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147IIC in pair-bondingrelationship maintenance (Levenson & Gottman, 1983). Thus, IIC might 8be a crucial element for successful bond formation and maintenance, andconsequently, bi-parental care.Non-human animals also show IIC (Duranton & Gaunet, 2016), as canfor example be observed in turn-taking (Pika, Wilkinson, Kendrick, &Vernes, 2018) or facial mimicry (Palagi, Celeghin, Tamietto, Winkielman,& Norscia, 2020). Specifically, species with bi-parental care, such asmany bird species, might be suitable models to study the role of IIC inrelationship initiation and maintenance. Indeed, species with bi-parentalcare display IIC in mutual courtship behaviors, such as vocal duetting(Haimoff, 1986), and parental care (Griffith, 2019) (Figure 1). In thisreview, we outline evidence suggesting that IIC is prevalent in pair-bondingspecies and, from an adaptationist point of view, might confer reproductivebenefits, such as more offspring or higher offspring survival. We set outto answer two main questions. First, how is IIC reflected in differentcomponents of pair-bonding (i.e., initiation, maintenance, and bi-parentalcare)? Second, how is IIC manifested on a behavioral and physiologicallevel? Our goal is to integrate findings from psychology and ethologyand create an inter-disciplinary framework for studying the role of IIC inpair-bonding.HumansBehavioral levelIt is difficult to envision romantic interactions without coordination witha partner. Indeed, as outlined below, evidence shows that humans exhibitsubstantial IIC in the context of romantic love. In particular, patterns ofbehavioral coordination during first romantic encounters have been referredto as the human courtship dance (Birdwhistell, 1970). For example, Grammer and colleagues (1998) describe a pattern of synchrony between couplemembers, where women, when interested in their partner, synchronize theirmovements with their partner. Moreover, in a recent study (Birnbaum et al.,2019), participants were more interested in meeting a stranger again afterengaging in synchronized activity together compared to a non-synchronizedactivity. Given that IIC is associated with shared intentionality (Kurtz,Rennebohm, Teal, Charleson, & Thoburn, 2019), these findings suggest thatIIC enables bond formation perhaps by facilitating the establishment of acommon motivational framework.IIC is also crucial in the maintenance of a pair bond. Recently, SharonDavid and colleagues (2019) demonstrated that participants who imaginedhaving a synchronous interaction with their partner reported higher levels ofintimacy in their relationship, while this was not the case for imagined outof-sync interactions. Even more convincingly, Maister and Tsakiris (2016)asked participants to perform one of two behaviors: either open or closetheir mouths. Simultaneously, participants were presented with pictures oftheir romantic partner or friends (as a control group) performing the sameexpression or not. Their results showed that participants imitated their romantic partner more often and faster than a platonic friend; suggesting thatspecifically romantic affiliation is more contingent on IIC. Crucially, similarevidence supports these findings based on real-life interactions satisfied couples exhibited more movement coordination compared to dissatisfied couples(Julien, Brault, Chartrand, & Bégin, 2000). It is likely that these findingsmight also extend to emotional contagion, which is more prevalent amongstaffiliated individuals (Preston & de Waal, 2002). For example, new parentsthat report higher relationship satisfaction are also more empathic towardseach other (Rosen, Mooney, & Muise, 2017). Altogether, these findingssuggest that IIC plays an important role in relationship maintenance.Studies investigating bi-parental care and IIC in humans are at presentlimited. Two main patterns become apparent in the literature. First, maritalsatisfaction affects coordination within couples, and disruptions in coordination might consequently reduce paternal investment (Belsky, Youngblade,Rovine, & Volling, 1991; Kitzmann, 2000). Second, decreases in paternal investment might reduce parental reciprocity (Feldman, 2007), meaning thatparental behaviors are more authoritative and less responsive to the infant’sIliana Samara 17x24.indd 147 08-04-2024 16:36