Intelligence is the one human trait that many continue to believe is genetically determined. The big question is whether or not intelligence is hereditary and therefore fixed, or due to the environment and changeable. In 1994, the controversy over this issue came up again with the publication of "The Bell Curve: Intelligence and Class Structure in American Life" written by Richard Herrnstein and Charles Murray. The authors argue that cognitive ability is substantially inherited, and therefore, it is virtually impossible to raise an individual's Intelligence Quotient, or IQ. Critics over this issue, however, state that the environment has an important role in determining intelligence and that if a child's environment is changed, their IQ can be changed as well. .
Herrnstein and Murray also state that there is a correlation between IQ and behavior, and that because of this, the national IQ average is declining. They say the performance of a group can be predicted based on the group's IQ average. Groups with low IQ tend to be poor, unemployed, on welfare, involved in criminal activities, and have more illegitimate children. Groups with a high IQ, however, are usually upper- to middle-class, do better in school and at work, and have fewer children. Herrnstein and Murray worry that the national IQ average is declining, however, because low IQ individuals have more children than high IQ individuals, which therefore brings down the national IQ average. Critics of this idea, site what is known as the "Flynn effect" which states that average IQ scores have risen three points per decade since the development of IQ tests. Researchers have noticed that test takers earn higher scores on IQ tests each year until the tests are re-standardized, which then makes it more difficult to get an average score of 100. .
The greatest controversy in their book however, is that cognitive ability varies among races.