This introduction asks what Darwin’s writings still offer to contemporary criticism and humanist thought, highlighting the implications of his work on ecologies, human difference, and aesthetics.

Darwin’s theory of sexual selection has presented queer and trans theorists with a number of stumbling blocks, in that it centers heterosexual reproduction orchestrated between aggressive males and coy females. Some critics have suggested rectifying this problem by imagining sexual selection only with the pursuit of pleasure and aesthetics. But because it severs sexual selection from the “economy of nature,” this split is not able to offer a robust theory of how sexual object choices come into being or circulate. This chapter suggests that Darwin’s thinking about domestication may prove more useful for queer theory because it encompasses criteria pertinent to both sexual and natural selection and entails theorizing how it is that aesthetic criteria matter within the economy of a world in which an organism finds itself.

Tumultuous nineteenth-century political debates, fears of violent revolutions, and the rise of women’s rights campaigns in Britain, the United States, and France provide a context for considering Darwin’s theory of sexual selection and its engagement with feminism. The Descent of Man, and Selection in Relation to Sex (1871) identified sexual differences, male–male combat, and female choice in courtship as key elements of animal copulation, while insisting that male choice controls human sexual relations, ideas that inspired radically different reactions from feminists, who objected to what they regarded as Darwin’s sexism, and fiction writers, who highlighted women characters resisting patriarchal expectations and making independent decisions. The long history and profound consequences of the concepts of sexual difference and sexual selection call for careful consideration of the intertwining of Darwin’s scientific theories about sexual difference and choice with divergent cultural formations, ranging from social Darwinism to feminist theory, and propose a more fluid understanding of sex and gender that supersedes the earlier two-sex model.

Although largely accepted in animal biology, sociobiology has proven to be a controversial model for explaining human behavior. Nowadays sometimes termed ‘behavioral ecology’, the application of biological models to human behavior has the potential to explain a wide array of human instincts and actions. This chapter reviews the models for such putative human universals as violence, sexual reproduction strategies, coalition-building, and altruism, and compares them with similar models applied to animals. It also emphasizes that environment and culture provides critical influences on the ultimate expression of behavior: for example, across societies, mate preferences are partially mediated by society’s economic opportunities, so that culture can act as a buffer for underlying biological instincts. Since this topic is controversial, the chapter review the historical antecedents, starting with Plato’s theory of universals, through John Locke’s Enlightenment ideal, the ‘tabula rasa’, to Margaret Mead and Napoleon Chagnon in modern anthropology. Much of this debate has an underlying moral element, so the chapter discusses the naturalistic fallacy and point out the fact that our morality need not be determined in any way by potential evolutionary influences on instincts or behaviors. However, it also notes the potential logical pitfalls of treating humans as ‘special case’ animals.

This chapter provides an outline of Mendelian and molecular genetics, with a particular emphasis on how DNA provides evidence of a single origin for all living things. It discusses the role of genetics in serial homology, and how that demonstrates genetic relatedness among vertebrates especially, and it shows how homologous HOX genes provide the head–tail orientation and body plan segmentation for animals as diverse as the house fly and human, and that we see this same pattern of segmentation as far back as the Cambrian. It also explains how the independent DNA of mitochondria and chloroplasts provide evidence for a deep history of symbiosis between the host and parasite for both plants and animals. This chapter provides a support for later chapters reviewing principles of evolution and the evolutionary history of complex organisms.

Many vertebrate animals engage in masturbation and it is also prevalent in primates. Given the gregarious nature of this order, this is perhaps surprising, since, by definition, it occurs to the exclusion of others. Our research maps the masturbatory landscape of the primate order, highlighting the distribution and diverse forms self-stimulation of the genitalia takes: from an infant vervet monkey grasping his own penis in his mouth, to female chimpanzees using water spigots to stimulate their clitorises. We also examine the causation of this behavior. While autosexual behavior can be a substitute for allosexual interactions, many acts of masturbation seem to serve functions, which fall broadly under two categories: avoidance of pathogen transmission and reduction of mate competition. In terms of implications for human public health, the finding that masturbation is ubiquitous throughout the primate order, practiced by wild-living members of both sexes and all age-groups is a strong counter-argument to voices who condemn human masturbation as "unnatural."

