How and why is political fact-checking spreading across journalism? The research presented in this report suggests that the challenge of disseminating the practice is significant -- mere proximity does not appear to be sufficient to drive adoption. However, we find that factchecking can be effectively promoted by appealing to the professional values of journalists. Our first study considers whether journalists might emulate their colleagues in emphasizing fact-checking, following the practices of professional peers in the way that other journalistic innovations have disseminated. However, the practice does not appear to diffuse organically within a state press corps. While fact-checking coverage increased dramatically during the 2012 campaign, these effects were concentrated among outlets with dedicated fact-checkers. We find no evidence that fact-checking coverage increased more from 2008 to 2012 among outlets in states with a PolitiFact affiliate than among those in states with no affiliate. However, it is possible to effectively promote fact-checking. In a field experiment during the 2014 campaign, we find that messages promoting the genre as a high-status practice that is consistent with journalistic values significantly increased newspapers' fact-checking coverage versus a control group, while messages emphasizing audience demand for the format did not (yielding a smaller, statistically insignificant increase). These results suggest that efforts to create or extend dedicated fact-checking operations and to train reporters are the most effective way to disseminate the practice of fact-checking. While audience demand is an important part of the business case for the practice, newsrooms appear to respond most to messages emphasizing how fact-checking is consistent with the best practices and highest aspirations of their field.