City-Data Forum Amount of 8th cousins or 7th cousins once removed
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12-31-2010, 08:21 PM
 Location: Pacific NW 6,413 posts, read 11,353,666 times Reputation: 5839

Quote:
 Originally Posted by Westerntraveler How many 8th cousins or 7th cousins once removed does the average person has because Ive only tracked down one and I would like to track down long-lost family.My 8th cousin was related to my grandfathers stepmom who was the mother of my Papas half-brother.My papa's half brother was John Roe Ivanhoe who was a prominant dentist in either the New Orleans or Atlanta areas.I know he went to dental school in New Orleans but I never got to meet him.I'm an Ivanhoe.
Well, if you presumed an average of 4 children per family per generation -- (which is just a guess at an average) in the generation of 8th cousins, there would be 393,216 descendants of your common ancestor. Of those, one-fourth (or 65,536) would be 8th cousins. The other three-fourths would be closer cousins, or siblings.

04-01-2011, 04:43 PM
 356 posts, read 788,668 times Reputation: 290
Quote:
 Originally Posted by EnricoV Well, if you presumed an average of 4 children per family per generation -- (which is just a guess at an average) in the generation of 8th cousins, there would be 393,216 descendants of your common ancestor. Of those, one-fourth (or 65,536) would be 8th cousins. The other three-fourths would be closer cousins, or siblings.

I think it's actually (196,608 - 1) = 196,607 8th cousins. At least from my math at 4 kids/person, b/c while 4^8 = 65,536, that only takes you back to 7th cousins, so you must multiply that # by 3 (ignoring the one person you are descended from).

And that's just from one ancestor. If you include all 256 couples you're descended from 9 generations back (1st=parents, 2nd=grandparents, etc), you get a grand total of (50,331,648 - 1) = 50,331,647, or approximately 16.2% of the US population and 0.7% of the current world population. The -1 btw, is so you don't count yourself.

Now, obviously not all of your ancestors had 4 living children who each had 4 living children, etc, not to mention there is a very real chance that some of your 8th cousins are "doubly 8th" or more related to you. Especially in small towns, perhaps you both are descended from 2-sets of the same 9th generation relatives. Doesn't mean there was any inbreeding, just that at some point perhaps two brothers married two sisters, which happened quite often.

Presumably your true # of 8th cousins is much less than that.

9-generations removed, at an average 30-yr generation, takes you back 270 years before you were born. So, if you were born abt 1950, approximately back to 1680.

For perspective, in 1650 there were only about 500 million people in the world. Today there are 6.9 billion.