The book reviews selected literature on IQ testing and argues that genetic racial differences exist, with a discussion of the causes and consequences.

The book claims to represent the largest collection and review of the global Intelligence Quotient (IQ) data, surveying 620 published studies from around the world, with a total of 813,778 tested individuals.

For instance, the "data point" for Suriname was based on tests given to Surinamese who had migrated to the Netherlands, and the "data point" for Ethiopia was based on the IQ scores of a highly selected group that had emigrated to Israel and, for cultural and historical reasons, was hardly representative of the Ethiopian population.

While there appears to be rather little evidence on cohort effect upon IQ across the developing countries, one study in Kenya (Daley, Whaley, Sigman, Espinosa, & Neumann, 2003)[5] shows a substantially larger cohort effect than is reported for developed countries.

"A review by Nicholas Mackintosh, Emeritus Professor in the Department of Experimental Psychology, University of Cambridge, expresses astonishment that Lynn infers that Kalahari bushmen, with an average measured IQ of 54, should be regarded as mentally retarded; and that an 8-year-old European child with the equivalent mental age would have no problems surviving in the same desert environment.

Mackintosh argues that the cranial capacity of early Homo sapiens, 100,000 or more years ago, was rather greater than that of modern Europeans.

He also writes:[6] A more egregious example is provided by his treatment of the Eyferth (1961) study of two groups of illegitimate children fathered by (mostly) American black and white servicemen and brought up by their (carefully matched) German mothers.

Lynn reduces the former number to 94 to compensate for use of an old test, and compares it, not with the score of the white sample, but with an average IQ of 100 for German children.