1. Center for Science of Science and Innovation, Northwestern University, Evanston, IL, USA.
2. Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL, USA.
3. McCormick School of Engineering, Northwestern University, Evanston, IL, USA.
4. Kellogg School of Management, Northwestern University, Evanston, IL, USA.
5. National Bureau of Economic Research, Cambridge, MA, USA.
6. Brookings Institution, Washington, DC, USA.