Abstract

Hamilton introduced two conceptions of social fitness, which he called neighbour-modulated fitness and inclusive fitness. Although he regarded them as formally equivalent, a re-analysis of his own argument for their equivalence brings out two important assumptions on which it rests: weak additivity and actor's control. When weak additivity breaks down, neither fitness concept is appropriate in its original form. When actor's control breaks down, neighbour-modulated fitness may be appropriate, but inclusive fitness is not. Yet I argue that, despite its more limited domain of application, inclusive fitness provides a distinctively valuable perspective on social evolution.