@Article{nowak-1998a,
  author         = {M. Nowak and K. Sigmund},
  title          = {Evolution of indirect reciprocity by image scoring},
  year           = {1998},
  volume         = {393},
  pages          = {573--577},
  annote         = {First paper of two. TODO: Read Nature version and write summary},
  read-status    = {read},
  journal        = {Nature},
  url            = {http://www.ped.fas.harvard.edu/pdf_files_old/Nature98.pdf},
  key            = {nowak-1998a}
}