Haplodiploidy alone does not predict the evolution of eusociality

As a corollary to inclusive fitness theory, W.D. Hamilton's haplodiploid hypothesis proposed that eusociality evolves more readily in haplodiploids because full sisters can be more closely related to each other than to their own offspring¹. This inflated sister-sister relatedness means that a female may gain more inclusive fitness by helping rear sisters than by reproducing independently. For the past sixty years, theoreticians have developed models clarifying how haplodiploidy might favor the evolution of eusociality^2,3,4,5. Despite this sustained theoretical interest, the predicted relationship between haplodiploidy and eusociality has only very recently been empirically tested^6,7. Here, we use two large species-level insect phylogenies with comprehensive sociality and ploidy trait data to test if eusociality evolves more readily in haplodiploids. We find that estimated transition rates to eusociality are higher in haplodiploids, but that this pattern is driven by a much higher transition rate within a single haplodiploid lineage, the aculeate Hymenoptera. Thus, the apparent links between haplodiploidy and eusociality reflect clade-specific dynamics rather than a general effect of haplodiploidy.