It has often been observed that people like each other more after synchronising actions. This is referred to as the synchrony-bonding effect and it has been observed in a wide variety of settings - from dance to rowing. It has been suggested that music may have evolved alongside human rhythmic abilities in order to support synchronised action to promote social bonding and group cohesion. However, the precise cognitive mechanisms underpinning the synchrony-bonding effect remain opaque. This thesis aims to bring new insights to the synchrony-bonding effect by testing a novel hypothesis: that the social bonding effects of synchrony arise from processing fluency. To do so, I bring together the general literature on processing fluency as it applies to perceptual preferences and aesthetic judgements, together with existing studies of interpersonal synchrony. I then conduct three empirical studies using novel experimental designs to test the fluency through synchrony hypothesis. Study 1 found that synchrony reduces processing load on a simple visual perception task, when compared with non-synchrony. Study 2 found that synchrony reduces processing load when participating in a rhythmic tapping task, when compared with non-synchrony. Study 3 built upon Study 2 and found that perceived difficulty of a rhythmic tapping task mediated the social bonding effects of synchrony. These studies reframe synchrony as a means for reducing processing load, which could explain the synchrony-bonding effect as being an effect of processing fluency, rather than of synchrony per se. Thus, the act of synchronising may be considered an act of making oneself easier to process. I discuss these findings in the context of the broader synchrony-bonding literature, consider the implications for the evolution of rhythmic abilities, and attempt to highlight the importance of understanding synchrony in an increasingly de-synchronised world.