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.

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.