danryan.us

One logic might go this way: we would expect that the more related people are, the more sacrifice they'd be willing to make to benefit one another. And we can imagine saying "the costs outweigh the benefits given how related we are; if we were more related I might cooperate at higher cost or with lower benefit." All of these suggest $r \gt \frac {C}{B}$ as the condition for cooperating. (see Nowak & Sigmund, p.8)