A Rutgers University professor estimates that 80 percent of historical marriages took place between second or first cousins.
How many ancestors you have doesn't seem like a difficult question, yet genealogists have been attempting to answer it for years. You should be able to double the number of ancestors in each generation (two parents, four grandparents and so on) for 1,024 ancestors in 10 generations. Go back 40 or 50 generations, though, and this formula results in more than a trillion ancestors -- more than the number of people who've ever lived.
What gives? The explanation is pedigree collapse. Rutgers University professor Robin Fox has estimated that 80 percent of historical marriages have taken place between second or closer cousins, causing the same folks to occupy multiple spots on pedigree charts. That means your family tree starts shrinking once you've gone back so many generations.
This genealogical shrinkage makes you cousins with just about everyone. English genealogist, physicist and computer programmer Brian Pears says that "If every single marriage was between second 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."
In 1215, an estimated 2.5 million people lived in England. Pears concludes that each of England's residents in 1300 was an ancestor to nearly every modern Brit. (See his "ancestor paradox" essays here.)
Scholarly research backs him up: In a 1980 Genealogical Demography article "Ancestors at the Norman Conquest," demographer Kenneth Wachter calculated that out of the 1.11 million residents of England at the Norman conquest in 1066, about 86 percent are ancestors to all current residents of England.
In a 1999 paper, Yale University statistician Joseph Chang used a mathematical model to show how all modern Europeans, except recent transplants, have a common ancestor who lived about 1400. Go back to 1000, and 20 percent of adult Europeans alive then have no descendants today, while each of the remaining 80 percent is an ancestor of every European alive today. (See The Atlantic's website for more details.) That's a lot of cousins to look for.
From the March 2011 issue of Family Tree Magazine.