An important case of the absence of the Jensen Effect is that of the secular increase in test scores. The g Factor, Jensen's monumental tome on the reality of g, actually has little to do with race, as such. The first five chapters deal with the intellectual history of the discovery of g and various models of how to conceptualize intelligence. The fact that psychometric g has many physical correlates proves that it is not just a statistical reality. Among biological variables, g loads on heritability coefficients determined from twin studies and inbreeding depression scores calculated in children born from cousin-marriages.
G is a product of human evolution and is also found in non-human animals.
Despite the fact that The g Factor is not primarily about race, Jensen's discussion of g and race will strike many as being its central theme. All the issues Jensen dared to voice in 1969 are still with us today. Indeed, much of the opposition to IQ testing and heritability research would probably disappear if it were not for the stubborn and unwelcome fact that, despite extensive programs of intervention, the mean Black-White difference in g does not diminish. The book describes how the difference between Blacks and Whites in average IQ scores has scarcely changed over the past 80 years. It can be observed as early as three years of age.