Social interactions between young and senior colleagues might have consequences for union membership uptake of young workers, thus influencing public policies on unions. We apply Norwegian administrative register data to test this claim about the influence of social interactions on unionization, while addressing threats of homophily bias, contextual, and network confounding. Leveraging exogenous spillover shocks by colleagues’ siblings’ unionization to colleagues’ unionization, we find causal evidence supporting the notion that social interactions with close colleagues are important for unionization, mainly driven by social costs and information sharing. Our results suggest that one standard deviation increase in the union density of close colleagues, causes the uptake of union membership for young workers to grow by 20-23 percent. Our analyses thus reveal one source of additional spillover impacts from the implementation of public policies supporting unions. Furthermore, our results have important implications for unions’ mobilization strategies.