The quantitative and molecular genetics of human intelligence.

General cognitive ability or g as measured by IQ tests has been the most intensively studied trait in human behaviour genetics. Although there has been much debate about how best to describe and measure intelligence, this is addressed elsewhere in this volume and here I will assume that there is utility in the concept of general intelligence and review the data showing that, whatever it is, g is familial and influenced by genes. Next, I will briefly describe some of the main quantitative genetics models that have been used in trying to tease out the genetic and environmental sources of variation. Finally, I will outline current molecular genetic research aiming to locate and identify genes influencing human intelligence and try to predict the directions in which such research will lead.