That states an intuitive statistical fact about spacial unevenness of economics but it does not necessarily correspond to a reasoning why you should add it as a

That states an intuitive statistical fact about spacial unevenness of economics but it does not necessarily correspond to a reasoning why you should add it as a variable. Based on the abstract of this paper, we don't know whether adding a variable will decrease or increase the bias: http://www.sas.rochester.edu/psc/clarke/405/Omit.pdf http://www.sas.rochester.edu/psc/clarke/405/Omit.pdf