After spending many years treating emotional disorders, I realized I knew hardly anything about normal emotions. Why do they exist at all? How are they regulated? How many are there? Reading the emotions literature was a slogging trek through the same New Hampshire woods that so discouraged William James on a similar journey. But my 1990 paper had a main point that has helped it become widely cited. Different emotions do not map to different functions, many functions are served by several emotions and many emotions serve several functions. To the extent there is structure to the emotions, it is because they have been shaped to cope with the challenges that regularly arise in SITUATIONS that have recurred over evolutionary time. Those situations are not entirely separate, and the coordinated patterns of response we call emotions are therefore not entirely separate from each other. They have evolved from each other, only partially differentiated. Neither basic emotions nor dimensions can accurately describe the organic complexity of the emotions. Confronted by such a situation one cannot help but experience disappointment! However, it is very satisfying to start to understand why confusion persists after more than a century of intense work on emotions.
Nesse RM, Ellsworth PC. Evolution, emotions, and emotional disorders. American Psychologist, 64(2):129-39, 2009. (a modern statement that emphasizes appraisal)
Nesse RM. Evolutionary origins and functions of emotions. In: Scherer K, Sander D, editors. The Oxford Companion to Emotion and the Affective Sciences. Oxford: Oxford University Press. p. 159-164, 2009.