I Would Die for Two Brothers or Eight Cousins

You share half your genes with a full sibling and a quarter with a half sibling. That single fact turns out to explain a strange kind of math: sometimes the best way to pass on your own genes is to not reproduce at all.

The Napkin Math That Started It All

If you have a sibling who shares half your genes, and you help that sibling have two extra children instead of having one child yourself, you have left exactly the same number of gene copies in the next generation either way. From your own reproduction, it is 1x. From your sibling's, it is 2 times one half, which is also 1x. By the math of evolution, the two options are worth exactly the same.

This is the idea behind kin selection: sometimes it makes more evolutionary sense to help a relative reproduce than to compete for your own reproduction. The early 20th century biologist J.B.S. Haldane summed it up with a joke that has outlived him: "I would gladly lay down my life for two brothers or eight cousins."

Two brothers each carry half your genes, so saving two of them equals saving one full copy of yourself. Eight cousins, who each carry an eighth of your genes, add up to the same thing. The scientist W.D. Hamilton later turned this joke into an actual formula, now called Hamilton's rule, which calculates exactly when helping a relative is worth the cost to yourself, based on how closely related you are.

Where Kin Selection Shows Up

Once you know to look for it, favoring relatives shows up across an enormous range of life:

• Bacteria: closely related bacterial strains cooperate with each other far more than unrelated strains do.
• Trees: trees share nutrients through their root systems with nearby trees, but only when those neighbors are likely to be relatives, for example in forests where seeds fall close to the parent tree rather than being carried far away by birds.
• Vervet monkeys: two researchers, Dorothy Cheney and Robert Seyfarth, hid a speaker in the bushes and played a recording of a baby monkey's alarm call. Every adult in the group turned to look at that baby's own mother, not just any adult, to see how she would react.
• Baboon troops: when a predator suddenly appears, every mother with a young infant sprints to grab her own child first, ignoring every other baboon around her.

Animals across wildly different branches of life keep making the same calculation: how closely related is this individual to me, and how much should that change what I do for them?

Vervet monkeys also track relatedness in subtler ways. If one young monkey bullies another, the victim's mother is more likely than chance to retaliate later against the mother of the bully, showing that she has connected her own child's suffering to a specific other family. This is kin selection working in real time, not calculated consciously, but built into behavior that consistently favors the individuals who share the most genes with you.

Helping a relative is one way to pass on genes without reproducing yourself. But there is a stranger version of this same idea, one that does not even require being related at all, just sharing a single, recognizable trait.

Further Reading

• Book: Behave: The Biology of Humans at Our Best and Worst by Robert Sapolsky, which covers kin selection and Hamilton's rule in more depth.
• Wikipedia: Kin selection · Hamilton's rule · J. B. S. Haldane