"Are You Your Own Cousin?"

We are one big happy family

Here’s an interesting problem – for every generation back you go, you should be able to double your number of ancestors; 2 parents, 4 grandparents, 8 great-grandparents, and so on. At 12 generations back, you should have 4,096 10th great-grandparents; at 20 generations back, over 1 million 18th great-grandparents. At 30 generations back, or roughly around the time of the “Black Death” in Europe in the middle of the 14th century, you should have 1,073,741,824 28th great-grandparents – that’s over 1 billion. The problem is that the world population just before the plague is estimated to be around only 450 million people. So what happened to a half-billion of my ancestors?

The answer is a phenomenon called pedigree collapse. It occurs when people from the same family tree procreate, and the result is a shrinking of the tree instead of the standard expansion. It has occurred in everyone’s family history – if you go back far enough (for some not so far back as others) you should find it common that 3rd, 2nd or even 1st cousins have married, thus pulling the branches of the family tree back inward, reducing the overall number of ancestors you have. British social theorist Robin Fox recently estimated that 80% of historical marriages have taken place between 1st or 2nd cousins, causing many people to take up more than one spot in their pedigree charts.

It actually had to be that way, if not more so. British genealogist Brian Pears figured out that "if every single marriage was between 2nd cousins, then 30 generations ago [residents of Britain] would all have needed exactly 4,356,616 ancestors - still more than the English population at the time" and concludes that each of Britain’s citizens around the year 1300 was an ancestor to almost every Briton currently living. Mr. Pears’ fascinating essays on “The Ancestor Paradox” can be read here.

Also, check out this article by Joelle Steele that attempts to uncover the truth about cousin marriages.

So the next time you meet up with someone of questionable character, and feel the need to fling the insult “that’s what happens when cousins marry”, just remember that somewhere in your past, it happened to you too.