How trait confidence and communication shape dyadic decision outcomes and confidence matching.

When individuals collaborate, they often rely on momentary estimates of their own and their partner's confidence (decision confidence) to guide collective decisions and achieve their goals. Through interaction, these confidence estimates tend to align over time. This process is known as confidence matching. More stable, dispositional trait confidence is also emerging as a key factor shaping the dynamics and outcomes of collaborative action. We examined how trait confidence and type of communication impact the accuracy of dyadic decisions, decision confidence, and the dynamics of decision confidence, including decision-specific confidence matching. In this study, 210 participants completed general knowledge tests individually and collaboratively, forming 105 dyads. The tests were completed under three communication conditions: isolated (no interaction), passive (viewing the partner's response and numeric confidence rating), and active (verbal discussion). Participants assessed as high-trait or low-trait confidence were allocated to three types of dyads: low-trait (two low-trait members), mixed-trait (one low-trait and one high-trait member), or high-trait (two high-trait members) confidence dyads. Statistically controlling for cognitive ability, trait confidence moderated decision accuracy and decision confidence gains: dyads with mixed-trait or high-trait confidence showed greater decision accuracy improvements in the active than the passive communication condition compared to their individual decisions. Whereas low-trait confidence dyads benefited equally from active and passive communication. Collaboration increased decision confidence overall, especially for high-trait confidence dyads under active communication. Decision-specific confidence matching occurred rapidly in both passive and active communication but predicted decision accuracy gains only in the passive condition where participants had limited social information. Although active verbal communication led to the greatest overall decision accuracy, these gains were not driven by decision-specific confidence matching. Our findings highlight the critical role of trait confidence in shaping collaborative outcomes in dyads and extend previous research by showing that decision-specific confidence matching occurs naturally during verbal communication. SIGNIFICANCE STATEMENT: When two people collaborate to make decisions, we often assume that "two heads are better than one." However, the benefits of dyadic decision-making depend on how effectively group members share and interpret their confidence in judgments. Our study highlights trait confidence, an individual's stable tendency to express confidence, as a critical yet often overlooked factor that shapes the success of dyadic decisions. We found that trait confidence moderates dyadic improvements in both decision accuracy and decision confidence. Importantly, the effectiveness of dyadic collaboration depends on the type of communication: verbal discussions maximized accuracy gains for dyads with high or mixed levels of trait confidence, whereas simpler, non-verbal exchanges were sufficient for low-trait confidence dyads. Additionally, we demonstrated that dyad members naturally align their levels of confidence (a process known as confidence matching) during verbal discussions. This extends prior research showing that the language used to express confidence becomes more similar over time and that confidence matching has previously been observed only under artificial, numeric rating contexts. These insights enhance our understanding of how individual differences in trait confidence and communication modes influence collaborative decisions, providing practical guidance for effectively structuring collaborative interactions and pairing partners in applied settings.