This introductory chapter serves multiple purposes. Its primary aim is to introduce psychiatrists and other mental health professionals who are new to Darwinian thinking to some of the basic concepts and terminology of evolutionary science in order to ease their progress through the remaining chapters of this volume. Another aim is to provide a distillation and update of some significant theoretical and other developments in a variety of evolutionary disciplines relevant to psychiatry and psychology that would be of benefit to all readers, including existing evolutionists. Given the constraints of space, there will inevitably be significant omissions. We have elected to cover the basics of standard evolutionary theory, as well as some of the basic principles of evolutionary psychology and medicine. We also briefly survey some of the recent developments in the evolutionary literature on cultural evolution and related fields. We recognise that a balance needs to be struck between covering as wide an area as possible without the chapter becoming a glossary of terms. Readers unfamiliar with specialised evolutionary terms are advised to consult the glossary on the Evolutionary Psychiatry Special Interest Group at the Royal College of Psychiatrists’ website: www.epsig.org (click on ‘About us’ then ‘Resources’).

The currently dominant model of health and disease in psychiatry and medicine is Engel’s biopsychosocial (BPS) model, proposed in the 1970s to advance reductionistic biomedicine by integrating psychological and social factors. Although the BPS model represented progress, its scientific and philosophical foundations remain questionable and it cannot be considered complete or sufficient. In this chapter, we provide a historical and conceptual analysis of the BPS model before showing that the integration of evolutionary theory can provide a suitable next step from the BPS model, much as the BPS model was a step forward from the biomedical approach. Evolutionary theory justifies and enhances the BPS model’s recognition of multiple levels of causation and expands it by recognising both ultimate and proximate causation. It allows a clearer distinction of biological function from dysfunction and encourages a phylogenetic perspective on biology, which can guide research in new directions. In connecting the model of health with the most fundamental theory of biology, this approach provides the philosophical and scientific coherence that the BPS model sorely lacked.

An evolutionary perspective on drug use and addiction poses two primary questions that complement the proximate models of mainstream medicine. These are: why are humans motivated to repetitively seek out and consume non-nutritional substances, and why do plants (which are the sources of the majority of such chemicals) manufacture substances that can alter the functioning of the human nervous system? We propose that these questions can have a real bearing on our understanding of the phenomena of abuse and addiction that complements models of proximate causation. The evolutionary perspective recognises that addiction can only arise through the interaction of substances with evolutionarily ancient systems designed to promote the pursuit of rewards associated with increased fitness in the ancestral environment. Thus, neglecting the phylogenetic history and function of such systems necessarily results in an incomplete understanding of this phenomenon. Evolution can also help us to understand human uniqueness and especially the role of cumulative culture and gene–culture co-evolution in shaping the human body and mind. Hence, the evolutionary perspective enables a deeper understanding of the human vulnerability to substance abuse and addiction. The chapter concludes by considering the clinical and public policy implications of the evolutionary perspective presented.

Psychopharmacology is the scientific study of the effects of drugs on thoughts, emotions and behaviour as well as the therapeutic implications of their role in treating mental disorders. Psychopharmacology focuses on understanding relevant mental processes as the key to finding new medications and improving clinical outcomes in mental disorder. Interconnected with this, neuropsychopharmacology is the complementary discipline of the study of the basic neural mechanisms that drugs act upon to influence behaviour. Progress has been slow in recent decades with no major new classes of medication being added to the psychiatric formulary. We suggest that evolutionary thinking brings novel additional scientific perspectives to psychiatry and its basic sciences that highlight the evolutionary history of cell communication, neurotransmission and substances that can alter the brain in various ways. Evolutionary perspectives of function and phylogeny also provide a deeper understanding of how natural as well as artificial chemicals (i.e. psychotropic medications) utilise evolved neuronal pathways for their actions. Evolutionary theory can thereby help us to understand the psychological effects and side effects of psychotropic medications as well as assist in the discovery and testing of new drugs.