Plutchik and I agree that the capacity for mood is a product of natural selection and needs an explanation in terms of its function. He, however, has several less fundamental objections which I reply to.
2. Plutchik's objections to my use of the word "mood" have been answered in my earlier replies to Green (PSYC 3.1.3.2 1992) and Morris (PSYC this issue) and will not be repeated here. Plutchik also objects that I do not tackle depression and grief but then notes one reason I excluded them when he says "there is obvious disagreement about the meanings of these different concepts." There is enough trouble in trying to explain ordinary happiness and sadness. Another reason is the difficulty in determining whether extreme states are adaptive or maladaptive. For example, I would not necessarily assume that seizures have an adaptive function related to arousal, even though extreme states of arousal may cause seizures. I remain, even in my own mind, quite undecided about the adaptive significance of depression.
3. Second, Plutchik objects to my argument that happiness and sadness have related functions; he suggests this implies that all pairs of opposite emotions have related functions. Especially in a circumplex model of the sort carefully studied by Plutchik (1980) I agree that it is important to consider that apparently opposite emotions may have unrelated functions. In the case of happiness and sadness, however, there are special reasons to expect that they have related functions: first, their characteristics remarkably mirror each other; second, the existence of manic depressive illness, with its regular cycles of high and low mood, virtually assures that happiness and sadness are regulated by the same brain mechanisms. Thus, they are likely (but not certain) to have closely related functions.
4. Plutchik objects to what he sees as my attempt to propose one function for mood. I am not proposing one function, but one situation. It seems to me that the big problem in evolutionary studies of emotions is that people keep trying to explain emotions by proposing one or several functions. I have tried to take a whole different approach, seeking explanations for emotions not in one or several functions but in the situations in which each emotion offers a selective advantage (Nesse, 1990). In the case of mood, the situation is not simple, but a variable aspect of the environment which I call propitiousness. Starting with the situation instead of the functions, it is possible to integrate the many functions of an emotion: communicative, physiological, and cognitive.
5. Plutchik cites Darwin and ethologists in support of his contention that "the communicative aspect of emotion is the most significant for survival." Well, maybe. But we sure seem to feel a lot of intense emotions when we are alone, and we sure hide a lot of emotions when we are in public. Emotions do have important communicative functions, but I see no reason to give these primacy over other functions.
6. Plutchik suggests that wide variation in the subjective experience of emotions means they are unimportant to fitness, but the same argument would apply to the wide variation in the communication of emotions, wouldn't it? Although variable traits are often of lessor adaptive significance, there are substantial consistencies in both subjective and objective aspects of emotions. As Ekman (1989) and others have shown, this consistency extends across cultures. As for the defenses and repression: far from their being evidence for the nonsignificance of introspective emotions, I have argued that they were shaped precisely because subjective awareness of emotions has such a large impact on fitness and because of the selective advantages of being unaware of certain of our own motives (Nesse, 1990).
7. Plutchik feels that my argument is circular, but I explain that propitious circumstances are those in which "small investments have a high likelihood of a large payoff." I mean a fitness payoff, of course, not a subjective payoff, although a close correspondence is to be expected. The propitiousness of a situation can be defined independently of its effects on mood. This is not to say that our minds track fitness itself. As emphasized by Cosmides and Tooby (1987, Tooby & Cosmides 1990), emotions are products of specific psychological mechanisms that track variables correlated with fitness, not fitness itself. It is the study of these mechanisms that keeps adaptationist approaches to emotion from being circular.
8. I share Plutchik's view that an ethological approach is crucial to understanding emotion, but I think only a partial solution is to be found if we "identify universal events found at all evolutionary levels whose presence or absence influence survival." Such factors are the foundation but not the structure of a theory of emotion. They do not take individual human goals into account. News of pregnancy may induce depression at age 16, but elation at age 26. It all depends on the person's life situation, values, and plans. There are events such as eating, having sex and receiving praise that are relatively hard-wired to emotional centers, but we must also confront all the messiness and confusion of human cognition if we are to have an adequate theory of mood. Still, I support Plutchik's basic point: it would be lots easier to explain mood if we had a better grasp of the fundamental cues that organisms monitor to assess their situations and adjust their behaviors. Why, I wonder, is this so difficult?
Cosmides, L. and Tooby J. (1987). From evolution to behavior: Evolutionary psychology as the missing link. The Latest on the Best: Essays on Evolution and Optimality. Cambridge, MS, MIT press.
Ekman, P. (1989). The argument and evidence about universals in facial expressions of emotions. In H. Wagner and A. Manstead, Eds. Handbook of Sociopsychophysiology, New York, Wiley, pp. 143-163.
Lazarus, R. S. (1991). Emotion and adaptation. New York: Oxford University Press.
Nesse, RM. (1990). The evolutionary functions of repression and the ego defenses. The Journal of the American Academy of Psychoanalysis, Vol 18, pp. 260-285.
Plutchik, R. (1980). A general psychoevolutionary theory of emotion. Emotion: Theory, Research, and Experience. New York, Harper and Row, pp. 3-33.
Tooby, J. and Cosmides L. (1990). The past explains the present: Emotional adaptations and the structure of ancestral environments. Ethology and Sociobiology 11: 375-424.