Symmetrical distribution, peaks at center.

The Bell Curve: Intelligence and Class Structure in American Life is a 1994 book by psychologist Richard J. Herrnstein and political scientist Charles Murray, in which the authors argue that human intelligence is substantially influenced by both inherited and environmental factors and that it is a better predictor of many personal outcomes, including financial income, job performance, birth out of wedlock, and involvement in crime than are an individual's parental socioeconomic status.

a 1995 book edited by the historian Russell Jacoby and the writer Naomi Glauberman.