[Xavier Gabaix][1] has several papers (eg [here][2] and [here][3]) that look at the emergence of Zipf's law in city sizes as a natural phenomenon.


  [1]: http://pages.stern.nyu.edu/~xgabaix/
  [2]: http://www.mitpressjournals.org/doi/abs/10.1162/003355399556133
  [3]: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B7P64-4FFPH84-2F&_user=142623&_coverDate=12/31/2004&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1466283797&_rerunOrigin=scholar.google&_acct=C000000333&_version=1&_urlVersion=0&_userid=142623&md5=dfc67114398bfe4c3dcdf4b7529cb7b1&searchtype=a