International differences in the speed of cognitive development: A systematic examination of the existence of the Simber Effect.

The Simber Effect refers to the phenomenon whereby, in Arabic countries, young children have an IQ that is little different from that of Western children but that these differences increase throughout childhood culminating in a difference of around 20 points by adulthood. The true nature of this phenomenon is revealed by an examination of 125 samples from all around the globe measured with Raven's Progressive Matrices. We show that in many cases different speeds of cognitive development increase the IQ score differences between countries mostly between 4 and 9 years of age, and that these increases can in part be explained by poor environmental conditions.