Many human biological processes – from breathing to walking – are rhythmic. Savage et al. propose entrainment as one neurobiological underpinning of music's social effects. Here, we clarify the concept of entrainment and its neurobiological mechanisms, arguing that entrainment should take the central role in the discussion of music's social origins.

Entrainment is the process by which two – or more – oscillatory (fluctuating) processes become coupled via phase or frequency adjustment (Pikovsky, Rosenblum, & Kurths, Reference Pikovsky, Rosenblum and Kurths2003). When two oscillators are entrained, they are temporally aligned, such as when musicians synchronize to another's tone onsets (Demos, Layeghi, Wanderley, & Palmer, Reference Demos, Layeghi, Wanderley and Palmer2019). This synchronization is sustained beyond direct physical coupling, which differentiates entrainment from resonance, a term that is often (and incorrectly) used interchangeably with entrainment (Helfrich, Breska, & Knight, Reference Helfrich, Breska and Knight2019). In fact, the intention and attention required to maintain behavioral entrainment (Leow, Waclawik, & Grahn, Reference Leow, Waclawik and Grahn2018) may create the “social” nature of the signal: entrainment in musical group contexts is not simply reflexive or the result of unintentional mirroring.

When an individual entrains their actions with an external oscillation – such as the music of a partner – they achieve behavioral synchrony. Growing evidence suggests that behavioral entrainment is governed by neural entrainment, the alignment of rhythmic brain activity to external rhythmic stimuli (for a review, see Lakatos, Gross, & Thut, Reference Lakatos, Gross and Thut2019). For music, neural entrainment may alter auditory perception by guiding attention toward rhythmically salient acoustic events (Henry & Herrmann, Reference Henry and Herrmann2014; Jones, Moynihan, MacKenzie, & Puente, Reference Jones, Moynihan, MacKenzie and Puente2002). Moreover, neural entrainment to perceived rhythms facilitates detection of auditory stimulus features (Bauer, Bleichner, Jaeger, Thorne, & Debener, Reference Bauer, Bleichner, Jaeger, Thorne and Debener2018; Henry & Obleser, Reference Henry and Obleser2012), suggesting that entrainment facilitates more than simply moving along to music.

Merely hearing an auditory rhythm activates motor brain areas (Grahn & Brett, Reference Grahn and Brett2007; Grahn & Rowe, Reference Grahn and Rowe2009; Zatorre, Chen, & Penhune, Reference Zatorre, Chen and Penhune2007), particularly when the rhythm features a salient beat (Grahn & Rowe, Reference Grahn and Rowe2013); a regular, psychologically salient, recurring event in the rhythm. The beat is central to entrainment, as the beat is what people generally synchronize to, and it enables synchronization even to novel music. Perhaps unsurprisingly then, entrainment of neural oscillations to the beat occurs not only in auditory, but also in motor regions (Fujioka, Trainor, Large, & Ross, Reference Fujioka, Trainor, Large and Ross2009, Reference Fujioka, Ross and Trainor2015; Morillon & Baillet, Reference Morillon and Baillet2017). Auditory-motor entrainment may be the neural driver behind moving to music; entrainment of beat-related auditory-motor activity influences behavioral synchronization (Mathias, Zamm, Gianferrara, Ross, & Palmer, Reference Mathias, Zamm, Gianferrara, Ross and Palmer2020; Nozaradan, Zerouali, Peretz, & Mouraux, Reference Nozaradan, Zerouali, Peretz and Mouraux2015). We propose that this tight auditory-motor coupling, translating musical rhythms into action, is critical for group music-making.

