What scientists cannot agree on is why. Most attribute the gap to differences in education, health and other environmental influences. Hereditarians, on the other hand, view the black-white difference as largely genetic in origin. They note, among other indirect evidence, that the disparity persists across time and around the world--a permanence that is crucial to the debate over what explains group dif?ferences. "If black-white differences converged--if there wasn't this whopping big difference everywhere--there'd be no debate left," says J. Philippe Rush?ton, a psychologist at the University of Western Ontario and an outspoken hereditarian.
This much is uncontested: for most of the 20th century, blacks worldwide have scored, on average, 15 points lower on most IQ tests than whites have.