Purpose: There is a clear gap in the literature regarding the factors that determine social conductivity (extent of interactions) in project relationships. This study aims to use social capital theory to answer the research question -- What factors determine social conductivity in project relationships? Design/methodology/approach: In the study, the authors use fuzzy synthetic evaluation based on the findings from a survey of 203 project practitioners to quantify the impacts of three principal components of factors determining social conductivity in project relationships. Findings: The study findings reveal the structural, relational and cognitive dimensions of project social capital are all significant (impact levels exceeding 3.50) for determining social conductivity in project relationships. However, the study's main finding reveals -- issues relating to the cognitive dimension of project social capital are the most critical factors determining social conductivity in project relationships (a criticality index of 4.326). Practical implications: This study provides evidence of varieties of ways social capital can boost the conductivity of project relationships. Originality/value: The key contributions of this study are linked to the manner in which project social capital determines social conductivity in project relationships. The findings add to previous research by extending the dimensions of the factors that increase social conductivity from relational project social capital to include two new dimensions (the cognitive and structural dimensions of project social capital).