not be large enough to support the numbers! 

In the Diamond Theory we see that as you keep going back through the generations you will eventually find cousins marrying cousins which narrows the number of unique ancestors and results in a diamond shape rather than a pyramid. 

One probability model created by a demographer for genealogists, is that a child born in 1947 in England tracing back to 1492 would have 60,000 ancestors. Going back further to 1215, this child would find that 80% of the entire population of England at that time would be on his or her family tree! So anyone living in present-day England who traces his/her lineage back through English history would theoretically be related. This is why genealogists find so many people searching for the same families in the 1600s and earlier, and why we find so many "cousins" out there in our search.

Here's a different take on how many ancestors we have.

In Your Family Past, Present and Future the author, Tim Urban of Wait But Why has some very informative and interesting ideas on this topic. At the end he comes up with four conclusions. I love this one

Writing this post has really hammered home the point that humans are mainly a temporary container for their genes. In 150 years, all 7,100,000,000 people alive today will be dead, but all of our genes will be doing just fine, living in other people.

Tim's article has illustrations and sources and is well worth the read. 

What's Your Number? Don't Be Too Shocked if It's Below 30%!

Did you know that with each generation your number of ancestors doubles? For example you have 2 parents, 4 (2x2) grandparents, 8 (4x2) great-grandparents, 16 (8x2) 2nd great grandparents and so on.

By the time you reach your 7th great-grandparents you will have a total of 1,024 ancestors. Phew!

Now that doesn't mean these are all unique individuals, in fact is is almost certain that you will find cousins marrying cousins, perhaps uncles marrying nieces as you go further back. That means that even though we all have 1,024 ancestors at this point in our family tree, there are probably duplicates. See Pyramid vs Diamond Theory of Ancestry for more on this fascinating topic, but I want to talk about something different in this blog post.

Recently my friend Lisa B. Lee of GotGenealogy posted on her Facebook Status that of the 1,024 possible ancestors in her family tree (going back 10 generations to her 7th great-grandparents) she has only found 77. That's 7.5%.

Now Lisa is not a novice at genealogy research. She's a very good genealogist and has been researching for many years. But the number not found surprised her! I was intrigued but before I could figure out my own numbers, I happened to read Crista Cowan's blog post called "Family History All Done? What's Your Number?" on the Ancestry.com newsletter. Crista also figured out how many ancestors she has found so far, again going back the same 10 generations.

Before I tell you the numbers remember that Crista is an experienced professional genealogist. She knows what she's doing! Well Crista's total number out of 1,024 was 365. That's 36% Crista still has 64% not yet found.

Okay it was my turn. I'll set the scene by saying I consider myself an experienced genealogist. I've been researching for more than 30 years. Like Crista and Lisa, I know what I'm doing and I am fanatical about finding my ancestors. Well I did the numbers and here's a chart showing my results:

Generation

Number of Ancestors

My Numbers

7th great

512

81

6th great

256

60

5th great

128

47

4th great

64

46

3rd great

32

30

2nd great

16

16

1st great

8

8

Grandparents

4

4

Parents

2

2

Out of 1024 possible ancestors I have found 295 for a total of 29%. What a surprise and yes, a shock! I knew I was stuck on my Irish ancestors. I knew I could only get back to my 2nd great grandparents on several of those ancestors from Ireland but I had no idea I had so many not found.

I'm doing okay with my family tree until I hit my 4th great grandparents. That takes me back to about 1800 so I've pretty much got my family tree covered for the last 200 years. After that it gets sketchy!

Not having a high percentage of found ancestors is obviously not related to how experienced a genealogist you are or how long you've been researching, so there's no shame attached to a low number. It's just an interesting exercise that points out that genealogy research is never done! And that's what makes genealogy so much fun and so addictive. There's always something more to find.

And now if you'll excuse me I'm off to find my missing 71%!