Contents lists available at ScienceDirect Biological Psychology journal homepage: www.elsevier.com/locate/biopsycho Sync to link: Endorphin-mediated synchrony effects on cooperation Martin Lang a,b, ⁎ , Vladimír Bahna b,c , John H. Shaver d , Paul Reddish e , Dimitris Xygalatas f,g a Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, 02138, USA b LEVYNA, Masaryk University, Brno, 60200, Czech Republic c Institute of Ethnology, Slovak Academy of Sciences, Bratislava, 81364, Slovakia d Religion Programme, University of Otago, Dunedin, 9012, New Zealand e Department of Psychology, National University of Singapore, 177572, Singapore f Department of Anthropology, University of Connecticut, Storrs, CT, 06269, USA g Interacting Minds Centre, Aarhus University, Aarhus, 8000, Denmark ARTICLE INFO Keywords: Synchrony Prosociality Pain threshold Endorphins Cooperation Self-other overlap ABSTRACT Behavioural synchronization has been shown to facilitate social bonding and cooperation but the mechanisms through which such effects are attained are poorly understood. In the current study, participants interacted with a pre-recorded confederate who exhibited different rates of synchrony, and we investigated three mechanisms for the effects of synchrony on likeability and trusting behaviour: self-other overlap, perceived cooperation, and opioid system activation measured via pain threshold. We show that engaging in highly synchronous behaviour activates all three mechanisms, and that these mechanisms mediate the effects of synchrony on liking and investment in a Trust Game. Specifically, self-other overlap and perceived cooperation mediated the effects of synchrony on interpersonal liking, while behavioural trust was mediated only by change in pain threshold. These results suggest that there are multiple compatible pathways through which synchrony influences social attitudes, but endogenous opioid system activation, such as β-endorphin release, might be important in facilitating eco- nomic cooperation. 1. Introduction Across cultures, people engage in collective activities that involve the matching of behaviour in time, such as music production and singing, dancing, and collective rituals (Hagen and Bryant, 2003Hagen & Bryant, 2003; Merker et al., 2009). It has long been speculated that such synchronous activities function to increase group cohesion (Durkheim, 1964; Marsh, Richardson, & Schmidt, 2009; McNeill, 1995), and experimental research has supported this con- jecture: synchronous behaviour has been shown to facilitate rapport and interpersonal liking (Hove & Risen, 2009; Lang et al., 2016; Miles et al., 2009); entitativity (Lakens & Stel, 2011; Reddish et al., 2013); cooperation in economic games (Launay, Dean, & Bailes, 2013; Reddish, Bulbulia, & Fischer, 2014; Wiltermuth & Heath, 2009); and helping behaviour (Kokal, Engel, Kirschner, & Keysers, 2011; Valdesolo & Desteno, 2011). Despite this convergent evidence, the me- chanisms mediating these effects are still poorly understood. Recently, two potential mechanism were proposed by Tarr and colleagues: one related to self-other overlap and one related to the endogenous opioid system (Tarr, Launay, & Dunbar, 2014; see also Mogan, Fischer, & Bulbulia, 2017). The self-other overlap mechanism builds on the literature describing the tight integration of perception and action systems (Gallese, Fadiga, Fogassi, & Rizzolatti, 1996; Gallese, Gernsbacher, Heyes, Hickok, & Iacoboni, 2011). Common neural encoding can be amplified by behavioural synchrony, which may, at some level of cognitive pro- cessing, lead to the blending of other-generated and self-generated behaviour (Paladino, Mazzurega, Pavani, & Schubert, 2010). Perceiving interaction partners as part of oneself may lead to feeling closer to them (Overy & Molnar-Szakacs, 2009), as shown by studies on synchronous movement and singing (Reddish et al., 2013; Weinstein et al., 2015). However, the evidence for overlap-mediated synchrony is mixed, with some studies failing to find a direct relationship (Cohen, Mundry, & Kirschner, 2013; Fischer, Callander, Reddish, & Bulbulia, 2013; Reddish et al., 2013; Wiltermuth & Heath, 2009). The β-endorphin release hypothesis, on the other hand, places em- phasis on the biochemical basis of human sociality, suggesting that synchrony leads to increased affiliative and socially rewarding beha- viour because it activates the endogenous opioid system (Loseth, Ellingsen, & Leknes, 2014; Machin & Dunbar, 2011). β-endorphin neu- rotransmitters and related μ-opioid receptors (MOR) have been pre- viously implicated to play an important role in mother-infant http://dx.doi.org/10.1016/j.biopsycho.2017.06.001 Received 30 March 2017; Received in revised form 2 June 2017; Accepted 2 June 2017 ⁎ Corresponding author at: Department of Human Evolutionary Biology, Harvard University, 11 Divinity Ave, Cambridge, MA 02138, USA. E-mail address: martinlang@fas.harvard.edu (M. Lang). Biological Psychology 127 (2017) 191–197 Available online 06 June 2017 0301-0511/ © 2017 Elsevier B.V. All rights reserved. MARK