Human offspring require intensive and extended parental care. While the needs of offspring have been argued to drive paternal investment, the evolution of male care and its patterning across cultures defy any simple story. Because human paternal investment, while often substantial in relation to other animals, is facultative rather than obligatory, there is considerable cross-cultural variability in how and how much fathers invest in their children. While cooperative and flexible parenting strategies are present across human societies, male investment is heavily influenced by life history tradeoffs. Specifically, when fitness payoffs towards time and energy allocated to mating effort outweigh parenting effort, men often invest less in their children. Moreover, although men appear to be physiological responsive to childcare responsibilities (through testosterone adjustments), paternal loss often has little effect on child survival. Rather than signifying the unimportance of fathers, it highlights the remarkable flexibility in the human family in terms of how mothers are assisted in raising multiple dependent offspring at the same time. Although some form of pair-bonding is observed cross-culturally, with mothers as the primary infant caregiver, extended kin and alloparental support are also important for offspring success. I review paternal investment from across the animal kingdom, discuss the evolution of paternal care in humans, and describe variability across individuals and groups in the ways and amounts men invest.

Sexual conflict arises from differences in the fitness interests of males and females. A trait that is beneficial for the reproductive success of one sex reduces the fitness of the other sex, resulting in opposing selection pressures on the two sexes. The two sexes need each other for reproduction, but their dependence is asymmetric. Males benefit from a higher mating rate than females, as their reproductive success is usually constrained by the number of receptive mates, while female reproductive success is limited by egg production. Sexual conflict can occur at any stage of reproductive interactions – before or during copulation, or after insemination – and over almost any aspect of reproduction, from the decision to mate to the investment into parental care. The conflict results in a perpetual tug of war between the sexes. Each sex attempts to maximize its fitness at a cost to the other sex, which results in sexually antagonistic selection. This can cause the rapid evolution of sexual traits, and ultimately results in the diversification of traits, and possibly even in speciation. Sexual conflict can manifest in two ways, intra- and interlocus sexual conflict. Intralocus sexual conflict occurs when a trait expressed in both sexes (determined by alleles in the same locus in the two sexes) has opposite effects on male and female fitness. Interlocus sexual conflict occurs when the conflicting traits are determined by alleles in different loci in the two sexes and the optimal outcome of male–female interactions differs between the two sexes. Intralocus sexual conflict generates a genetic tug of war between the sexes over the optimal trait expression, while interlocus sexual conflict can lead to open-ended cycles of sexually antagonistic coevolution. Sexual conflict before mating has resulted in a diversity of tactics and strategies that males use to overcome female unwillingness to mate, from forced copulations and sneaky fertilization to the emission of love darts. Sexual conflict after mating has favored the evolution of male traits that increase success in sperm competition, such as postcopulatory mate guarding, toxic seminal substances that destroy the sperm of other males, mating plugs that prevent other males from mating with the female, and morphological and physiological traits that harm females, such as spines on the intromittent organ that pierce the reproductive tract of the female. Females have evolved cryptic choice of sperm to influence which males fertilize her ova. Whether the sexual conflict between males and females can be resolved depends on the type of conflict. Several mechanisms may reduce the strength of intralocus conflict, such as sex-limited expression of traits, but the interests of males and females are unlikely to become aligned when it comes to interlocus conflict. The relative influence of sexual conflict on the fitness of organisms, and the degree to which it can be resolved, are open questions.

