The ‘social glue’ function of nonverbal mimicry has received much support in the
empirical literature, with research demonstrating its prosocial consequences, including increased cooperation. When looking to explain why nonverbal mimicry effects behaviour, some research has pointed to interpersonal closeness. However, in these studies, a robust measurement of nonverbal mimicry and closeness is absent, making it impossible to confidently argue that the observed mimicry resulted from increased closeness and not a third
factor. Likewise, without a reliable measure of nonverbal mimicry it is not possible to determine that nonverbal mimicry was manipulated sufficiently. This thesis addresses this by testing the impact of nonverbal mimicry on cooperation through closeness, using rigorous measures. In chapter 3 I use high-resolution motion tracking—Xsens MVN systems—todemonstrate that an increased closeness towards a partner is associated with more nonverbal mimicry of that partner. It also identified regions of mimicry (discreet body movements) that
are related to closeness. Chapter 4 showed a positive relationship between nonverbal mimicry and closeness but found no mediation effect of closeness on the relationship between mimicry and cooperation. In chapter 5, I controlled for methodological limitations in Chapter 4 and found a positive relationship between nonverbal mimicry and cooperation andsupported a mediating effect of closeness. Extending beyond mimicry within the dyad, chapter 6 showed that third-party observers would be more willing to engage in conversation
with dyads who showed increased nonverbal mimicry compared to lower amounts of nonverbal mimicry. The effects of third-party nonverbal mimicry were mediated by closeness towards the dyad. Overall, this thesis provides robust evidence for closeness as one of the psychological mechanisms underpinning how nonverbal mimicry works to increase cooperation and provides new insight into the relationship between nonverbal mimicry and
social judgements.