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Legend has it that, one day in a pub in Bloomsbury in 1955, a lively conversation with Haldane over a few pints turned to the serious subject of the lengths that one would go to save another’s life. Would, for example, Haldane risk his life to save a drowning man? Haldane, after a few moments’ consideration, including some scribbling on the back of an envelope, replied, “No, but I would do it for two brothers or eight cousins.”

In this way, Haldane built on and extended the sturdy idea that parents look after their offspring and thus their own genes. He had given the world an intoxicating insight into cooperation that would seduce and beguile generations of biologists: if genes were the key entities vying to get to the next generation, then it made sense for individuals to incur a cost if at the same time they conferred a benefit on relatives who carried the same genes.

This theory of kin selection acknowledges that a gene can propagate itself through two routes. The first is familiar—a gene can thrive by increasing the likelihood that the body in which it resides will survive and produce offspring carrying more copies. The second route is by increasing the reproduction of close relatives (kin) who also possess copies of the same gene. So brothers who share half of their genes are more likely to put themselves out for each other than cousins, who share one-eighth of their genes.

Haldane explained the idea as follows: “Let us suppose that you carry a rare gene that affects your behavior so that you jump into a flooded river and save a child, but you have one chance in ten of being drowned, while I do not possess the gene, and stand on the bank and watch the child drown. If the child’s your own child or your brother or sister, there is an even chance that this child will also have this gene, so five genes will be saved in children for one lost in an adult. If you save a grandchild or a nephew, the advantage is only two and a half to one. If you only save a first cousin, the effect is very slight. If you try to save your first-cousin-once-removed, the population is more likely to lose this valuable gene than to gain it. It is clear that genes making for conduct of this kind would only have a chance of spreading in rather small populations when most of the children were fairly near relatives of the man who risked his life.”

Excerpted from page 91 of ‘Super co-operators ’ by Martin Nowak