During decades of watching both collegiate and professional football, I have seen hundreds of touchdowns scored by black players -- but not one extra point kicked by a black player.
Is this because blacks are genetically incapable of kicking a football or because racists won't let blacks kick a football?
Most of us would consider either of these explanations ridiculous. Yet genes and discrimination were the predominant explanations of black-white differences offered by intellectuals in the 20th century.
It was genes that were the preferred explanation in the early decades of that century and discrimination in the later decades, as I show in my recent book, "Intellectuals and Race."
The intelligentsia did not simply offer these as possible explanations among others. On the contrary, each was offered as the predominant, if not exclusive, explanation. Anyone who said otherwise risked being dismissed as a "sentimentalist" in the early 20th century or denounced as a "racist" in later years.
Out of such dogmatic insistence on some one-size-fits-all theory came racial quotas and "disparate impact" lawsuits in our times, based on the presumption that racial differences in outcomes show that somebody did somebody else wrong.
In earlier times, the prevailing theory was that differences in outcomes show that some races are inferior to others. This led to such things as eugenics and ultimately to the Holocaust.
In both eras, the prevailing theory flattered the egos of the intellectuals -- first as saviors of their race, and later as rescuers of victims of racism.
Among the alternative explanations of group differences that were ignored were geography, demography and culture.
For example, people with the geographic handicap of living in isolated mountain valleys have seldom, if ever, produced world-class achievements that advanced science, technology or philosophy. People geographically isolated on islands far from the nearest mainland or people isolated by deserts or other geographic features have likewise seldom kept up with the progress of others.
Conversely, urbanized peoples have often been in the vanguard of progress, producing far more of the historic advances of the human race than a similar number of people scattered out in the hinterlands -- even when both were of the same race.
Isolation can be man-made, as well as created by nature. Centuries ago, when China was the most advanced nation in the world, its leaders decided to isolate the country from other peoples, all of whom they regarded as barbarians. After a few centuries of isolation, China was shocked to find itself overtaken by others, and to some extent at the mercy of those others.
Demography is yet another reason why some groups have very different outcomes than others. Age differences between groups within a nation, or between whole nations, have often been a decade or even two decades.
There are many differences between races that have nothing to do with either genes or discrimination, but have much to do with their educational, economic or other outcomes. However, it is a much harder job to examine these many factors, and their complex interactions, than to seize upon whatever happens to be the prevailing theory of the day that may be both easier to grasp and more self-flattering.
Thomas Sowell is a senior fellow at the Hoover Institution, Stanford University