At what point does my family tree stop doubling and start decreasing?

Your family tree doubles with each generation you trace backward. Two parents, four grandparents, eight great-grandparents. Continue this pattern far enough, and you hit a mathematical wall that forces the doubling to stop and reverse.

The limit emerges from a simple fact: human population size remains finite. If everyone had exactly two children and no one ever married a relative, your ancestors would eventually outnumber all humans who ever lived. That's impossible.

This constraint creates what genealogists call "pedigree collapse." Ancestors start appearing multiple times in your family tree. A great-great-grandmother on your father's side might also be your great-great-grandmother on your mother's side, through different pathways. The further back you go, the more common this overlap becomes.

Readers debating the New Scientist article highlighted how transportation technology shaped when this collapse occurred. Before bicycles and trains, communities remained isolated. People married neighbors, increasing the likelihood of distant shared ancestry. Marriages between cousins, common centuries ago, accelerated the collapse.

The Industrial Revolution changed this dynamic. Better transportation expanded the pool of available partners beyond walking distance. People could marry strangers from distant towns, temporarily slowing the rate of pedigree collapse.

Royal dynasties provide extreme examples. European royal families married within their own circles for generations, creating dense webs of shared ancestry. King Charles II of Spain descended from the same ancestor so many times through different paths that his inbreeding coefficient matched that of a child born to siblings.

Researchers estimate that for most modern populations, pedigree collapse becomes severe around 20 to 30 generations back, roughly 600 to 900 years. At that point, your family tree stops growing exponentially and begins shrinking. The doubling halts. Instead of discovering new ancestors, you find the same individuals appearing in multiple branches.

This doesn't mean