What are love drugs? Basically, drugs that affect love—or romantic relationships more broadly. This chapter begins with an account of drugs, explaining that they are essentially just chemicals—clusters of molecules that work on the brain to produce certain effects—and that our choice to regard them as medicine versus recreation, or as a means to personal or spiritual development, is up to us. It is a question of values. The chapter then gives an account of love, explaining that it has both biological and psychosocial dimensions. When there is a tension between love and well-being, it may make sense in certain cases to intervene in either or both of those dimensions to improve our relationships and our lives.

This final chapter has two main goals: to address lingering worries about the medicalization of love—that is, bringing love and relationships into the domain of medicine in a way that threatens to undermine their value—and to put forward a positive vision of love as something we can partially choose, or improve, through science and technology. Will knowing how love works, and even shaping it through hormones and chemistry, rob it of its importance in our lives? Or will it empower us to make our most intimate relationships more reliably consistent with real human flourishing?

Drug-supported couples therapy is not a new phenomenon. In fact, MDMA was widely used for this purpose, to good effect in many cases, into the 1980s—before it was banned for largely political reasons. This chapter discusses the history of MDMA-assisted psychotherapy, making clear that MDMA is not just ‘emotional glue’ that holds romantic partners together, no matter how incompatible. Rather, professionally guided, drug-enhanced counseling may help some individuals or couples realize that they need to end their relationship, and may allow them to do so in a more loving and healthy way. The chapter asks whether MDMA poses a threat to authenticity or personal identity and raises other risks that may be associated with its use under certain conditions. It concludes with a call for careful, controlled scientific research into the potential of MDMA as an aid to couples counseling

This chapter highlights the recent burst of controlled, scientific research on medical and non-medical uses of psychedelic drugs and MDMA to improve individual welfare, and argues that this research should be extended to couples in romantic relationships. It questions the line between ‘drugs’ and ‘medicine’ and argues that such distinctions often reflect dubious social and historical factors, rather than a clear-eyed assessment of actual benefits and harms. It introduces the idea that love drugs might help strengthen certain relationships, and that anti-love drugs might help other relationships end. But there are serious risks that might be associated with such drugs, and the wider social implications will be hard to predict. To minimize this risk and uncertainty, careful ethical deliberation and nuanced policy measures will be key.

So much of our lives has been subsumed by drugs and medicine—do we really need another ‘pill’ to add to the mix? This brief epilogue argues that the answer is, actually, no. We need fewer, but better drugs—drugs with less severe side-effects, and more power to genuinely improve our well-being. The potential of MDMA and some psychedelics to replace a range of harmful medications is discussed, with a renewed call for high-quality research into this possibility as applied to relationships.

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.