Introduction

Psychologists who adopt the evolutionary paradigm (see Nettle & Scott-Phillips, 2021) seek to document how information processing of the mind has been engineered to address the unremitting challenges of survival and reproduction. Its predictive framework has been broadly adopted within the social sciences, including anthropology (Fessler et al., 2015; Gibson & Lawson, 2015), sociology (Tanskanen & Danielsbacka, 2018), consumer research (Otterbring, 2021; Saad, 2017), decision science (Morris et al., 2021), animal behavior and cognition (Vonk, 2021), political science (McDermott & Hatemi, 2018; Petersen, 2020), and law and social policy (James et al., 2020; Palomo-Vélez & van Vugt, 2021). Natural selection of genes is a popular level of analysis at which to propose and test evolutionary hypotheses because DNA is the fundamental unit of inheritance in sexually reproducing organisms (see Williams, 1966). Biological evolution occurs when populations undergo cross-generational change in heritable trait frequency. These traits are pitted against the reproductive and survival demands of life, and those which better promote self-replication compared to competing alternatives become more prevalent. However, evolutionary change occurs in any cyclic system where modification by competitive replacement occurs over time, such as in neural network modeling (Badcock et al., 2019; Hasson et al., 2020; Stanley et al., 2019), cellular growth (Aktipis, 2020), decision-making (Morris et al., 2021), or multilevel selection (Hertler et al., 2020; Wilson & Coan, 2021). Each application shares the common premise that if you study how evolution has engineered a system, you will discover that system’s functional design.

The evolutionary study of romantic relationships has accordingly uncovered the adapted psychology underlying intimate relationships (Bode & Kushnick, 2021; Buss & Schmitt, 2019; Durante et al., 2016). For example, it is well documented that reproduction entails unique adaptive challenges for men and women (e.g., paternal uncertainty and minimum investment in gestation), which create unique mating optima for each sex (Trivers, 1972; see Mogilski et al., 2021). These optima can conflict (see Kennair et al., this volume) causing sexual selection for different ideal mating strategies for men and women (Buss & Schmitt, 1993; Luoto 2019; Puts, 2016). Understanding how attraction and competition occurs between the sexes thus becomes a useful framework for predicting how people initiate, maintain, and dissolve their relationships. That is, the collaboration and conflict that people experience within their relationship(s) may follow computationally adaptive scripts that—at least across deep evolutionary time—alleviated the exigencies of men’s and women’s unique reproductive challenges. These foundations and their empirical support are reviewed in Chapter 1 and referenced throughout this volume.

Of course, good theory reliably and expansively predicts the phenomena that it explains. Relationship researchers who use the evolutionary paradigm have rapidly integrated it with gender and women’s studies (Fisher et al., 2020; van Anders, 2013), sexuality (Diamond, 2021; Sommer & Vasey, 2006), marriage and family studies (Aspara et al., 2018), neuroendocrinology (Welling & Shackelford, 2019), mating cognition (Joel & MacDonald, 2021; Lenton & Stewart, 2008; Miller & Todd, 1998), intelligence (Baur et al., 2019; Miller, 2000), and comparative psychology (Fraley et al., 2005). This wellspring of novelty has matured to create robust, replicable models of mate choice (Conroy-Beam et al., 2019, 2021; Walter et al., 2020), same-sex competition (see Krems et al., in this volume; also Ayers, 2021; Bradshaw & DelPriore, 2022; Reynolds et al., 2018), friendship (Seyfarth & Cheney, 2012; Williams et al., 2022), jealousy (Buss, 2018; Edlund et al., 2018), face and body perception (Antar & Stephen, 2021; Brown et al., 2021; Fink et al., 2018), and interpersonal deception (Desrochers et al., 2021; Redlick & Vangelisti, 2018; Trivers, 1991). Research once dominated by self-report and forced-choice paradigms has developed multivariate solutions for describing the logic of partner choice (Brandner et al., 2020; Csajbók and Berkics, 2022  Li et al., 2002; Mogilski et al. 2014, 2017, 2018; Jones, 2018; Stephen et al., 2017), relationship maintenance (Vowels et al., 2021). Behavioral genetics (e.g., twin studies) have disentangled the contributions of genes and environment to variation in human development (Kupfer et al., 2022), and applied sciences, such as medicine and mental health (Nesse, 2019; Giosan et al., 2020; Hollon et al., 2021) have advanced how knowledge of evolutionary design can improve personal and relational outcomes.

