Geography, Ties, and Knowledge Flows : Evidence from Citations in Mathematics

Using data on academic citations, career and educational histories of mathematicians, and disaggregated distance data for the world's top 1000 math departments, we study how geography and ties affect knowledge flows among scholars. The ties we consider are coauthorship, past colocation, advisor-mediated relationships, and alma mater relationships (holding a Ph.D. from the institution where another scholar is affiliated). Logit regressions using fixed effects that control for subject similarity, article quality, and temporal lags, show linkages are strongly associated with citation. Controlling for ties generally halves the negative impact of geographic barriers on citations; the distance effect became insignificant after 2004