There are times where you really wonder why authors would even think that some variables could be correlated and how they then come up with a story that can explain this statistical relationship coming from seemingly nowhere. The paper by Jose Fernandez and Lisa Stohr is one of these.
To quote their abstract, "this paper uses variation in traffic safety laws and obesity rates to identify substitution patterns between living and cadaveric kidney donors." Despite reading this sentence ten times, I could not make any theoretical sense of it. But reading through the paper, a good story can be made. Tightening traffic safety laws reduces the number of fatalities, and thus the number of cadaveric organ donors. An increase in obesity increases the demand for organs, in particular kidneys. Thus one can instrument for supply and demand using these measures. With this in mind, one can then study how variations in the supply of supply of cadaveric organs (which are of poor value) and demand can motivate living donors to come forward, as they trade off the usefulness of their donation with the personal harm it will inflict upon them. Fernandez and Stohr find that donors respond indeed to cadaveric supply and to the increase in demand due to obesity.
But finding such ways to interpret data is difficult, you need the talents like those of John Donohue and Steven Levitt.
Update: I just came across another paper making essentially the same points, by Randolph Beard, John Jackson, David Kaserman and Hyeongwoo Kim.