the source. Age, Cohort and Co-Authorship

at several universities, and especially John McDowell for helpful comments, and to James Brand and Callum Porter-Harris for superb research assistance. Financial support for data collection was provided only by Royal Holloway University of London. The views expressed herein are those of the author and do not necessarily reflect the views of the National Bureau of Economic Research. NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications