Unequal allocation alters the benefit of interactive decision-making in novices: A hyperscanning study

Equality is often considered fundamental for effective social interaction, while inequality seems to be counterproductive. Although interaction benefits under equal sharing are well-documented, how unequal reward allocation shapes such benefits and their neural basis remains unclear. This study examined dyads consisting of one "expert" and one "novice" (classified based on individual performance in baseline task) performing a joint dot-location estimation task during simultaneous functional near-infrared spectroscopy (fNIRS) hyperscanning. Three reward conditions were tested: equal reward distribution (ED), unequal distribution-expert advantage (EA), and unequal distribution-novice advantage (NA). Behaviorally, novices benefited from interaction in the ED and NA conditions, but not in EA, while experts showed no gains across conditions. Moreover, dyads compromised more under ED and NA, indicating reflecting greater mutual influence and cooperation. Neurally, inter-brain synchronization (IBS) was highest in EA between experts' frontal pole (FP) and novices' right temporoparietal junction (rTPJ) and dorsolateral prefrontal cortex (DLPFC). In contrast, both EA and NA elicited greater IBS than ED in fronto-executive pathways (expert-novice: FP-DLPFC, DLPFC-DLPFC), with NA in particular supporting novice benefit through enhanced coordination. Notably, IBS was lowest in the ED condition, despite behavioral benefits, suggesting that equal allocation may foster more streamlined or efficient neural collaboration. These findings indicate that unequal reward allocation modulates expert-novice neural coupling differently than equal allocation. Crucially, allocation favoring novices preserves behavioral interaction benefits alongside distinct fronto-executive neural synchrony patterns, revealing adaptive social and neural dynamics shaped by reward structures.