(2nd edition 2001)
Anthony Stevens and John Price
"...rank theory proposes that depression is an adaptive response to losing rank and conceiving of oneself as a loser. The adaptive function of the depression, according to rank theory, is to facilitate losing and to promote accommodation to the fact that one has lost. In other words, the depressive state evolved to promote the acceptance of the subordinate role and the loss of resources which can only be secured by holding higher rank in the dominance hierarchy. The function of this depressive adaptation is to prevent the loser in a status conflict from suffering further injury and to preserve the stability and competitive efficiency of the group by maintaining social homeostasis.
In circumstances of defeat and enforced subordination, an internal inhibitory process comes into operation which causes the individual to cease competing and reduce his level of aspiration. This inhibitory process is involuntary and results in the loss of energy, depressed mood, sleep disturbance, poor appetite, retarded movements, and loss of confidence which are typical characteristics of depression.
The selective advantage of an evolved capacity for the recognition and acceptance of rank difference in social groups is that it reduces aggressiveness and establishes precedence in granting rights of access to indispensable resources such as territory, food, and potential mates. It follows that gaining rank is associated with elevated mood and losing rank with depressed mood. The evolutionary advantage of living in groups is the protection it provides from predators. For Homo sapiens it also afforded protection from other hominid groups. Living in a group became crucial for safety, for access to resources, for co-operative hunting of large game, and for reproductive success. A sense of belonging has thus become indispensable to our physical and mental security. To be popular and hold rank within a group are immensely desirable accomplishments; to perceive oneself as unpopular and without rank are causes of misery and unhappiness; while to be rejected from the group altogether is one of life's greatest disasters. It is in terms of these factors that joy and sorrow, mania and depression, contentment and anxiety can be most readily understood.
One important contribution of rank theory is that it has proposes a hypothesis of how depression actually evolved: it emerged as the yielding 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.
Both yielding and winning subroutines thus ensure that social change is accomplished relatively quickly without too much disruption of group activities and that once it has occurred it will prove lasting. The object of the losing strategy is damage limitation, that of the winning strategy is status preservation. Inevitably, such subroutines carry greater significance among group-living species than among those living a solitary existence. A solitary animal fights for possession of a territory. If he loses a contest for one territory, then he must be able to withdraw and move on to fight for another. Yielding should involve only a brief "disappointment" on the basis that he who fights and runs away live to fight another day. Group-living individuals, on the other hand, require more prolonged and complex winning and losing subroutines, for a loser may have to give up a position in the hierarchy that he has held for many years. Should he not greatly modify his behaviour he could be expelled from the group. Chance's concept of "reverted escape" becomes relevant here.
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..."