Going back to earlier and earlier generations results in a smaller and smaller number of ancestors when compared to the ancestry charts of the offspring of “unrelated” couples.
This reduction in the number of ancestors is called pedigree collapse.
To be sure, all of us are related to each and every other person somewhere back in history. Some lists claim that 26 states allow cousin marriage. In Leviticus, Chapter 18 (KJV), God tells us that we are not to have sexual relations with a long list of relatives, but the word “cousin” is absent from the list.
The confusion apparently stems from the laws of North Carolina, where double cousin marriage is specifically prohibited, but marriages amongst “normal” cousins (those who are not double cousins) are allowed. Leviticus does deny marriage to “any close relative,” and scholars have long debated whether or not that includes cousins.
Rudy Giuliani, former mayor of New York, married his second cousin once removed, Regina Peruggi Samuel F. Morse, inventor of the Morse Code, took his first cousin once removed, Sarah Elizabeth Griswold, as his second wife.
Johann Sebastian Bach married his second cousin, Maria Barbara Bach.
A single individual occupies multiple places in the family tree when the parents of an ancestor are cousins (sometimes unbeknownst to themselves).
Of course Albert Einstein is the Poster Boy for cousin marriage.
Anyone whose parents were cousins only has six great-grandparents instead of the normal eight.
One set of great-grandparents will show up twice in the ancestors chart.
John Adams married his third cousin, Abigail Smith. Fitzgerald, former mayor of Boston and grandfather of John F.
Kennedy, married his second cousin, Mary Josephine Hannon.