For 104 of the 185 nations, no studies were available. In those cases, the authors have used an estimated value by taking averages of the IQs of neighboring or comparable nations. For example, the authors arrived at a figure of 84 for El Salvador by averaging their calculations of 79 for Guatemala and 88 for Colombia. Including those estimated IQs, the correlation of IQ and GDP is 0.62.
It's not hard to find corroborating evidence, you can just make it up!
[1] http://en.wikipedia.org/wiki/IQ_and_the_Wealth_of_Nations#Na...