1. Sloan School of Management, Massachusetts Institute of Technology, Cambridge MA 02139, USA,
2. Department of Human Communication, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
3. Laboratory for Computer Science, Massachusetts Institute of Technology, Cambridge MA 02139, USA
4. New England Complex Systems Institute, 24 Mt. Auburn Street, Cambridge MA 02138, USA