Hamilton’s rule attempts to link costs, c, benefits, b, and genetic relatedness, r.
Hamilton’s rule does not apply to spouses, because r is 0.
Everyday life tells us that costly benefits are more likely to be given because of expediency, or because of mutuality, than because of consanguinity. A necessary condition for mutuality is that the potential donor of the benefit feels accurately perceived by the potential recipient.

Hamilton’s untestable rule construes a subset of human relationships, in which an individual behaves constructively towards a relative. A different subset of human relationships is one in which an individual behaves destructively towards a relative: other subsets are ones in which an individual behaves destructively and constructively towards a relative at the same time, or after provocation by the relative or with a rejoinder by the relative. And so on. Does Hamilton’s rule front social defensiveness about the degree to which relatives can inhibit offspring, who then act out the characterisations they have been given, with a reduction in their reproductive fitness? For example, a parent may give a child a phenotype based on the parent’s own image, thereby at least failing to cultivate the child’s genotype, and at most inhibiting the child’s genotype: when reproductive age is reached, the now adult has a confused identity which reduces reproductive fitness, both direct and indirect.

How does the rearing of unrelated offspring compare with the rearing of the offspring of relatives, within individuals, between individuals, and between species?

Hamilton’s rule endures because of the numinous tendencies and reductionist needs of humans.