The prevailing paradigm used by most researchers and physicians to understand depression appears myopic and deficient, and the treatments derived from it are ineffective. A broader perspective is required for understanding why people in contemporary societies are vulnerable to chronic depression. Evolutionary psychoneuroimmunology synthesizes insights from evolutionary biology, evolutionary psychiatry, neuroscience, immunology, and psychology to provide a broad scientific framework for understanding the relatively high prevalence of depression in people with contemporary western lifestyles. A range of findings indicates that the mismatch between the environments in which Homo sapiens evolved and the environments it currently habits – characterized by sedentary lifestyles, obesogenic diets, fragmented communities, loneliness – contributes in several ways to depression prevalence in contemporary societies. Understanding depression in contemporary Homo sapiens requires not just analyzing the proximate mechanisms contributing to it, such as chronic stress, poor diet, and low-grade systemic inflammation, but also the evolutionary question of why a mind that has undergone hundreds of thousands of years of evolution can be so vulnerable to such severe and debilitating mental health problems. Tinbergen’s four questions, a fundamental conceptual framework used in ethology, evolutionary biology, and evolutionary psychology, organizes these different levels of analysis in a coherent framework for this book.

There have been several attempts to subtype depression in prior research, but none of them have gained sustained traction – with the exception of separating seasonal depression as its own subtype. Prior attempts to subtype depression have lacked a convincing theoretical framework. We present the rationale for subtyping depression based on evolutionary psychiatry, arguing that depression is not a single disorder but a heterogeneous syndrome comprising multiple evolutionarily distinct subtypes. From an evolutionary psychiatric perspective, a depressive episode may be (1) a prolonged adaptative response to a specific adaptive problem (thus representing a prolonged version of an adaptive short-term mood change); (2) a maladaptive state arising from an environmental mismatch as a result of modern lifestyle; or (3) a by-product of other adaptations. Depressive episodes caused by the latter two can lead to malfunctioning of the mood-controlling system. This can produce a pathological depressive state without any adaptive function. The 12 subtypes of depression discussed in this chapter are: depression induced by (1) infection, (2) long-term stress, (3) loneliness, (4) traumatic experiences, (5) hierarchy conflict, (6) grief, (7) loss of romantic relationship or romantic rejection, (8) postpartum events, (9) the season, (10) chemicals, (11) somatic diseases, and (12) starvation.

An evolutionary perspective on bipolar disorder integrates evolutionary history with psychoneuroimmune mechanisms to explain the various factors that contribute to mood changes between depression and mania. It also highlights the evolutionary novelty of many of these factors. Manic and hypomanic episodes are often triggered by stressful life events, partially because stress can impair the functioning of the internal circadian clock. Stress can also induce gut dysbiosis, low-grade systemic inflammation, and neuroinflammation. However, increased dopaminergic activity observed during mania can suppress inflammatory processes. The anti-inflammatory effect of dopamine can explain the absence of typical manifestations of sickness behavior (such as slow psychomotor activity, lethargy, weakness, and anhedonia) that characterize depression and other inflammatory states but that are absent in mania. In the depressive phase of bipolar disorder, in contrast, the dopaminergic system is downregulated, and symptoms of sickness behavior tend to be present. Astrocytes are another proximate mechanism that dysregulates circadian rhythm, leading to bipolar mood shifts via low sleep quality and quantity. The evolutionary psychoneuroimmunological model presented in this book indicates that bipolar disorder is a result of an environmental mismatch between the environments in which humans evolved and the environments and lifestyles that are typical for contemporary western populations.

An evaluation of existing treatment practices for depression shows that they do not represent effective ways of treating this disorder. For instance, there is no definitive epidemiological evidence that antidepressants would be beneficial as a depression treatment. Moreover, antidepressants can have many side effects. The so-called monoamine hypothesis – according to which depression is caused by a lack of serotonin or other neurotransmitters in the brain – is outdated. Psychotherapy is only marginally more effective than placebo treatments, and this difference is below cut-offs for clinical significance. As there are different subtypes and underlying causes of depression, the same treatment is not expected to work for every patient. Classifying depression into subtypes based on evolutionary psychiatry is a novel approach – so novel that the effectiveness of specific treatments for all different subtypes of depression has not yet been tested. This chapter summarizes current knowledge on possible subtype-specific treatments for depression. Grounded in an evolutionary psychoneuroimmunological framework, the chapter emphasizes the importance of identifying the triggers, functions, and biological pathways underlying each depressive episode. By aligning treatment strategies with individual etiology and pathogenesis, this approach aims to improve treatment outcomes, reduce chronicity, and move toward more precise and intelligent depression interventions.

