The question about the fitness value of mood also relates to the broader question of the fitness value of emotion. Communication media, such as emotion, create adaptive functioning. Thus, natural selection will select them in the long run through the return communication channel, its DNA will move on to later generations and will replicate all the structures.
2. The external approach is behavioral and fits with behavioral psychology whereas the internal approach is physiological and fits more with neurophysiology and brain science. But because mood is an emotion that is diffusely related to behavior, the external approach also poses some difficult problems. Can one link mood -- whatever it is -- definitely with behavior? For example, among the vervet monkeys, could not "low mood" lead to irrational bursts of frustrated aggression with negative consequences as well as submissiveness?
3. Nesse's target article adopts the external approach and makes some good arguments for the fitness value of mood as a form of communication, signalling social position for example. Such behavioral consequences of mood are recognizable. Yet, in situations where an individual is alone, mood cannot communicate well. (Perhaps that is why psychotherapists suggest that depressed people talk about their problems.) The internal approach, which is not incompatible with the external one, fills out the evolutionary picture of emotion and helps to complete the understanding of its adaptive value: Mood is also a means of internal communication within the central nervous system that helps the organism mount a wide ranging set of behavioral responses to an environment.
4. What are emotions? They can be regarded as communication media within the CNS that summarize entire states of the system resulting from many physiological processes. This would make psychotherapy and other forms of symbolic healing more understandable (Dow 1986). There is a small difference between Nesse's "high mood" and the pleasure resulting from some goal accomplishment. Mood in this sense is a perception of anticipated pleasure. (Pleasure was of course a subject dear to the heart of Freud.) Mood may not be so different from other emotions. Both high mood and the emotion of pleasure are symbolic messages from the CNS to the conscious mind that the state of the organism is good and that the directions in which behavior is taking the organism are positive, in terms of fitness. Thus, they can be seen as symbolic media facilitating the integrity and functioning of the organism as an adaptive system, again in terms of fitness. The answer to the question "What is mood for?" can accordingly be formulated in terms of the basic organization of the CNS and the adaptive functions of organ for the organism as a whole.
5. The fitness functions of pleasure and other emotions lie in the way they put together and allow a complex organism with a CNS to confront an environment. Many somatic systems, perhaps thousands, integrate and send a message to higher levels of the CNS produce emotion. The communicative function of emotion has fitness value by itself. Emotions are among the least behaviorally and perceptually linked psychological processes. They are regulators of the entire nervous system. For example, pleasure encapsulates the essence of survival and adaptation. It is beyond behavior, containing a higher level message to the cognitive functions of the CNS. And although it can be seen in terms of adaptive external behavior, pleasure is not always the best guide to survival (as many religions and Freud teach). The internal approach has the advantage of generating more neurophysiological hypotheses. Chemicals that diffuse through relatively large ares of the brain are often linked with emotions. How do these neurochemicals function?
6. One can look at the structure of communication created by evolution on many levels. Emotions belong to the level at which somatic systems are communicating with the consciousness of an organism. This communication is adaptive not simply because it leads to fitness enhancing behavior, but because it holds the whole organism together as a system and allows it to interact adaptively with its environment. There are analogies with other communication media operating at other levels in the hierarchy of living systems created by evolution (Dow 1986). For example, social systems, aggregating many organisms of the same species, generate forms of communication that regulate the behavior of the group. These communication systems are as difficult to understand in evolutionary terms as emotion. Among human being, messages are encoded in symbols maintained by sacred mythologies, value systems or other media such as money. Money symbolizes value in a socio-economic system the same way pleasure symbolizes value in a psychological system. I am not arguing for group selection here, but for the tendency of evolution to create hierarchies of control: molecule, cell, organ, organism, society, ecosystem.
7. All these communication media, such as emotion and money, create adaptive functioning. Thus, natural selection will select them in the long run through the return communication channel that reaches from the highest level, the ecosystem, back to the lowest, the molecule. If the organism reproduces in the ecosystem, its DNA will move on to later generations and will replicate all the structures.
Darwin, Charles. (1872, 1965). The Expression of the Emotions in Man and Animals. Chicago: University of Illinois Press.
Dow, James. (1986). Universal Aspects of Symbolic Healing: A Theoretical Synthesis. American Anthropologist 88(1), 56-69.