Rainfall: not such a great instrument after all... 

 Every discovery of a plausible instrumental variable sparks a cottage industry of papers all using the same instrument to ask different questions.  A working paper by Heather Sarsons, titled  "Rainfall and Conflict"  calls one of these cottage industries into serious question.  From the abstract: 

 Starting with Miguel, Satyanath, and Sergenti (2004), a large literature has used rainfall variation as an instrument to study the impacts of income shocks on civil war and conflict. These studies argue that in agriculturally-dependent regions, negative rain shocks lower income levels, which in turn incites violence. This identication strategy
relies on the assumption that rainfall shocks affect conflict only through their impacts on
income. I evaluate this exclusion restriction by identifying districts that are downstream
from dams in India. In downstream districts, income is much less sensitive to rainfall
fluctuations. However, rain shocks remain equally strong predictors of riot incidence
in these districts. These results suggest that rainfall affects rioting through a channel
other than income and cast doubt on the conclusion that income shocks incite riots. 

 It's a short, readable paper -- worth checking out if you're into this kind of thing.