This diaspora has developed alongside larger trends within relationship science, such as the rising utility and rigor of personality measurement (Del Giudice, 2017; Durkee et al., 2022; Lukaszewski et al., 2020). Personality science has become central to relationship science because it models how natural variation between individuals impacts interpersonal functioning across time and ecology. Life history theory, for example, meaningfully explains how human cognition is altered by environmental unpredictability and harshness (Simpson et al., 2017). When environments are less predictable, planned investments are unsteady and cost prone. This favors immediate over delayed reward and thereby disposes individuals to present-oriented decisions (Frankenhuis et al., 2016; also see Fenneman & Frankenhuis, 2020; Fennis et al., 2022), less deliberation (Wang et al., 2022), more unrestricted sociosexuality (Szepsenwol et al., 2017), greater interpersonal antagonism and detachment (Jonason et al., 2017), and poor emotional control (see Szepsenwol et al., 2021)—traits which may aid lone survival amid environmental irregularity. As an exemplar of evolutionary theorizing, life history has been successful in integrating findings from varied disciplines (Nettle & Frankenhuis, 2019) and across cultures (Pelham, 2021), but it has accordingly inspired controversy and revision (see Dinh et al., 2022; Woodley et al., 2021; Zeitsch & Sidari, 2020).

Neuroendocrinological research has complemented evolutionary relationship science’s focus on individual differences (Baugh et al., 2017; Trillmich et al., 2018) because hormones mediate sex/gender differences and the intimate processes that develop within and between people (Edelstein & Chin, 2018; Roney & Simmons, 2018; also see Welling & Shackelford, 2019). Corticosteroids guide responsivity to relationship stressors (Mogilski et al., 2019b), oxytocin promotes pair-bonding formation (Walum et al., 2012), and sex hormones shape the morphological and psychological characteristics that systematically differ between men and women (Gurvich et al., 2018; Rehbein et al., 2021)—traits to which people are sensitive when assessing a potential intimate partner (Jones & Jaeger, 2019; Marcinkowska et al., 2014). Indeed, morphometric analyses have allowed researchers to quantify developmental change in sex/gender to study how its fluctuation impacts relationship process (e.g., Stephen et al., 2017). Others have documented how sexual behavior and preferences shift with hormone deficiency (Shirazi et al., 2021), hormonal contraception use (Hill, 2019), and temporal fluctuations in hormone levels, such as across the menstrual cycle (see Havlicek & Roberts, this volume). Endocrinology has thus become a fundamental level of analysis for the evolutionary study of romantic processes (Denes et al., this volume; Makhanova, this volume).

International differences are a substantial source of natural variation in relationship behavior (Kline et al., 2018; Segall et al., 1990; Silan et al., 2021). Evolutionary theorizing has revealed pervasive, culture-sensitive psychological effects related to gender (Lippa, 2010), kin favoritism (Schulz et al., 2019), game theory (Pan, Gelfand, & Nau, 2021), and social organization (see Henrich & Muthukrishna, 2021). Broadly, relationship behaviors are expected to adaptively shift to address local and historical demands on individuals’ survival and well-being. This thereby influences how people initiate (see Karandashev, this volume) and maintain (Adair & Ferenczi, this volume) relationships across cultures.

Studies of sexually and gender diverse people (e.g., those who are lesbian, gay, bisexual, transgender, queer, etc.; i.e., LGBTQ+) have revealed unique variation in human bonding and competition (see Holland & Lannuti, this volume; Pachankis et al., 2020; Semenyna et al., 2021; Valentova et al., this volume). For example, Diamond and Alley (this volume) argue that safety concerns are salient among LGBTQ+ relationships because same-sex attraction and gender nonconformity are targeted more often for condemnation and violence. Eliminating prejudice and wrongful discrimination against LGBTQ individuals (Blair & Hoskin, 2019) may be aided by a technical knowledge of which adaptive concerns these beliefs and attitudes have historically addressed—and whether they still do. Computation that ancestrally enhanced reproduction may be mismatched to modern circumstances (Li et al., 2020). Similarly, studying how people form and maintain multiple, concurrent intimate relationships (i.e., consensual nonmonogamy) (Mogilski et al., this volume) may reveal novel strategies for managing extra-pair romance (also see Brady & Baker, 2022; Hunter & Stockwell, 2022).

By harnessing the insights of interdisciplinary collaboration, evolutionary relationship scientists have identified novel features of human mating, have expanded durable theories and perspectives of human development, and strengthened the methodological robustness of its core predictions. The editors assembled this handbook to showcase the empirical and theoretical progress of the evolutionary study of intimate relationships. We dedicate this volume to future generations of relationship scientists. It is our intent that this collection will be a primer for those seeking to incorporate contemporary evolutionary reasoning and methodology into their research program. Many of its contributors self-identify as evolutionary psychologists. Others do not but are familiar with the evolutionary sciences and have successfully incorporated its reasoning into their work. All have challenged orthodoxy to improve how evolutionary psychology studies intimacy. The authors’ words are their own, but the editors offered feedback for improving the interdisciplinary scope of their writing. Our reflections on each chapter precede each of the three major sections of this handbook: relationship initiation, maintenance, and dissolution.

We hope that readers of this volume walk away feeling that their views on intimacy and interpersonal relationships have been enriched.