Anthony Stevens, John Price:

"Whereas attachment theory proposes that depression is an adaptive response to losing an attachment figure and conceiving of oneself as unlovable, 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."