The K-values of Speculative Kinship

we’ll share descendants

Sometimes someone can feel like family, though you don’t share any known ancestors and it’s by no means clear you’ll co-parent with them. No matter what happens in this generation, as long as everyone has children, you’re probably family via mutual descendants.

You probably have 0-1 co-parent(s)—people you’ll have kids with—though few these days blink at 1-2 more, especially if you donate genetic material.

But let’s say you have two kids—and, if the world and their heads keep turning, each has two kids of their own—your family tree will look like this:

You’ll have four co-grandparents outside your co-parent!

In fact, things get out of control pretty quickly. Each grandchild who has children contributes at least four additional co-great-grandparents. So you might have at least twenty co-great-grandparents outside your co-parent.

Substack doesn’t support inline LaTeX, so read this:

So it takes ~17-18 generations for every person in your generation to be a co-ancestor of some descendant with you. Assuming 30 years / generation, that’s ~510 years.

So if every person in 1515 had two children, each descendant had two children, and no one overlapped—they would all be in family with each other via the people alive today.

The people alive today can be seen as belonging to nested sets:

Coparents ⊂ Cograndparents ⊂ ... ⊂ 17th degree coancestors ≃ everyone

K_AB: The number of generations that must live for A and B to become co-ancestors of some descendant

Social deduction / getting to know someone is figuring out K_AB.

Falling in love is the process of driving K_AB to 1.

Your nearest and dearest probably have K_AB of 2-5.

Looking at people alive with you today, the question is one of degree of relation, not whether.