 A quite different ambiguity concerns the distinction between weak and strong altruism, in the terminology of D.S. Wilson (1977, 1980, 1990). This distinction is about whether the altruistic action entails an absolute or relative fitness reduction for the donor. To count as strongly altruistic, a behaviour must reduce the absolute fitness (i.e., number of offspring) of the donor. Strong altruism is the standard notion of altruism in the literature, and was assumed above. To count as weakly altruistic, an action need only reduce the relative fitness of the donor, i.e., its fitness relative to that of the recipient. Thus for example, an action which causes an organism to leave an additional 10 offspring, but causes each organism(s) with which it interacts to leave an additional 20 offspring, is weakly but not strongly altruistic. The action boosts the absolute fitness of the ‘donor’, but boosts the absolute fitness of other organisms by even more, thus reducing the donor's relative fitness.