Musical partners mutually synchronize auditory and motor brain activity to the sounds that they produce (Müller, Sänger, & Lindenberger, Reference Müller, Sänger and Lindenberger2013; Sänger et al., Reference Sänger, Müller and Lindenberger2012, Reference Sänger, Müller and Lindenberger2013; Zamm et al., Reference Zamm, Debener, Bauer, Bleichner, Demos and Palmer2018). In fact, when two brains are synchronized through electrical stimulation, spontaneous (Pan et al., Reference Pan, Novembre, Song, Zhu and Hu2021) and intentional (Novembre, Knoblich, Dunne, & Keller, Reference Novembre, Knoblich, Dunne and Keller2017) bodily movements also become more synchronized. Moreover, physiological activity is coupled during synchronous musical behavior (Gordon et al., Reference Gordon, Gilboa, Cohen, Milstein, Haimovich, Pinhasi and Siegman2020; Mueller & Lindenberger, Reference Müller and Lindenberger2011). Thus, interpersonal synchrony operates at behavioral, neural, and physiological levels, all of which may support the social effects of synchronous music-making.

Interpersonal synchrony has been suggested to play an evolutionary role in promoting social cohesion (Launay, Tarr, & Dunbar, Reference Launay, Tarr and Dunbar2016) by facilitating trust (Launay, Dean, & Bailes, Reference Launay, Dean and Bailes2013), affiliation (Hove & Risen, Reference Hove and Risen2009), and prosocial behavior (Cirelli, Einarson, & Trainor, Reference Cirelli, Einarson and Trainor2014). Consistent with this view, interpersonal behavioral, physiological, and neural synchrony occur between mothers and infants (for a review, see Wass, Whitehorn, Haresign, Phillips, & Leong, Reference Wass, Whitehorn, Haresign, Phillips and Leong2020), between certain animal species' movements and computer-generated or conspecific rhythms (Cook, Rouse, Wilson, & Reichmuth, Reference Cook, Rouse, Wilson and Reichmuth2013; Lameira, Eerola, & Ravignani, Reference Lameira, Eerola and Ravignani2019; Patel et al., Reference Patel, Iversen, Bregman and Schulz2009). These links between entrainment and social cohesion across adults, children, and certain animals, suggest that entrainment may be the musical feature evolutionarily selected to facilitate bonding.

Interestingly, both infants and animals are poorer at synchronization than adult humans, raising the question of whether precise synchronization and well-honed prediction mechanisms are critical for experiencing the social benefits of interpersonal synchrony, the latter being a central argument of Savage et al. Even adults show a wide range of synchronization abilities (Grahn & Schuit, Reference Grahn and Schuit2012), yet there is little evidence that poor synchronizers – adults or children – do not enjoy dancing or moving to music, nor that they experience less social affiliation afterward. In fact, anecdotally, the social affiliation within an amateur, less synchronized musical group may surpass that of a professional, highly synchronized group. Therefore, bonding can arise in social entrainment contexts regardless of individuals' ability to synchronize accurately. Thus, the ability to experience the social benefits of entrainment in group music-making is not necessarily dependent on accurate entrainment ability.

Overall, auditory entrainment to musical rhythms activates the motor system. During music-making, synchronization of actions, brain rhythms, and physiological activity occurs between musical partners. However, many non-musical behaviors involving entrained actions, such as walking side-by-side (Nessler & Gilliland, Reference Nessler and Gilliland2010), rowing (Cohen, Ejsmond-Frey, Knight, & Dunbar, Reference Cohen, Ejsmond-Frey, Knight and Dunbar2010), and dance (Chauvigné, Walton, Richardson, & Brown, Reference Chauvigné, Walton, Richardson and Brown2019), facilitate interpersonal synchrony of behavior and likely also neural and physiological rhythms. We argue that entrainment – of bodies and minds – is the key evolutionary mechanism underlying the social bonding that arises in music. The highly rhythmic structure of music – which arises from both well-defined temporal and spectral patterns – makes music a superior facilitator of interpersonal synchrony and bonding to other entrained activities; however, this hypothesis remains to be definitively proven. We hope that future research inspired by this special issue will test how group music-making specifically enhances social bonding beyond other forms of interpersonal entrainment.