Beyond Hamilton's rule

Why do worker bees give up their own reproduction in favor of other offspring of the queen? Does this make sense from a Darwinian point of view, which prescribes maximization of reproductive success? Ever since Darwin, evolutionary biologists have time and again revisited this problem of how social behavior evolved. There must be some benefit to the donor in terms of fitness, otherwise the trait would vanish. However, how to evaluate this fitness benefit remains controversial because confusion about which models to use abounds.