  Explaining the evolution of cooperation in the strong altruism scenario,
where a cooperator does not benefit from her contribution to the public goods,
is a challenging problem that requires positive assortment among cooperators
(i.e., cooperators must tend to associate with other cooperators) or punishment
of defectors. The need for these drastic measures stems from the analysis of a
group selection model of temporarily formed random groups introduced by
Hamilton nearly fifty years ago to describe the fate of altruistic behavior in
a population. Challenging conventional wisdom, we show analytically here that
strong altruism evolves in Hamilton's original model in the case of biparental
sexual reproduction. Moreover, when the cost of cooperation is small and the
amplified contribution shared by group members is large, cooperation is the
only stable strategy in equilibrium. Thus, our results provide a solution to
the `problem of origination' of strong altruism, i.e. how cooperation can take
off from an initial low frequency of cooperators. We discuss a possible
reassessment of cooperation in cases of viral co-infection, as cooperation may
even be favored in situations where the prisoner's dilemma applies.