An evolutionary approach to depression seeks to analyze whether similar behavioral patterns that characterize depression in humans also occur in nonhuman animals – and if so, whether the events or circumstances that trigger such behaviors are analogous between humans and nonhuman animals. In nonhuman animals, triggers fvof depression-like behaviors include circumstances like being kept at zoos, being exposed to air pollution, experiencing sleep disturbances, or experiencing various stressors like electric shocks or separation from mother. In humans, evolutionary psychiatrists have criticized the diagnostic methods of depression used in psychiatry, partially because depressed mood is automatically classified as a disorder regardless of what caused it. An evolutionary perspective recognizes the possibility that negative emotions and mood states can be beneficial to the individual under some circumstances. This approach can help clarify the contexts, causes, adaptive functions (if any), and possible maladaptive manifestations of depressive symptoms, providing a framework for more intelligent, personalized, effective treatment strategies. Anthropological evidence indicates that depression is much more common in populations with contemporary western lifestyles than in populations with traditional or ancestral lifestyles. Understanding depression through an evolutionary lens helps clarify its biological roots, environmental triggers, and the reasons for its increasing prevalence in the modern world.

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’).

Discovering why natural selection has left humans vulnerable to mental disorders will make psychiatry more sensible and effective, but defining the appropriate objects and kinds of explanation remains challenging. Asking how a disorder increases fitness is a mistake; disorders are not adaptations and they do not have evolutionary explanations. The correct objects of explanation are the traits that make all members of a species vulnerable to a disorder. Task 1 is to describe the evolutionary origins and functions of the traits involved. Task 2 is to describe the proximate processes that result in the disorder. Task 3 is to discover why natural selection left the traits vulnerable to malfunction. Five main kinds of explanation need to be considered: stochasticity, path dependence, mismatch, trade-offs that benefit the individual and traits that benefit gene transmission at a cost to the individual. Depression, addiction, eating disorders, autism and schizophrenia are used to illustrate the opportunities and challenges of framing and testing hypotheses about vulnerability. Multiple explanations are often needed for a single disorder, frustrating the wish for simplicity. However, recognising the fundamental differences between organic and designed systems offers opportunities for resolving – or at least understanding – some enduring controversies in psychiatry.

In Western culture, both the lay public and mental health professionals tend to believe that mothers evolved to love all of their children instinctually and unconditionally. In contrast, any mother who feels ambivalence or hostility towards her child is typically seen as unnatural, and a mother who maltreats her child is seen as behaving pathologically. This chapter draws on evolutionary research to challenge this widespread view of motherhood. In particular, it describes how raising children has required mothers to negotiate a series of complex, precarious and layered trade-offs, and it argues that maternal negativity and child maltreatment can arise from this. The goal of this chapter is to foster a more evolutionarily valid, nuanced and compassionate understanding of motherhood. Such an understanding has the potential to contribute to clinical work with faltering mothers as well as to programmes focused on preventing maternal maltreatment of children.

Anxiety disorders make sense only in the evolutionary context of the origins and functions of normal anxiety. Anxiety is an adaptation that adjusts diverse aspects of individuals in ways that increase fitness in dangerous situations. Subtypes were partially differentiated by different dangers. Anxiety is not fully differentiated from other aversive emotions, especially low mood. Anxiety disorders result when regulation systems fail. Explaining them requires considering five possible reasons for vulnerability. However, much harmful anxiety arises from normal mechanisms. These insights are valuable in the clinic, and they suggest new research initiatives.

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.

Psychiatric therapeutics is facing a major crisis that originates from its limited efficacy and dubious scientific credibility. Such a crisis requires a radical change in the paradigm that inspires psychiatric research and clinical practice. The new paradigm should integrate in a meaningful way all of the variables that mediate the aetiology and pathogenesis of psychiatric disorders: biological, psychological, developmental, behavioural and social. The essence of the evolutionary study of human psychology and behaviour is just such an integration. Therefore, evolutionary thinking has great potential to improve the clinical care of psychiatric patients. According to a modern view of medicine, the aims of therapy are not only to lessen symptoms and to reverse the pathogenic mechanisms, but also to restore the congruence between a patient’s functional capacities and the conditions of the environment. Evolutionary thinking suggests replacing symptoms with functional capacities as the primary targets of psychiatric treatment. Most mental disorders are conditions of compromised functional capacities. Therapy should aim at improving patients’ capacities necessary to enact behaviours associated with goal achievement. When treating patients, clinicians should distinguish between symptoms caused by dysfunctional mechanisms and symptoms that are adaptive reactions to environmental situations with negative cost–benefit outcomes.

