The Journey of Humanity by Oded Galor: A Review Essay.

What does it mean to assume it is orthogonal to HT <math display="inline" altimg="urn:x-wiley:00987921:media:padr12566:padr12566-math-0025" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>R</mi><mrow><mi>i</mi><mi>j</mi></mrow></msub><mo>? First, the credibility of the necessary identification condition depends on the time interval HT <math display="inline" altimg="urn:x-wiley:00987921:media:padr12566:padr12566-math-0014" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>T</mi><mo>-</mo><mi>K</mi></mrow><annotation encoding="application/x-tex">$T - K$</annotation></semantics></math> ht . One possibility is that one should interpret (2) as looking for explanations that were fixed at time HT <math display="inline" altimg="urn:x-wiley:00987921:media:padr12566:padr12566-math-0023" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>T</mi><mo>-</mo><mi>K</mi></mrow><annotation encoding="application/x-tex">$T - K$</annotation></semantics></math> ht . $</annotation></semantics></math> ht Since there are underlying facts of geography that affect each of these factors, it is difficult to see how one could make an argument that each of these factors is ex post uncorrelated with HT <math display="inline" altimg="urn:x-wiley:00987921:media:padr12566:padr12566-math-0026" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><msub><mi>R</mi><mrow><mi>i</mi><mi>j</mi></mrow></msub><annotation encoding="application/x-tex">${R {ij}}$</annotation></semantics></math> ht given the common geography. [Extracted from the article]