Rank theory is an evolutionary theory of depression, developed by Anthony Stevens and John Price, and proposes that depression promotes the survival of genes. Depression is an adaptive response to losing status (rank) and losing confidence in the ability to regain it. The adaptive function of the depression is to change behaviour to promote survival for someone who has been defeated. According to rank theory, depression was naturally selected to allow us to accept a subordinate role. The function of this depressive adaptation is to prevent the loser from suffering further defeat in a conflict.
In the face of defeat, a behavioural process swings into action which causes the individual to cease competing and reduce his ambitions. This process is involuntary and results in the loss of energy, depressed mood, sleep disturbance, poor appetite, and loss of confidence, which are typical characteristics of depression. The outward symptoms of depression (facial expressions, constant crying, etc.) signal to others that the loser is not fit to compete, and they also discourage others from attempting to restore the loser's rank.
This acceptance of a lower rank would serve to stabilise an ancestral human community, promoting the survival of any individual (or individual's genes) in the community through affording protection from other human groups, retaining access to resources, and to mates. The adaptive function of accepting a lower rank is twofold: first, it ensures that the loser truly yields and does not attempt to make a comeback, and second, the loser reassures the winner that yielding has truly taken place, so that the conflict ends, with no further damage to the loser. Social harmony is then restored.