Lynn and Vanhanen test the hypothesis on the causal relationship between the average national intelligence (IQ) and the gap between rich and poor countries by empirical evidence. Based on an extensive survey of national IQ tests, the results of their work challenge the previous theories of economic development and provide a new basis to evaluate the prospects of economic development throughout the world. They begin by reviewing and evaluating some major previous theories. The concept of intelligence is then described and intelligence quotient (IQ) introduced. Next they show that intelligence is a significant determinant of earnings within nations, and they connect intelligence with various economic and social phenomena. The sociology of intelligence at the level of sub-populations in nations is examined, and the independent (national IQ) and dependent (various measures of per capita income and economic growth rates) variables are defined and described. They then provide empirical analyses starting from the 81 countries for which direct evidence of national IQs is available; the analysis is then extended to the world group of 185 countries. The hypothesis is tested by the methods of correlation and regression analyses. The results of statistical analyses support the hypothesis strongly. The results of the analyses and various means to reduce the gap between rich and poor countries are discussed. A provocative analysis that all scholars, students, and researchers involved with economic development need to confront.