In this introductory chapter, we discuss the nexus between evolutionary theory and behavioral genetics, using it to elucidate the biological origins of human behavior and motivational predispositions. We introduce relevant behavioral genetics methods and evolutionary theoretical background to provide readers with the necessary conceptual tools to deepen their engagement with evolutionary behavioral genetics – as well as to help them take on the challenge of building a scientifically and evolutionarily more consilient account of human behavior. To demonstrate the utility of behavioral genetics in evolutionary behavioral science, our analytical examples range from personality, cognition, and sexual orientation to pair-bonding. We conclude by presenting a few recent landmark studies in behavioral genetics research with a particular focus on two aspects of sexual behavior: assortative mating and same-sex sexual behavior. This chapter considers behavioral genetics methods and their connection with evolutionary science more broadly while providing a succinct overview of recent advances in understanding the evolutionary genetic underpinnings of human sexual behavior, mate choice, and basic motivational processes. It is a sine qua non of scientifically principled evolutionary behavioral scientists to acknowledge the distal evolutionary and proximal genetic processes which, interlinked, underlie the psychobehavioral predispositions that form the variegated fabric of human societies and, more broadly, the diversity of life found in nature. These evolutionary processes operate from distal selection pressures acting on genetic material through hundreds of millions of years of natural selection –and from individual and population differences in genotypes to their manifestations in complex behavioral phenotypes and life outcomes in contemporary humans – which, in turn, enact concomitant selection pressures on the genetic material underlying and arising from them.

Over human evolutionary history, women have benefited from competing with same-sex mating rivals to acquire and retain desired mates. Winning a rivalry may lead to direct advantages, such as securing an attractive, healthy mate who has the ability and willingness to invest in a relationship, as well as possessing important resources that may help sustain future children. Simultaneously, such competition is associated with potential costs, such as jeopardizing alliances, being victimized within social networks, becoming the target of malicious gossip, or, in the case of a loss, wasting one’s time and effort that could have been allocated elsewhere. Here I first present the evolutionary framework that underpins women’s intrasexual mating competition, and then review the existing literature on the specific ways that this competition is manifested. Attention is especially paid to competition via physical attractiveness, namely women’s efforts to improve or enhance their attractiveness, given men’s universal tendency to prefer attractive mates. I focus on how this competition typically utilizes indirect aggression tactics and relies on women behaving in a strategic manner that depends on the local environment, such as the number of available mates, the mate value of potential rivals, and concerns about maintaining one’s reputation.

The present chapter advances the view that women’s mate preferences can be grouped into at least two overarching domains: competitiveness and fatherhood. Theoretical and empirical considerations suggest that female mate preferences evolve in contexts of male competitiveness and often amplify the effects of male–male competition. Evidence for the importance of male–male competition and female choice for competitiveness in humans is reviewed. Evidence is likewise offered for the importance of human fatherhood as an additional domain of female choice outside of male competitiveness. Implications of more inclusive mate preferences for the evolution of cognitive architecture are discussed alongside the social and ethical implications of female choice for competitiveness.

To understand the sexual behavior of chimpanzees (Pan troglodytes), we must focus on factors such as physiological characteristics, ecological constraints, and the different social behaviors found in individual groups. This chapter provides an overview of some central ideas in the study of chimpanzee sexual behavior. We begin with an overview of the main physiological characteristics of chimpanzees, including the characteristic sexual swelling (i.e., pronounced perineal swelling during the estrous period of females) and the complex behavioral patterns around these physiological characteristics. Then we consider the different sexual behaviors observed across different sites. These descriptions include several categories of behaviors commonly observed, such as courtship behavior, consortship, sexual interference, and mating calls. We include, when appropriate, a description of the different ways in which these behaviors are displayed by males and females and by mature and immature individuals. Then we examine the interplay among sexual behavior, social patterns, and natural selection. We examine how understanding sexual behavior in the context of sexual selection helps us appreciate the interactions that occur in mating, including the relationship between rank and mating choices. This section includes a brief discussion of how rearing conditions, in particular isolation and human-rearing, influence the development of sexual behavior in chimpanzees. Next, this chapter provides examples of variations in sexual behaviors across different chimpanzee communities. The aim of this section is twofold, first, to show how, despite similarities across groups, we must be careful when asserting generalizations about chimpanzee sexual behavior and, second, to underscore the importance of considering variation across different sites so that we can obtain a more accurate picture of sexual behavior. Finally, we provide a brief discussion of the limitations of using chimpanzee sexual behavior to understand the evolution of human sexual behavior. Despite the origins of the study of chimpanzee sexual behavior as a tool to understand the evolution of human sexual behavior, it is necessary to recognize the limitations of extrapolating from chimpanzee sexual behavior to human sexual behavior. Given the ecological differences among these two species and variations among different populations of chimpanzees, extrapolations must acknowledge potential differences.

