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# For those familiar with Hamilton´s rule... Tweet

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1. For those familiar with Hamilton´s rule...
(I posted this problem in Bio Society as well - but decided to create this new thread in case it might be spotted sooner...)

Is Hamilton´s rule specifically used only for when one individial helps/saves exactly one individual (rB > C)? I got across the example when one brother saves the other one from drowning. The rule was applied like this: B=2 (average number of offspring for humans), r=0.5 (since they are brothers) and C = 0.25 (probability the brother will drown if the other will not save him) times 2 (the number of expected offspring if the altruist had stayed on the shore). Thus, it is beneficial for one brother to save the other. But, what would happen if two of his brothers were drowning (and he could save both)? I realize that if it is beneficial to save one, then it follows that it is advantageous to save both (if trying to save the second, does not endager them all - again, assume both can be saved), but how would the rule be applied? How would it be like mathematically?

When it comes to the cost of "the provider", you can figure out the average probability they both will die, but how would “the-benefit-to-the-recipients” side of the inequality look like? Because, I don´t think you can just sum it up (r1B1 +r2B2; B1=B2) (or, to say it differently, multiply the benefit of one brother by two) as 50% of genes they share with the altruist is (if only in the minor part) different set of 50% (yes, r1 and r2 are both 0.5 mathematically, but when comparing "the biological character" = gene content of what r1/r2 respresent, then they are not the same). Thus – if we think of drowning brothers in the terms of genes, the altruist will – in total – save more than 50%…be it even 50.00000001%…

We don´t even have to consider this ("drowning") example - I am simply interested in the maths of Hamilton rule when "the altruistic act" of an individual saves more than one individual (for this purpose, all of them can be siblings of the altruist).

So? I am curious to know how this works...
2. For those familiar with Hamilton´s rule...
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