Friday, 16 February 2018

Generation lengths

Generation lenghts - the number of years between a person's birth and the birth of their child - have always fascinated me. The fact that some people have very old parents and, by extension, very old grandparents and great-grandparents (or, by contrast, extremely young ones), is something I find extremely interesting.

In my families, generations have been rather short, especially on my mother's side, where they are in fact unconventionally short.

Between myself and my oldest great-grandparent (Fredrik M. Pedersen born 1904), the distance is 86 years.
Between myself and my youngest great-grandparent (Gerd O. Hansen born 1924), the distance is 66 years.

Between myself and my oldest great-great-grandparent (Nils E. D. Lister, born 1857), the distance is 133 years.
Between myself and my youngest great-great-grandparent (Dagny G. A. Hellerud, born 1901), the distance is 89 years. Note that we are five generations born within the same century.

Here are some of the "longest" lines I've found in my tree so far:

Gunvor Cederholm, born 1915 (my great-grandmother)
Sven Mikal Cederholm, born 1872
Sven Hansson Cederholm, born 1824
Hans Svensson Sandberg, born 1781
Sven Hansson, born 1750

Gunvor to her great-grandfather: 134 years.
Gunvor to her great-great-grandfather: 165 years.

JP, born 1987
BP, born 1946
Ingrid Gunhild Olsen, born 1905 (my great-grandmother)
Gunerius Olsen, born 1864
Ole Sivertsen Svegjerdet Okkelberg, born 1826
Sivert Olsen Fordal, born 1781
Ola Gunnarsen Fordal, born 1723

Gunerius to his great-grandfather: 141 years.
Ingrid to her great-great-grandfather: 182 years.
BP to his great-great-grandfather: 165 years.
JP to his great-great-grandfather: 161 years.
I was wrong about Sivert O. Fordal being the father of Ole Sivertsen!

Here's another "long" line, from my stepfather's tree:

Jenny Andrea Hammerås, born 1909 (my stepfather's grandmother)
Emma Janette Pedersen, born 1872
Edvard Pedersen Orvik, born 1826
Peder Eriksson Orvik, born 1776
Erik Hansen Harbaken, born 1737

Jenny to her great-grandfather: 133 years.
Emma to her great-grandfather: 135 years.
Jenny to her great-great-grandfather: 172 years.

In contrast, here is a "short" line from my own tree:

MH, born 1988 (my first cousin)
My aunt
Kari Johansen, born 1946 (my grandmother)
Gerd Ovidia Hansen, born 1924
Dagny G. A. Hellerud, born 1901

MH to her great-grandmother: 64 years.
MH to her great-great-grandmother: 87 years.

Both these numbers are less than half of the longest distances given in the lines above!

This shows that:

(A) There really is extreme variation in generation lengths;
(B) That this variation accumulates over the course of several generations, becoming more and more extreme.

This is especially important to take into consideration when working with autosomal DNA matches. It is possible that your match's great-grandparents were born twice as long ago as your own, and that your match could be, say, a second cousin of your great-great-grandmother. The opposite is also possible: Your (seemingly distant) match may be a descendant of your own great-great-grandparents, but that couple may be your match's 6th great-grandparents!

The last example is taken from real life. I myself have a 6th great-grandmother who was born in 1792, while I know several living people whose great-great-grandparents were born in that year or even earlier.

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