Parental Investment Theory, as an overarching theory in evolutionary biology, has not only deepened our understanding of sexual selection and mate preferences, as is evident in this chapter, but has also contributed to our understanding of the underlying mechanisms producing sexual behavior in all sexually reproducing species, including humans. In evolutionary biology, parental investment, as formulated by Robert Trivers in 1972, is any cost or expenditure (e.g., resources, time, energy) associated with raising offspring that increases that offspring’s chances of survival or reproductive success, and reduces a parent’s ability to invest in other or future offspring. There are many applications of Parental Investment Theory when considering the behavior of sexually reproducing species; however, the current chapter focuses on the implications of the evolution of asymmetrical parental investment for human sexual psychology. Parental investment theory has inspired new theories in the human evolutionary sciences addressing sexual preference and mating behaviors, including sex difference in sexual preferences and attraction tactics, emergence of intrasexual competition and intersexual selection, cognitive biases in perceptions of sexual intent, sexual coercion and rape, female coyness and sexual regret, mate guarding and sex difference in sexual jealousy, and sex differences in the consumption of sexually explicit content and the psychology of extramarital relationships. This chapter considers how knowledge of asymmetric parental investment, which is a result of the evolution of sexual reproduction, has contributed to our understanding of sexual behavior and psychology. Finally, ecological variation in parental investment across different human populations due to environmental harshness and demand, as well as the importance of cross-cultural research in human sexual psychology, are discussed.

Darwin found that many animals had characteristics that were difficult to explain in terms of natural selection (i.e., the gradual process in which organisms better adapted to their environment survive and reproduce more successfully over sufficiently long time periods). He proposed a new selective force, sexual selection, which refers to the process generated by differential sexual access to opposite-sex mates. The process of sexual selection, in its classic conceptualization, consisted of two components: male–male competition, resulting in built-in weapons, and female choice, resulting in ornaments. Following Darwin, sexual selection is often divided into two forms: (1) intrasexual selection, in which members of one sex, most often males, compete with one another to gain sexual access to opposite-sex mates; and (2) intersexual selection, in which individuals of one sex, most often females, choose among individuals of the opposite sex as mates. The two forms of sexual selection have been investigated in humans across cultures, producing a large body of work on psychological similarities and differences between women and men in the context of mating. Post-mating sexual selection and its effect on sexual psychology have also gained increasing research attention in the last two decades. Two post-mating strategies in sexual selection are discussed: sperm competition (the competition between the sperm of two or more males to fertilize the egg(s) of a single female) and mate guarding (behaviors used to maintain reproductive opportunities and sexual access to a mate). Previous applications of sexual selection to sexual psychology and future directions in integration of multiple perspectives in evolutionary social sciences are discussed.

Why do men go to such trouble to shave off their facial hair? A man’s beard represents an especially salient secondary sexual characteristic: a physical feature that develops at puberty and distinguishes men from women. In nature, such sexual dimorphism has been believed, since Charles Darwin, to participate in so-called sexual selection – females choosing the fittest males based on such characteristics. Thus, according to Darwin’s account, a lion cropping his mane would be tantamount to committing evolutionary suicide. But, that’s exactly what human males do. Why and when they do so raises interesting questions concerning the origins of shaving equipment and technique as well as the changing tastes of both women and men. Cultural change can be a matter of both function and fancy; such change can even trump the physical results of nature itself.

