In everyday life people commonly refer to each other as being smart or slow. The perception that individuals differ widely in mental adeptness—in intelligence—long preceded development of the IQ test, and there is indeed a large vernacular for brilliance, stupidity, and the many points in between. There has been much sparring over the scientific meaning and measurement of intelligence, both in the rowdy corridors of public debate and in the sanctums of academe. But what do we actually know about intelligence? A lot more in the last decade, and some of it surprising even to experts. Moreover, the data form a very consistent pattern showing that differences in intelligence are a biologically grounded phenomenon with immense sociological import.

Measurement of Intelligence

The effort to measure intelligence variation among individuals is a century old. Two strategies for measuring such differences have emerged, the psychometric and the experimental...