## 4 Conclusions  
We systematically identified, critically appraised, and synthesized 126 relevant studies on inter-brain neural synchrony during social interactions since the first publication in 2002, which fitted the pre-specified eligibility criteria. We used an explicit systematic method, the preferred reporting items for systematic reviews and meta-analyses (PRISMA) to address three specific questions regarding the neuroimaging method (RQ1), the application domain (RQ2), and the experiment paradigm (RQ3). First, it is important to understand what neuroimaging methods have been frequently used, because that information can show us which hyperscanning methodologies researchers should focus on improving first to further neural synchrony research. To answer RQ1 (What is the frequency of the four major neuroimaging methods being used in hyperscanning studies? ), we systematically categorized the selected 126 hyperscanning studies by the four major neuroimaging methods: EEG/MEG, NIRS/FNIRS, MRI/fMRI and hyper-tACS. We found MEG/EEG (47%) the most widely used neuroimaging method with hyperscanning, and the hyper-tACS method was rarely applied. Second, we further categorized the papers by the application domains of cognition, decision-making, motor synchronization, and education. Applications in cognition accounted for almost half the studies (48%), while educational applications accounted for less than 5% of the studies. Applications in decision-making tasks were the second most common (26%), shortly followed by applications in motor synchronization (23%). Finally, we found that hyperscanning research studies have been experimentally designed in various settings, orientations, and group sizes within each neuroimaging method and application area. Measuring brain activity simultaneously from more than two people interacting, or \"hyperscanning\", has enhanced our understanding of the neurobiological aspects of human social interaction. However, hyperscanning literature has been narratively reviewed in that the reviews have only recorded and assessed the state of knowledge on a particular topic in hyperscanning. PRISMA is known to minimize bias and thus provide reliable findings from which conclusions can be drawn [71]. To the best of our knowledge, this is the first systematic review study that used PRISMA to compile all relevant and cutting-edge hyperscanning research to address the current state-of-the-art hyperscanning research since the first publication in 2002. Future challenges and directions from this review of hyperscanning studies include:  
* Various neuroimaging methods (e.g., hyper-tACS) are still required to assess the synchronization between multiple brains. * The inter-brain synchrony study that compares social interactions between normal individuals and groups with social cognitive impairment would be able to explore research findings on the corresponding brain regions to social cognitive impairment. * Future studies should pay more attention to more factors that may influence the anatomical and functional similarity of the two brains as well as the inter-brain synchrony. * Simple and easily interpretable experiment paradigms are still required and need to be extended to a wide range of fields (e.g., natural settings) if this is to be a useful design of social interactive experiments to conduct hyperscanning studies. This systematic review, based on the findings of documented, transparent, and reproducible searches, should help build cumulative knowledge and guide future research regarding inter-brain neural synchrony during social interactions, that is, hyperscanning research. **Author Contributions:** Conceptualization, C.S.N. ; methodology, C.S.N., S.C. and J.H. ; validation, C.S.N., S.C. and J.H. ; formal analysis, C.S.N., S.C., J.H. and J.P.; investigation, C.S.N., J.H. and J.P.; data curation, C.S.N., J.H. and J.P.; writing--original draft preparation, C.S.N., S.C. and J.H. ; writing--review and editing, C.S.N. ; visualization, S.C. and J.H. ; supervision, C.S.N. ; project administration, C.S.N.