This chapter introduces the central ideas in Darwin’s Expression, poses the main interpretive questions that scholars have raised, and outlines my answers to those questions. Why does Darwin analyze expressions in terms of heritable habits, recalling Lamarck’s debunked theory of evolution, when his own theory of natural selection provides a superior alternative? My answer is that Darwin embraces Hartley’s associationist theory of mind, which posits habit as the basis of thought. I claim that multiple puzzling features of Expression are resolved once we view Darwin as an associationist philosopher.

This chapter examines Darwin’s taxonomy of emotions. I show that nineteenth-century associationists were divided on whether emotion categories exist in nature or are conventional. Herbert Spencer argues that emotion categories exist in nature, anticipating modern basic emotions theory. Thomas Brown argues that emotion categories are conventional, anticipating modern psychological constructionism. I show that Darwin sides with Brown and regards emotion categories as conventional. This finding is surprising, since Darwin is often viewed as a precursor to modern basic emotions theory, which adopts the opposite view.

This chapter argues that Darwin’s philosophical theory of emotion has been forgotten due to paradigm shifts in biology, psychology, and philosophy. These shifts have caused researchers to neglect associationist theories of emotions, including Darwin’s contributions to this school of thought. Having explained why Darwin’s philosophy was forgotten, I conclude by explaining why it should be remembered, given its relevance for contemporary emotions research.

This chapter traces the development of Darwin’s theory of emotion and expression from 1838 to 1872, emphasizing his many engagements with associationist philosophers. I demonstrate that all three of Darwin’s principles of expression are derived from the works of associationist philosophers, especially David Hartley, Erasmus Darwin, Herbert Spencer, and Alexander Bain.

The Introduction presents the main theses of this book: that Charles Darwin developed a philosophical theory of emotion, inspired by his reading of several associationist philosophers; that Darwin denied that emotional expressions evolved as social signals, designed to reveal emotions to others; and that Darwin’s theory of emotion has more in common with modern constructionist theories than with modern basic emotions theories, which often claim Darwin as their inspiration.

This chapter surveys associationist theories of emotion leading up to Darwin’s Expression. These theories analyze emotions as sequences of thoughts, feelings, and actions, linked together by principles of association. Thomas Hobbes contributes to this tradition the idea that emotions can be analyzed as “trains of thoughts.” John Locke contributes the idea that these trains are connected by the “association of ideas.” David Hume contributes the idea that association can occur via contiguity, resemblance, or cause and effect. David Hartley puts these ideas together to present the first full-fledged associationist theory of mind and emotion. Harley’s ideas are developed further by Joseph Priestley, Erasmus Darwin (Charles’s grandfather), Thomas Brown, James Mill, Alexander Bain, and Herbert Spencer, among others. This tradition in the philosophy of emotion has never before been described or analyzed.

This chapter examines Darwin’s analysis of emotional expression. It is widely accepted that Darwin wrote Expression to refute Sir Charles Bell’s theory that God created humans with special muscles to express their emotions. However, scholars have overlooked the fact that Bell developed his theory to refute Erasmus Darwin’s associationist analysis of emotional expression, inspired by David Hartley, and that Charles Darwin defends his grandfather’s analysis against Bell’s objections. I demonstrate that Charles’s defense of Erasmus’s associationist theory, which denies that expressions occur for the sake of communicating emotions, explains Charles’s puzzling reluctance to claim that expressions evolved to serve as signals in communication.

Chapter 3 explores the role of small group dynamics and collective emotions in facilitating group theorizing, provoking unconventional scientific thought, and facilitating the rise of new scientific movements. It describes RA’s idioculture, their context of theoretical discovery, and the socio-emotional practices Holling used to spark transformative scientific creativity – a process that he called “island time.” I show how holding short, energetic meetings on remote islands with group rituals, personality selectivity, social bonding, charismatic leadership, and inductions to a secret scientific society created what I call “hot spots and hot moments.” These are brief but intense bursts of collective emotion, intersubjectivity, group creativity, and exceptional scientific performance where transformative science is conducted and faith in the group and its ideas were generated. This highlights new aspects of theory group dynamics, including the bursty nature of creative production within them, and the role of collective emotional states, relationships, and evocative locations for producing innovative scientific knowledge that can support new scientific movements.

