One important contribution of rank theory is that it has proposes a hypothesis of how depression actually evolved: it emerged as theyielding component of ritual agonistic conflict. This has been called the yielding subroutine (Price and Sloman, 1987). The adaptive function of the yielding subroutine is twofold: first, it ensures that the yielder truly yields and does not attempt to make a comeback, and, second, the yielder reassures the winner that yielding has truly taken place, so that the conflict ends, with no further damage to the yielder. Relative social harmony is then restored.
Similarly, we may offer the hypothesis that mania evolved as the winning component of ritual agonistic behaviour: the winning subroutine. Here again, the adaptive function is twofold: first, it ensures that the winner truly wins and makes clear that any attempt at a comeback by the yielder will be successfully resisted, and, second, it ensures that should the yielder attempt to reopen the conflict, the winner will have such resources of confidence, determination, strength, and energy that he will force the yielder to yield for good and all.
That the incidence of depression is higher and its course longer than hypomania suggest that natural selection has favoured the prolonged yielding subroutine over its winning equivalent. This could reflect the evident fact that in any asymmetrical society there are potentially more losers than winners..."