What does this equation mean?

What the rule does not do is scale. Relatedness falls by half with each step of genealogical distance — a full sibling carries r = 0.5 , a first cousin r = 0.125 , and by the time two people are unrelated in any traceable genealogical sense, r is effectively zero. Set r $\approx$ 0 into Hamilton’s inequality and the right-hand side has to do all the work: either the benefit to the recipient must be extraordinary or the cost to the actor negligible, for almost every real transaction between strangers. That condition does hold for some human behavior — warning a stranger about to step in front of a car costs almost nothing and saves a great deal — but it manifestly does not hold for the transactions that build cities and nations: financing a stranger’s business, paying taxes into a fund one may never draw on, or absorbing the cost of punishing someone who cheated a person one has never met. Kin selection is not wrong; it is answering a narrower question than the one a market or a state actually poses. Any account of large-scale human cooperation has to start by admitting that this particular, well-confirmed mechanism runs out well before it gets there.