Inclusive fitness requires a careful accounting of all the fitness effects of a particular behaviour. Verbal arguments can potentially exaggerate the inclusive fitness consequences of behaviour by including the fitness of relatives that was not caused by that behaviour, leading to error. We show how this this ‘double counting’ error can arise, with a recent example from the signalling literature. In particular, we examine the recent debate over whether parental divorce increases parent-offspring conflict, selecting for less honest signalling. We found that, when all the inclusive fitness consequences are accounted for, parental divorce increases conflict between siblings, in a way that can select for less honest signalling. This prediction is consistent with the empirical data. More generally, our results illustrate how verbal arguments can be misleading, emphasising the advantage of formal mathematical models.
