The original academic article can be read here:
The Quarterly Journal of Economics http://qje.oxfordjournals.org/content/early/2013/02/19/qje.q... (open access full text, at least for now)
I haven't read it, it is a peer reviewed publication, so I imagine they do have some evidence.
So the bulk of the argument stems from some initial qualitative assessment of the region's agricultural state (i.e. hoe or plow) and then comparing it to present day stats for the metrics he outlines.
Your suspicions are probably right, but I don't think the author would even disagree necessarily. I think this is just an interesting look into one of the factors that may have been responsible.
Farming promoted development. It did not hold it back.
Thinking in terms of "development" is generally a bad approach. History doesn't have levels. Using generic terms like that implicitly assumes all sorts of correlations that don't actually exist.
Not that I'm aware of. My understanding is that agricultural societies were less violent. As I understand it, when humans are in hunter-gathering societies, we are more territorial and less accepting of those outside of our tribe. (On the order of 50 people.) I'd like to know if that understanding is wrong.
What I do know is that we have gotten much less violent over time. The 20th century, even with all of its wars, was still much less violent than all previous centuries.
Before significant agriculture some amount of warfare was simply a part of life as usual. This scenario suggests martial competence is fairly ubiquitous, and thus dominating enemies without paying a high cost in blood is difficult (e.g. you might need great political skills to stitch together a powerful local coalition).
However once significant cities were established, being skilled at warfare was potentially very lucrative. This means a soldier specialist may be a very rewarding career.
Is it really the plough itself, or that the larger cities the plough helped create were a big incentive for professional soldiers to become the ruling class?
Hegel would be very disappointed