Previous research has established that interpersonal similarity can influence knowledge sharing in such a way that similar people are more likely to share knowledge than those who are dissimilar. We contribute to the literature by showing that in the MNC context, cultural and functional similarity can become more powerful sources of bias than more commonly assumed demographic characteristics such as gender or seniority. This may be driven by the salience of such boundaries in the MNC: while demographic characteristics are more broadly distributed, cultural and functional barriers create more defined and observable faultlines, often coinciding with unit boundaries. We further argue that it may not be similarity as such that matters but rather its positive impact on different dimensions of social capital, which mediate the relationship between similarity and knowledge sharing. These microfoundations of inter-unit knowledge exchange point to important theoretical and practical implications for international management.