Why Does Depression Exist? A Review With New Predictions From Evolutionary Theories

Depression is highly prevalent, but its evolutionary origin is poorly understood. Evolutionary psychology generates hypotheses on the computational and functional design architectures of the human mind. Evolutionary medicine and evolutionary psychopathology study the design characteristics of adaptations that make humans vulnerable to diseases. These perspectives are necessary when attempting to answer why complex traits exist in living organisms. Some theoreticians postulate that depression is an adaptation that solves particular problems, such as reducing the risk of being socially excluded, elicit help from others, or to analyze complex problems. Others view depression as an evolutionary side-effect of pathogen host defense, and others still view depression as dysfunctions of adaptive mood-mechanisms. The purpose of this paper is to discuss predictions from all evolutionary theories, and make new testable predictions from them. I will also discuss predictions from evolutionary theories on why women are more likely to be depressed than men (~2:1). I argue that research on normal sadness, or low-mood" states are likely to illuminate our understanding of when depressed mood is working as designed and when it is malfunctioning