Eusociality
If the theory of inclusive fitness is accurate, the haplodiploidy makes kin selection easier. Sisters are more related to each other than to any offspring they might have. Hamilton called them supersisters. On average, two supersisters have 75 percent of the same genes. If they  breed, they would share only half of their genome with their offspring. From the selfish gene's point-of-view, it is better to raise more sisters. Even though workers often do not reproduce, they are potentially passing on more of their genes by caring for sisters than they would by having their own offspring.