Monogamy:

"in the huge modern eusocial insect colonies, such as with ants and termites, the interests of the different colony members, whether queen or workers, may be virtually identical. The reason is that all will realise reproductive success only through the rather small group of reproductively mature individuals that will emigrate and found new colonies. Not only are the workers and the queen likely to be similarly related to these reproductives (especially in forms with diplo-diploid sex determination, as with termites), but they are likely to have no other opportunity to reproduce.

A good comparison in familiar terms would be represented by a species in which the male and female tend to be obligately monogamous, bonded for life. If opportunities for differential assistance to non-descendant relatives, and for philandering, are rare or non-existent, then, even though the male and female may be completely unrelated, their reproductive interests are identical. Each will reproduce via the offspring they produce together; and in most cases the two parents will be equally related to offspring. In such cases the male and female are expected to behave as though their evolutionary interests are identical, as with members of clones, and queen and workers in eusocial colonies (Alexander, 1987)."

Richard D. Alexander, Evolution of the Human Psyche, 1989