That paper was not published in the JAMA - the lead author apparently works for the JAMA:
May also be a member of the AMA - from the abstract:
Furthermore, the Social Science Research Network (papers.ssrn.com) is just a pre-print repository, anyone can submit a paper to the site, nothing there is peer reviewed - most peer reviewed article are published in journals and once they are the journal has copyrights on them, so the final/official/peer reviewed papers are not on ssrn.com.