The Bell Curve at 30: A Closer Look at the Within- and Between-Group IQ Genetic Evidence

Abstract

The year 2024 marks 30 years since the publication of The Bell Curve: Intelligence and Class Structure in American Life by Richard Herrnstein and Charles Murray. The book created a storm of controversy after its 1994 publication, including its authors’ claim that it is “highly likely…that both genes and the environment have something to do with racial differences” in mean IQ scores between U.S. Whites and Blacks (African Americans). In this review, we evaluate not only Herrnstein and Murray’s ethnic-differences claims, but also the evidence they cited in support of “substantial” within-group IQ heritability (now in conflict with recent modest molecular genetic results). We show that the “twins reared apart” (TRA) studies supplying what the authors called the “most unambiguous direct estimates” of IQ heritability contain numerous biases, and that the results of the “most modern” TRA IQ study were based on omitted control group data, a reliance on false assumptions, and other “questionable research practices.” We then describe the major problems, biases, and unsupported assumptions found in IQ family, reared-together twin, and adoption studies while calling into question the validity of long-disputed concepts such as “heritability,” “IQ,” and “race.” Like many others before us, we conclude that Herrnstein and Murray presented no valid evidence in support of genetic causes of between-ethnic-group or social-class IQ score differences. And like few others before us, we also conclude that they produced no valid evidence that genes influence IQ score differences within groups. As in 1994, the authors’ social and political policy recommendations should be rejected because, among other reasons, they are based on faulty research coming increasingly to light in science’s replication crisis.