Population Differences in g
Because IQ is strictly a phenotype, as is every observable or measurable human characteristic, it does not, by itself, support any inference concerning the cause of either individual or group differences in IQ. Whatever their cause, IQ differences are related to variables of immense practical consequence in the modern world. The substantial correlation of IQ with many educational, economic, and social criteria has been well established. Largely for this reason, there has been a long-standing interest in the IQ differences between various populations in the United States that markedly differ, on average, on these salient criteria. By far the most extensively researched group differences in IQ are those between the two largest populations in the United States: persons of European ancestry who are socially identified as "white" and persons of some African ancestry who are socially identified as "black" or African-American.
The approximately normal distribution of IQ, as measured by nationally standardized tests, shows that, on average, the American black population scores below the white population by about 1.2 standard deviations, equivalent to eighteen IQ points. (Blacks in sub- Saharan Africa score about two standard deviations [approximately thirty IQ points] below the mean of whites on nonverbal tests.)
This statistical mean difference between the American black and white populations has scarcely changed over the past eighty years for which IQ data have been available. However, it varies across different regions of the country, being largest in the Southeast and decreasing in magnitude on a gradient running north and west. The mean difference, which is in evidence by about three years of age, increases slightly from early childhood to maturity. These are simply the phenotypic, psychometric, and statistical facts. The average dif-