Behavioral innovation, the ability to invent new behaviors and/or use preexisting behaviors in a new context to respond to a novel situation, can be critical to an individual’s survival (i.e., natural selection). Less studied is how innovation can be critical for mating success (i.e., sexual selection). Bowerbirds are an excellent system to study the latter, given the likely importance of sexual selection to their diversification. Bowerbirds are a family of birds that show remarkable diversity in their unique construction of courtship arenas out of sticks and use of various colored objects as decorations. In this chapter, I give background on what bowerbirds are and present inadvertent evidence from experimental manipulations of their off-body sexual displays that bowerbirds are extremely flexible in their behavior. The bulk of the chapter reviews experiments in which novel problem-solving tasks were presented to bowerbirds and then their performance was compared to their mating success. I conclude by suggesting that an important future research goal should be to study how innovativeness affects the speciation process via sexual selection.

From an evolutionary perspective, aggression is viewed as a flexible context-specific adaption that was selected for because it enhanced the survival and reproductive success of ancestral humans. Evolutionary pressures have impinged differentially on the sexes, leading to the hypothesis that sex differences should be manifest in aggressive behavior. Evidence to date supports key predictions made from sexual selection theory that women direct their aggression primarily toward same-sex competitors, which peaks as mate competition intensifies. Women demonstrate a notable preference across cultures for more indirect, as opposed to direct, forms of intrasexual rivalry as a likely consequence of heightened obligatory parental investment, lower lifetime reproductive potential, and the greater importance of maternal survival for the health and longevity of offspring. An evolutionary approach can yield unique insights into the sex-differentiated functions, development, and outcomes of aggressive behavior.

Chapter 6 argues that evolutionary theories of crypsis and display served as models for thinking through the positions of disempowered, marginalised groups at the turn of the century. Israel Zangwill sometimes invoked protective mimicry to decry Jewish assimilation as the degenerate defence mechanism of helpless dependents. Persecution, he complained, had driven Jews to become invisible and, thus, alienated from their own nature. Conversely, Charlotte Perkins Gilman made sense of women’s perceived weakness via the conspicuous display of sexual selection. Gilman argued that women’s eye-catching, impractical clothes reflected their feeble dependence on men. Through their rhetoric of standing out and blending in, both authors sought to recover the imagined authentic essences of their group identities, which regimes of Gentile or male surveillance had repressed. Yet they also imagined this self-realisation being asserted through visual display. The intersubjectivity of display rendered it inherently inauthentic, mediated by arbitrary symbols. This contradiction caused Zangwill’s vision of Jewish self-realisation to vacillate between essentialism and anti-essentialism, between a return to pure origins and progression toward open-ended, heterogeneous identity. Gilman’s vision similarly vacillated between the restoration of a primordial female ‘modesty’ and the progressive transcendence of visible sex distinctions.

The Introduction outlines the historical context in which evolutionary theories of animal mimicry, concealment and display (grouped under the term ‘adaptive appearance’) emerged, and how they interacted with broader Victorian culture. It is argued that adaptive appearance constituted a shared discourse in which scientific arguments overlapped with more general ideas about appearance, perception and semiosis that can be traced in the period’s literature. Adaptive appearance was symbiotic with new models of communication that located meaning in receiver interpretations instead of sender intentions. In this way, it challenged the Cartesian binary between semiotic humans and mechanistic animals, suggesting that non-humans might be imagined as performers, deceivers and interpreters. This implication problematised the ideal of scientific objectivity, framing subjective perceptions and misperceptions as intrinsic to nature’s processes. Equally, adaptive appearance inspired reimaginings of human social interactions involving appearance and interpretation as biological processes instead of moral acts. It is suggested that, by materialising mind and meaning, adaptive appearance in some ways prefigured contemporary theories of biosemiotics and zoosemiotics. However, such discourse was not necessarily politically progressive. Adaptive appearance was ideologically adaptable, triggering different connotations for different observers, at once divine presence and absence, progress and degeneration, creative individuality and mindless conformity.