Why Interindividual Variability in Intelligence Matters:

"A population with little intellectual variance but homgeneously centered around an average level of ability, say, with 90% of persons having IQs between 90 and 110 (the range that now contains the middle 50% in the United States Caucasoid population), if left to itself, would probably advance very slowly, if at all, beyond a Stone Age or simple agrarian culture. As a factor in cultural evolution, the amount of variability of talents in a population could be more important than its overall mean."

Arthur R. Jensen (1978)

Human Variation - Genetic and Behavioral Effects of Nonrandom Mating