[citation needed] To account for the Flynn effect (an increase in IQ scores over time), the authors adjusted the results of older studies upward by a number of points.

Similarly, the authors think that large resources of diamonds explain the economic growth of the African nation Botswana, the fastest in the world for several decades.

The authors argued that the People's Republic of China's per capita GDP of at the time roughly US$4,500 could be explained by its use of a communist economic system for much of its recent history.

"[4] Richardson (2004) argued, citing the Flynn effect as the best evidence, that Lynn has the causal connection backwards and suggested that "the average IQ of a population is simply an index of the size of its middle class, both of which are results of industrial development".

"[3] A review by Michael Palairet criticized the book's methodology, particularly the imprecise estimates of GDP and the fact that IQ data were only available for 81 of the 185 countries studied.

However, the review concluded that the book was "a powerful challenge to economic historians and development economists who prefer not to use IQ as an analytical input", but that it's likely those scholars will deliberately ignore this work instead of improving it.

[5] In a book review for the Journal of Economic Literature, Thomas Nechyba wrote that "such sweeping conclusions based on relatively weak statistical evidence and dubious presumptions seem misguided at best and quite dangerous if taken seriously.

Ervik stated, "The arguments put forward in the book to justify such comparisons [between the average IQ in different countries and their GDP] seem at best vague and unconvincing.

"[7] Edward Miller, an economist who has published many controversial papers on race and intelligence, gave the book positive reviews in two different white nationalist publications, the Journal of Social, Political, and Economic Studies and The Occidental Quarterly.

[10] University of Reading geographer Stephen Morse also criticized the book (as well as IQ and the Wealth of Nations), arguing that the authors' hypothesis rests on "serious flaws".

Morse also argued: "The central dilemma of the Lynn and Vanhanen case rests with their assumption that national IQ data are primarily (not wholly) a function of innate ability, which in turn is at least partly generated by genes.

As Lynn rightly remarked during the 2006 conference of the International Society for Intelligence Research (ISIR), performing a literature review involves making a lot of choices.

[3] Denny Borsboom argued that mainstream contemporary test analysis does not reflect substantial recent developments in the field and "bears an uncanny resemblance to the psychometric state of the art as it existed in the 1950s".

The review criticized the principal assertion of the authors that differences in intelligence, attributed to genetics, account for the gap between rich and poor countries.

[17] On July 27, 2020, the European Human Behavior and Evolution Association issued a formal statement opposing the utilization of Lynn's national IQ dataset, as well as all updated forms of it, citing various criticisms of its methodology and data collection.