Understanding Depressior1 A translational approach

Neuroscience provides proximate explanations based on mechanisms, but a full biological explanation of depression also requires an evolutionary explanation of the origins and functions of the capacity for low mood. Failure to recognize that both are essential slows progress. Mood regulates patterns of investment as a function of environmental propitiousness. When invest­ ments are not resulting in progress towards a goal, low mood gives a fitness advantage. If a person cannot give up an unreachable goal, low mood can escalate to clinical depression. There are several evolutionary reasons why brain systems that regulate mood are vulnerable to dysfunction. Introduction In the course of research for a book on why natural selection has left humans so vulnerable to depression, I asked three leading neuroscientists why the capacity for depression exists at alL One suggested that groups with a mix of optimists and pessimists do better than other groups. A second said that negative mood is harmful while good mood gets people to want to have sex and do other things that increase fitness. A third said that depression results from neurotransmitter-receptor abnormalities. These responses suggest that even some of the best neuroscientists are unfamiliar with evo­ lutionary principles that long ago transformed behavioural biology. Evolutionary explanations based on benefits to the group have been recognized as problematic for 40 years (Williams, 1966). The belief that positive emotions are useful but negative emotions are harmful is an illusion (Nesse, 2004). Finally, reductionist explanations for why some individuals get depressed do not address the question of why depression exists (Kendler, 2005). Perhaps more important than specific misunderstandings, however, is the larger problem of widespread failure to recognize that biological traits need both evolutionary and proximate explanations. Neuroscience can never provide a full biological explanation for depression, it can only explain mechanisms. Progress will speed up when we concurrently address the evolutionary question of why depression exists at alL This thesis is supported by considering how the appli­ cation of how three simple evolutionary principles can advance depression research. The first principle is that proximate explanations based on brain mechanisms are insufficient; every trait also needs an evolutionary explanation. The second is that diseases do not have evolutionary explanations, however evolution can explain why some aspects of the body have been left vulner­ able to failure. Third, many symptoms, such as pain, fever, cough, and negative emotions, are not usually the result of bodily …