We propose that major depressive disorder is not a unitary disease. Instead, different triggering factors causing periods of low mood can give rise to different and sometimes even opposite symptom patterns. Some of the symptoms of depression are maladaptive; others may be psychobehavioural adaptions to solve the adaptive problem that triggered the depressive episode. It is therefore logical to subtype depressive episodes according to their triggering factors. In evolutionary psychiatry, depressive episodes can be classified into discrete subtypes that are induced by infection, long-term stress, loneliness, traumatic experience, hierarchy conflict, grief, romantic relationship dissolution, post-partum events, season, chemicals, somatic diseases and starvation. In hunter-gatherers and in people who have traditional lifestyles, periods of low mood only rarely turn into episodes that fulfil the diagnostic criteria of major depressive disorder. Modern lifestyles cause low-grade inflammation and an increased susceptibility to chronic stress, which introduce symptoms of sickness behaviour into reactive short-term mood changes. Therefore, features of contemporary environments may prevent the normalisation of mood after adverse life events, resulting in major depressive disorder. An evolutionary approach to depression helps to identify the factors in our environments and lifestyles that contribute to greater susceptibility to this debilitating disorder, which can inform both prevention and treatment of depression. We further propose that the treatment of major depressive disorder should be tailored according to the patient’s depression subtype, focusing on the root causes of the disorder rather than alleviating symptoms with drugs.

Sexual concerns and dysfunctions are common, complex, and result in significant distress for individuals and couples.In spite of the vital role sex plays in our lives, defining sexual function and dysfunction is complicated by the multitude of variables impacting sexual behavior: biological, instinctual, emotional, interpersonal, social, cultural, as well as personal sexual history.Further, our understanding of human sexuality is currently in flux in response to changing sociocultural interpretations.The inconsistency with which we currently define typical sexual function is complicated by our incomplete understanding and acknowledgement of how evolutionary influences impact sexual function.Human sexuality is influenced not only by our socially constructed ideals, but also by a deeper physiology that transcends cultural ideals and understanding. In this chapter, we argue that some proportion of a person’s sexual experience is generated via unconscious means through evolutionary forces manifesting as innate reactions and desires.These evolutionary influences manifest in human sexual behavior in nuanced yet profound ways.Thus, conscious sociocultural decisions about what constitutes “appropriate” sexual function may be of limited value in the privacy of people’s bedrooms without embracing evolution’s impact. The unfortunate tendency to disregard evolutionary biology by the mental health community leaves patients with an incomplete understanding of their sexual and relationship struggles.Further, this lack of understanding risks our labeling typical female sexual function as dysfunctional, and translates into less effective treatments for those suffering with sexual concerns.Evolutionary theory does not offer insight into all human sexual function and dysfunction.However, it is a relevant aspect of the story for many individuals who struggle sexually.Acknowledging our sexual adaptations and instinctive tendences results in less pathologizing of sexual desires, less shaming of ourselves and others, and less self-blame and personalization of a partner’s sexual inclinations.Sexual satisfaction and self-acceptance have dramatic implications in people’s lives and relationships that extend far outside the bedroom.Sexual satisfaction impacts relationship satisfaction and even life satisfaction.Clinical experience reveals how encouraging the sexual vulnerability inherent in our more primitive sexual tendencies make sex more satisfying, and perhaps more importantly, it deepens lovers’ emotional connection. The most loving, open-minded thing we can do for ourselves is to embrace our humanness, rather than deny it.This chapter explores ways evolutionary forces may impact female sexual function and dysfunction.Greater clarity on evolutionary influences impacting people’s sexual experience can support women and their partners in creating the sex lives they long for, as well as assist medical practioners in developing more targeted and thus more effective treatment plans for their patients.

The main areas of male sexual disorders recognized by sexual medicine are paraphilic disorders and sexual dysfunctions This chapter offers an overview of the contributions of evolutionarily informed approach to the aetiology of male paraphilic disorders, with some space dedicated also to an evolutionary view of male sexual dysfunctions. First, the details of an evolutionary sexology (ES) approach are explained: (1) ES is relatively independent of current sexologic diagnoses. In contrast with the traditional medical and legal view, it is also independent of current sociocultural norms; (2) An advantage of ESis that symptoms themselves might be viewed as separate units of selection or parts of different complex evolutionarily relevant phenomena; (3) recent evolutionarily informed theories explain sexual disorders using a combination of nature and nurture reasoning; (4) ES combines the theoretical approach ofevolutionary psychology and ethology with knowledge of human phylogenetic history and selective pressures in the pleistocene environment of evolutionary adaptedness. An evolutionary framework views some male sexual disorders as adaptive behavioral strategies selected over evolutionary history because they increased the reproductive success of our ancestors, but they might also be viewed as maladaptive margins of the variability within the normophilic spectrum. A major part of the chapter is dedicated to evolutionary perspectives on paraphilic sexual interests (paraphilias) and disorders (according to current diagnostic manual (DSM-5, American Psychiatric Association [APA], 2013), the latter characterized by either paraphilia-related distress or antisocial behavior). Throughout the chapter, we distinguish between paraphilias and sexual offending. We provide evidence for the biological as well as developmental correlates of paraphilia and explain their possible placement in multidimensional sexual orientation space. We also summarize recent evidence on the prevalence of paraphilias in current populations. Finally, the dominant evolutionarily informed theories of the aetiology of common paraphilic interests and disorders (i.e., chronophilia, fetishism, nonconsent related paraphilias, and sadomasochism) and male sexual dysfunctions are described.