This table shows (theoretically) how many first cousins, second cousins, third cousins, etc. a person could have, up to 20th cousins--although everyone has hundredth and thousandth cousins. In theory, a person could have 4 trillion 20th cousins.
This table is built on two major assumptions. First, I assume that my parents each had two siblings, who each had four children--for a total of 16 first cousins. (In reality, I have a total of 28 first cousins, so I am being conservative). I then assume each of my four grandparents had two siblings each who had 4 children (who survived to adulthood), who each had four children, for a total of 64 2nd cousins. I use this same assumption back to the 20th cousins. Again this is a conservative assumption because people in the 19th, 18th, and 17th centuries had many more than 4 children survive to adulthood.
Second, I assume there is no inter-marriage among any of my ancestors, which is clearly not true. It is not true because I know 2nd and 3rd cousins married each other. It is also not true because there are not 4 trillion people living on the earth today, therefore there must have been inter-marriage. The alternative is that every living human on the earth is at least my 15th cousin or closer.
Note that these numbers are cumulative. That is, both my 1st and 2nd cousins are now living. and so are my 3rd cousins. Therefore, one must add all these numbers together to get a total of all the people I am related to.
One might ask, just for fun, what is the probability that one of my 20th cousins (or closer) is famous? The answer is 100%, since 4 trillion, plus 1 trillion, plus 274 billion, plus 68 billion, etc. is a pretty large number. What is the probability that you are related to someone famous. One hundred percent.
As an example, see how I am related Joseph Smith, the Prophet (4C,5R), the Prophet, or how I am related to Robert E. Lee, the General (3C,5R). Such family trees could be constructed between me and any famous person of European extraction (such as Prince Charles). Such family trees can be constructed for you and anybody famous in your ethnic group. Such a family tree could be constructed for me and you, whoever you are.
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