Does Knowledge Accumulation Increase the Returns to Collaboration?

We conduct the first empirical test of the knowledge burden hypothesis, one of several theories advanced to explain increasing team sizes in science. For identification, we exploit the collapse of the USSR as an exogenous shock to the knowledge frontier causing a sudden release of previously hidden research. We report evidence that team size increased disproportionately in Soviet-rich relative to -poor subfields of theoretical mathematics after 1990. Furthermore, consistent with the hypothesized mechanism, scholars in Soviet-rich subfields disproportionately increased citations to Soviet prior art and became increasingly specialized.