Purpose – The purpose of this paper is to contribute to a better understanding of the complex relationship between ethnic diversity, social capital, and objective career success in upward mobility systems over time. The authors conceptualize the underlying process of why intra-organizational career boundaries are more permeable for dominant ethnics compared to minority ethnics. Design/methodology/approach – The authors conceptually explore and model this relationship by elaborating on three mechanisms of social capital return deficit proposed by Lin (2000), building the argument based on four underlying principles (stereotype fit, status construction, homophily, and reciprocity). Findings – Based on a proposed reciprocal relationship between social capital and objective career success, the authors suggest the development of an upward career spiral over time, which is continuously affected by ethnic group membership. Consequently, the authors argue that dominant ethnics do not only advance to a higher level of objective career success, but that they also advance exponentially faster than minority ethnics. Research limitations/implications – The conceptualization provokes the question to what extent the permeability of intra-organizational boundaries constrains careers of some, while enabling careers of others. Originality/value – The contribution lies in the exploration of the relationship between social capital and objective career success over time, of the permeability of intra-organizational career boundaries, and how both are affected by ethnic group membership.