Recent studies have used sex ratios to quantify the mating systems of organisms, the argument behind it being that more female–biased sex ratios are an indication of higher local mate competition, which goes hand–in–hand with higher levels of inbreeding. Although qualitative tests of the effects of mating systems on sex ratios abound, there is a dearth of studies that quantify both the mating system and the sex ratio. I use a colour dimorphism with a simple Mendelian inheritance to quantify the mating system of an unusual fig–pollinating wasp in which males disperse to obtain matings on non–natal mating patches. In qualitative agreement with initial expectations, the sex ratios of single foundresses are found to be higher than those of regular species. However, by quantifying the mating system, it is shown that the initial expectation is incorrect and this species' sex ratio is a poor predictor of its mating system (it underestimates the frequency of sib–mating). The species has a very high variance in sex ratio suggesting that excess males can simply avoid local mate competition (and hence a lowered fitness to their mother) by dispersing to other patches.