In neither era were the intelligentsia receptive to other explanations. In each era, they were convinced that they had the answer -- and dismissed and disparaged those who offered other answers.
Differences in mental test scores among different racial and ethnic groups were taken as proof of genetic differences in innate mental ability during the Progressive era in the early 20th century. Progressives regarded the fact that the average IQ test score among whites was higher than the average among blacks as conclusive proof of genetic determinism.
A closer look at mental test data, however, shows that there were not only individual blacks with higher IQs than most whites, but also whole categories of whites who scored at or below the mental test scores of blacks.
Among American soldiers given mental tests during the First World War, for example, white soldiers from Georgia, Arkansas, Kentucky, and Mississippi scored lower on mental tests than black soldiers from Ohio, Illinois, New York, and Pennsylvania.
Among other groups of whites, those with average mental test scores no higher than the average mental test scores among blacks included those in various isolated mountain communities in the United States, those living in the Hebrides Islands off Scotland and those in isolated canal boat communities in Britain.
Looking at achievements in general, people living in geographically isolated environments around the world have long lagged behind the progress of people with a wider cultural universe, regardless of the race of the people in these isolated places. When the Spaniards discovered the Canary Islands in the 15th century, they found people of a Caucasian race living at a stone age level.
Many mountain communities around the world have also been isolated, especially during the centuries before modern transportation and communications.
These mountain communities were often not only isolated from the outside world but also from each other, even when they were not very far apart as the crow flies, but were separated by rugged mountain terrain.
As distinguished French historian Fernand Braudel put it, "Mountain life persistently lagged behind the plain." A pattern of poverty and backwardness could be found from the Appalachian Mountains in the United States to the Rif Mountains of Morocco, the Pindus Mountains of Greece and the mountains and uplands of Ceylon, Taiwan, Albania and Scotland.
Cultural isolation due to geographic factors afflicts not only peoples isolated in mountains or on islands far from the nearest mainland, but also peoples isolated by deserts or in places isolated by a lack of navigable waterways -- or even by a lack of animal transport, as was the situation in the Western Hemisphere when Europeans arrived and brought horses that were unknown to the indigenous peoples.
Cultural isolation can also be due to government decisions, as when the governments of 15th century China and 17th century Japan deliberately isolated their peoples from the outside world. At that time, China was the leading nation in the world. But it lost that lead during centuries of isolation.
Sometimes isolation is due to a culture that resists learning from other cultures. The Arab Middle East was once more advanced than Europe but, while Europe learned much from the Middle East, the Arab Middle East has not translated as many books from other languages into Arabic in a thousand years as Spain alone translates into Spanish annually.
Against this background, racial and ethnic leaders around the world who promote a separate cultural "identity" are inflicting a handicap on their own people. Isolation has held back many peoples in many lands, for centuries. But such social and cultural isolation serves the interests of today's ethnic leaders.
They have every incentive to promote a breast-beating isolation. It is a sweet-tasting poison.
(Daily Corinthian columnist Thomas Sowell is a senior fellow at the Hoover Institution, Stanford University, Stanford, CA 94305. His website is www.tsowell.com.)