Filed under: mischievous applications of genealogy
Your Sex Life is a Combinatorics Problem
B.M. Scott
1 Jan 2026
Everyone is someone’s cousin, and your sex life is a combinatorics problem. That line almost certainly has the right mix of “surely not” and “oh no” to do useful work on your intuitions. Population genetics and ancestry models show that human family trees overlap so heavily that almost any two people in the same broad population share at least one common ancestor within the last few hundred years, making them some degree of cousin, even if that degree is so distant no one bothers to name it.
Most people navigate romance and sex with an unspoken map in their heads: family is here—parents, siblings, a few cousins you see on holidays—and partners are over there—classmates, coworkers, app matches, one‑nightstands. Outside of concentrated family dynamics, those circles feel cleanly separated. From the perspective of ancestry though, what looks like a set of disconnected dots is actually a tightly woven mesh. The fact that we don’t experience it that way is a feature of human cognition and social narrative, not a feature of the underlying biology or mathematics.
Consider a concrete case. Suppose you have had sex with 30 people. The familiar questions are about what that number means morally, psychologically, or socially. The less obvious but more structurally interesting question is: how many of those people are blood relatives, even if distant? Combinatorially, 30 people generate 435 distinct pairs of partners. Each pair is a little test of whether those two individuals share an ancestor in the not‑so‑distant past. Ancestry models and theoretical work on shared ancestry indicate that, in realistic population sizes, the probability that two randomly chosen people share at least one ancestor within a few hundred years approaches 100 percent; once your sample includes a few dozen people, it becomes vanishingly unlikely that none of the 435 pairs share such an ancestor. In everyday language, if “relative” includes very distant cousins—10th, 15th, or more—then if you have had 30 partners it is overwhelmingly likely that some of them are, in that thin genealogical sense, relatives of each other.
This feels wrong for several reasons, and that felt wrongness is philosophically useful. First, there is a kind of genealogical myopia - we treat “family” as what fits on a small, tidy tree we can draw by hand, not the vast, overlapping forest of actual ancestors. Second, there is a local sampling bias: we infer global rules from our extremely parochial sample of the population, the few hundred or thousand people we ever meet. Third, the word “relative” drags strong emotional content with it—incest taboos, disgust responses, ethical concerns—that properly apply to close kin and then get illegitimately generalized to vanishingly small degrees of shared ancestry. Nothing of biological or moral consequence changes if two of your exes turn out to be 12th cousins; what changes is your sense of how thin your everyday map of “related” and “unrelated” really is.
It can help to picture networks instead of stories. Imagine each person as a point in a huge graph. Draw one kind of line between any two people who share at least one ancestor within a few hundred years; then draw another line wherever there has been a sexual relationship. Intuitively, most people picture the ancestral lines as sparse and confined to family units, while the sexual lines wander somewhat freely. At realistic scales, the ancestral lines are so dense that nearly everyone in a region sits in one giant, intricately connected component. Swiping right does not move you outside the “village”; it moves you through a village whose boundaries are simply larger and more temporally extended than your intuitions like to admit. Every new partner quietly increases not just the number of stories in your life but the number of hidden kin‑relations among people you think of as entirely unrelated.
On the surface, this observation is a light (almost absurd) claim about sex and relations - viz. something you can play for a laugh over drinks. Underneath, a cheap calculation and a shift in scale quietly pry apart our ordinary categories—family versus strangers, kin versus non‑kin—and show how much they depend on limited perspective rather than on the actual structure of ancestry and populations. Nothing about anyone’s lived experience changes when you recognize that your lovers are statistically entangled as distant cousins; what shifts is the confidence with which you sort the world into neat, non‑overlapping circles. The joke, if it lands, is that the life you feel yourself to be living sits inside a far stranger web than you ever (explicitly) signed up for.
Recommended Further Reading
Pemberton, Trevor J., et al. “The Genomic Analysis of Cryptic Relatedness in the
HapMap 3 Panel.” G3: Genes, Genomes, Genetics, vol. 2, no. 7, 2012, pp. 925–935.
Relethford, John H. Human Population Genetics. John Wiley & Sons, 2012.
Sykes, Bryan. The Seven Daughters of Eve: The Science That Reveals
Our Genetic Ancestry. W. W. Norton, 2001.