We combine two aspects of fitness – lifetime reproductive success and (population) growth rate (“propensity fitness”) – for defining a generation duration which, in combination with either of these fitness definitions, can quantitatively answer one of the classic questions of evolution: What is the effect, on later generations, of a single case of reproduction greater (or smaller) than average? Individual breeding success of offspring can be included by a simple multiplication of generations, or by referring the reproductive history directly to grandchildren or later generations.We define inclusive fitness by an inclusive lifetime reproductive history, combining the histories of two individuals sharing an altruistic act. HAMILTON’s rule should rather be expressed as a ratio of inclusive growth rate fitnesses. The termination of growth/decline after a few generations or within one generation, respectively, has different results so it is mandatory to distinguish clearly between these two cases of constant vs. waxing and waning populations. Observed life reproductive histories need a reduction to successful histories (i.e., reproductive offspring) for quantitative answers.