Changing institutions is an integral part of an academic life. Yet little is known about the mobility patterns of scientists at an institutional level and how these career choices affect scientific outcomes. Here, we examine over 420,000 papers, to track the affiliation information of individual scientists, allowing us to reconstruct their career trajectories over decades. We find that career movements are not only temporally and spatially localized, but also characterized by a high degree of stratification in institutional ranking. When cross-group movement occurs, we find that while going from elite to lower-rank institutions on average associates with modest decrease in scientific performance, transitioning into elite institutions does not result in subsequent performance gain. These results offer empirical evidence on institutional level career choices and movements and have potential implications for science policy. Despite their importance for education, scientific productivity, reward and hiring procedures, our quantitative understandings of how individuals make career moves and relocate to new institutions, and how such moves shape and affect performance, remains limited. Indeed, previous research on migration patterns of scientists [1, 2] tended to focus on large-scale surveys on country-level movements, revealing long-term cultural and economical priorities [3] [4] [5] [6] . At a much finer scale, research on human dynamics and mobility has emerged as an active line of enquiry [7] [8] [9] [10] [11] [12] [13] , owing to new and increasingly available massive datasets providing time resolved individual trajectories [14] . While these studies cover a much shorter time scale than a typical career, they uncover a set of regularities and reproducible patterns behind human movements [7, 10, 15] . Less is known about patterns behind career moves at an institutional level and how these moves affect individual performance. Here we take advantage of the fact that scientists publish somewhat regularly along their career [16, 17] , and for each publication, the institution in which the work was performed is listed as an affiliation in the paper, documenting career trajectories at a fine scale and in great detail. These digital traces, offering data on not only individual scientific output at each institution but also career moves from one institution to another, can provide insights for science policy, helping us understand how institutions shape knowledge, the typical moves of individual career development and help us evaluate scientific outcomes associated with professional mobility. We use the Physical Review dataset to extract mobility information, publication record, and citations for individual scientists. The data consists of 237,038 physicists and 425,369 scientific papers, out of which 4,052 different institutions are extracted after the disambiguation process for authors and affiliations (see SM for disambiguation process). To reconstruct the career trajectory of a scientist, we use the affiliation given in each of his/her publications (Fig 1) . For authors with multiple affiliations listed on a paper we consider the first affiliation as primary institution. We compute the impact of each paper by counting its cumulative citations collected 5 years after its publication [18] [19] [20] [21] . Three characteristics are computed for each institution i (Fig. 2) : the institution size (A i ), representing the total number of distinct authors that published at least one paper at institution i; the number of papers (P i ) published under affiliation i; the cumulative number of citations C i collected by all papers P i . We find that P(A) follows a fat tailed distribution, indicating significant population heterogeneity among different institutions (Fig. 2a) . While most institutions are small, a few have a large number of scientists, often corresponding to large institutes or universities. We observe similar disparity in P(C) (Fig. 2b) : few institutions acquire a large number of citations, while most research labs or universities receive few citations. Figures 2c-d show the correlation between the institution size A and both the average publications impact C/P and the average productivity P/A of institutions. The average productivity and impact of an institution are different but complementary measures of scientific performance. We find the institution size has little influence on productivity (R 2 = 0.43) (Fig. 2d ), yet it positively correlates with the impact of publications (R 2 = 0.85), indicating that large institutions offer a more innovative/higher impact environment than smaller ones as captured by citations per paper (Fig. 2c) . Also, as larger institutions have more internal collaborations, the number of co-authors in publications from large institutions might be larger and, as a consequence, attracts more citations [18] . Many institutions are small with few citations, hence they arXiv:1404.6247v1 [physics.soc-ph]
Analyzing the relocations and the scientific performance of scientists, Deville et al. REF find that while moves from elite to lower-rank institutions lead to a moderate decrease in scientific performance, moves to elite institutions does not necessarily result in subsequent performance gain.
Career on the Move: Geography, Stratification, and Scientific Impact
Deville, P.et al. Career on the Move: Geography, Stratification, and Scientific Impact. Sci. Rep. 4, 4770; DOI:10.1038/srep04770 (2014)
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