@InProceedings{sigmund-2001a,
  author         = {K. Sigmund and Ch. Hauert and Martin A. Nowak},
  title          = {Reward and punishment},
  year           = {2001},
  volume         = {98},
  booktitle      = {Proc. Natl. Acad. Sci.},
  pages          = {10757--10762},
  annote         = {},
  url            = {http://www.ped.fas.harvard.edu/pdf_files_old/PNAS01a.pdf},
  organization   = {NAS},
  key            = {sigmund-2001a}
}