The Franco-Prussian War was both unnecessary and imprudent. France initially received all that it demanded from King William of Prussia. Pressure from the legislature and the people of the boulevard, however, then caused French leaders – including Napoleon III – to expand French demands. This allowed Bismarck, the chancellor of Prussia, to manipulate France into declaring a war that he knew Prussia could win. Two characteristics proved fatal for France. First, the French polity and key French leaders succumbed to war hysteria. This allowed emotions to guide their decision-making, rather than rational calculations. Second, an arrogance among French leaders led to military overconfidence and an element of wishful thinking (e.g., about prospective allies). France was, in other words, less powerful than its leaders thought it would be. Finally, the war was imprudent from the French perspective as well. Within two weeks, as a result of French decisions, Napoleon III went into exile, France lost Alsace-Lorraine, a revolt occurred in Paris, and the German states unified.

Although people have been making decisions for many thousands of years, it was only since John von Neumann and Oskar Morgenstern wrote Theory of Games and Economic Behavior and Herb Simon wrote of satisficing and bounded rationality, that researchers started to analyze and understand how people make decisions. The mid- and late twentieth century saw an expansion in what is known about the making of decisions, but more recently new areas within decision theory have come under scientific study. This final chapter is forward-looking and considers possible future directions for understanding human decision making and also for the development of decision theory. Among these future directions are emotion, culture, artificial intelligence, and intuition itself.

There is growing evidence that language plays an important role in emotion because it helps people acquire emotion concept knowledge. In this chapter, we argue that language plays a mechanistic role in emotion because emotion concept knowledge, once acquired, is used by the brain to predictively and adaptively regulate a person’s subjective emotional experiences and behaviors. Building on predictive processing models of brain function, we argue that the emotion concepts learned via language during early development “seed” the brain’s emotional predictions throughout the lifespan. We review constructionist theories of emotion and their support in behavioral, physiological, neuroimaging, and lesion data. We then situate these constructionist predictions within recent neuroscience research to speculate on the neural mechanisms by which emotion concepts “seed” emotional experiences.

Psychiatric disorders are highly comorbid and are not separated by sharp biological boundaries. Understanding the common mechanisms that explain symptom overlap in mental disorders is therefore clearly needed. Here, we briefly review impaired emotional processing and emotional dysregulation in affective disorders, with a special focus on unipolar depression. Affective disorders are characterized by abnormal emotion intensity, changes in the temporal dynamics of emotion and difficulties to influence the trajectory of emotions. Disruptions in emotion processing and emotional regulation are underlined at the neural level by abnormal interactions between cortical and limbic structures in terms of increased variance in functional connectivity. Emotional processes are also tightly linked to cognitive processes, which constitute main targets for therapeutic interventions in affective disorders.

Decades of research demonstrate cultural variation in different aspects of emotion, including the focus of emotion, expressive values and norms, and experiential ideals and values. These studies have focused primarily on Western and East Asian cultural comparisons, although recent work has included Latinx samples. In this chapter, we discuss why studying culture is important for studies of emotion and what neuroscientific methods can contribute to our understanding of culture and emotion. We then describe research that uses neuroscientific methods to explore both cultural differences and similarities in emotion. Finally, we discuss current challenges and outstanding questions for future research.

This chapter introduces peripheral physiological measures of emotion as important tools for studying emotion in affective neuroscience. It examines responses across three systems: skeletal muscle activity, autonomic nervous system (cardiovascular and electrodermal), and respiration. It surveys measurement modalities, derived metrics, their neural control, timescales of expected response, and prominent findings in recent literature, linking them to central nervous system activity throughout. The chapter concludes by highlighting outstanding questions and future challenges in the field of peripheral physiological measures of emotion.