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 altrui