In many fig wasp species, flightless males fatally fight with rivals. In a recent comparative analysis, West and colleagues found no influence of male relatedness on the frequency of fatal fighting. Inspired by this study, I used a simple theoretical model to examine the conditions that should influence the probability of fatal fighting in a closed mating system. I show that, without kin recognition, relatedness can be expected to have no influence on the frequency of fatal fighting. Under such conditions, males should fight if their chance of winning is greater than a threshold determined by the number of competitors. If males recognize kin, a non–linear relationship between relatedness and the frequency of fatal fighting can be expected. However, two other factors should also be important when there is kin recognition: male number and variance in male fighting strength can be shown to have decisive influences on the frequency of fatal fighting. My results thus corroborate and explain one finding of the empirical study—that there is no significant influence of relatedness on fatal fighting—but point to a role for other factors besides foundress number—male number and male variance in fighting strength. These are probably important factors determining the occurrence of lethal combat in fig wasps.