An evolutionary perspective seeks to identify the ultimate evolutionary reasons why humans have evolved the predisposition to develop depressive symptoms, often as a response to specific circumstances or life events. For such psychobehavioral depressive adaptations to have evolved, similar circumstances should have arisen frequently enough in the human evolutionary history. Viewing depression symptoms as evolved responses to specific life adversities or other evolutionarily relevant circumstances contrasts with the traditional view in psychiatry of viewing depression symptoms primarily as indicators of disorder, associating them with distress or impairment. In contrast, an evolutionary psychoneuroimmunological perspective recognizes the possibility that some depressive symptoms can sometimes be beneficial by helping an individual cope with the adaptive problem that triggered their mood change. Yet people with unhealthy western lifestyles are more likely than ancestral humans to suffer from low-grade systemic inflammation and chronic stress, which can make adaptive depressive symptoms either too strong or chronic. A mixture of adaptive depressive symptoms and symptoms of sickness behavior can create prolonged episodes of depression that would be much less likely to arise in absence of low-grade systemic inflammation and chronic stress. For this reason, many instances of chronic depression may be maladaptive in contemporary depressogenic environments.