ABSTRACT
Many functions have been suggested for low mood or depression, including communicating a need for help, signaling yielding in a hierarchy conflict, fostering disengagement from commitments to unreachable goals, and regulating patterns of investment. A more comprehensive evolutionary explanation may emerge from attempts to identify how the characteristics of low mood increase an organism's ability to cope with the adaptive challenges characteristic of unpropitious situations in which effort to pursue a major goal will likely result in danger, loss, bodily damage, or wasted effort. In such situations, pessimism and lack of motivation may give a fitness advantage by inhibiting certain actions, especially futile or dangerous challenges to dominant figures, actions in the absence of a crucial resource or a viable plan, efforts that would damage the body, and actions that would disrupt a currently unsatisfactory major life enterprise when it might recover or the alternative is likely to be even worse. These hypotheses are consistent with considerable evidence and suggest specific tests. Arch Gen Psychiatry. 2000;57:14–20
Pain or suffering of any kind, if long continued, causes depression and lessens the power of action; yet it is well adapted to make a creature guard itself against any great or sudden evil.
The Life and Letters of Charles Darwin
Charles Darwin, 1887