Social anxiety mismatch is associated with reduced inter-brain synchrony and altered neural coordination dynamics...

People with high social anxiety (HSA) experience self-disclosure difficulties that disrupt interpersonal interaction. Inter-brain synchrony (IBS) has been proposed as a neural index of interpersonal coupling during social interaction, yet how dyadic social anxiety composition shapes IBS remains unclear. Using fNIRS hyperscanning, this study examined IBS during a closeness-generation task in 21 low-low (LL), 23 high-high (HH), and 23 low-high (LH) social anxiety dyads. Participants reported state anxiety, perceived closeness, and post-event processing (PEP). HSA individuals reported higher state anxiety and greater PEP than individuals with low social anxiety. PEP was shaped by the interaction between participants' own and partners' social anxiety, with higher PEP after same-anxiety interactions. LH dyads showed reduced IBS in the right superior temporal gyrus (r-STG) and right frontopolar area compared with LL dyads, and reduced IBS in the r-STG and right dorsolateral prefrontal cortex compared with HH dyads. HH and LL dyads showed comparable IBS. Associations between IBS and subjective measures differed across dyad types: higher r-STG IBS was associated with greater closeness in HH dyads, whereas in LH dyads, higher r-STG IBS and greater closeness reported by HSA individuals were associated with higher PEP. Dynamic IBS analyses showed that LH dyads had a lower probability of transitioning from lower-efficiency state to a numerically intermediate-efficiency state than matched dyads. These findings suggest that social anxiety mismatch is associated with reduced IBS, altered neural coordination dynamics, and distinct links between neural and subjective measures, highlighting the value of dyadic, dynamic, and multi-level perspectives on social anxiety.