Recent discovery of cryptic species in fig-pollinating wasps creates a puzzle for the ecological competition theory: how do two or more apparently identical species coexist? Conventional theory predicts that they should not. Chesson (Trends Ecol. Evol., 1991, 6, 26–28) identified one exception which he considered unlikely to occur in reality: coexistence might be possible if appropriate social behaviour was discriminately directed towards conspecifics and heterospecifics. Here we present an example of the exception by showing that two identical species with local mate competition and population size-dependent sex ratio adjustment may coexist. The new findings about fig-pollinating wasps provide a putative example of unexpected coexistence of identical competitors via this mechanism.