Yerkes reached three controversial conclusions based on data gathered with the Army intelligence tests. First, he claimed that the average mental age in the United States was a mere 13 years. Second, he said there were genetically based racial differences in intelligence, with whites outperforming blacks. Third, he said there were also genetically based ethnic differences in intelligence within the white population, with individuals whose ancestors came from northern Europe surpassing those from southern or eastern Europe. In an article quoted by Dewsbury, Yerkes wrote:
If we may safely judge by the army measurements of intelligence, races are quite as significantly different as individuals . . . Almost as great as the intellectual difference between Negro and white in the army are the differences between white racial groups.
Yerkes published his massive report on these findings in 1921. Two years later, Carl Brigham, a young psychologist who had been one of Yerkes's assistants, published his own book on the subject. Titled A Study of American Intelligence, Brigham's book repeated many of the same claims made by Yerkes. Brigham also noted that immigration from southern and eastern Europe had been increasing in recent years. Based on the eugenicist views that he and Yerkes shared, and that were common at the time, Brigham warned that growing numbers of presumably inferior immigrants would further taint the gene pool in the United States. He urged that immigration restrictions be imposed before it was too late.
The next year, Congress passed the Immigration Act of 1924, which limited the number of immigrants who could enter the country. Strict quotas were set for each national group. Since the quotas were based on the makeup of the U.S. population in 1890, before the recent wave of immigration from southern and eastern Europe, the quotas for those areas were quite low. Public sentiment against immigration was strong enough that the bill probably would have passed in any case. Nevertheless, the support of respected psychologists such as Yerkes and Brigham certainly bolstered the cause.
Even at the time, however, their conclusions did not go completely unchallenged. Walter Lippman, a columnist for New Republic magazine, wrote a series of articles in which he ridiculed Yerkes's claim that the average intelligence of recruits was on par with that of a typical 13-year-old. At the same time, several reviews published in psychology journals commented on Brigham's tendency to neglect or dismiss data that did not agree with his interpretations. They also noted statistical oddities that called into question the validity of the data.
The strongest challenge, however, came from psychologists who embraced the views of Franz Boas, the leading American anthropologist of the time. Boas argued that many racial and ethnic characteristics were passed down from generation to generation not by heredity, but by culture, through shared values, language, and childrearing customs. One of the first researchers to apply this culture concept to group differences in intelligence test scores was Otto Klineberg, a graduate student in psychology who happened to study anthropology with Boas.
In 1926, Klineberg began working on his dissertation. While giving intelligence test items to Yakima Native American children in the state of Washington, Klineberg noticed that the children were almost completely unaware of time. Even when urged to hurry, they still took their time, but they also made relatively few mistakes. Here was a clear example of a cultural, rather than genetic, difference that would put the Yakima children at a disadvantage on any timed intelligence test. Yet it was unrelated to any real difference in mental ability. Instead, it was rooted in cultural values that equated speed with carelessness.
This experience attuned Klineberg to cultural factors affecting intelligence test scores. Soon, he followed up on his dissertation with studies of the psychological characteristics of African Americans and
Native Americans. His 1935 book Negro Intelligence and Selective Migration argued that it was superior cultural and environmental advantages that caused northern blacks to score higher on intelligence tests than their southern black counterparts. He found that, when black students moved from racially segregated schools in the South, which usually were poorly funded, to integrated schools in the North, their intelligence test scores tended to improve. In fact, their scores rose to the level of northern-born blacks once they were in the integrated schools.
By the 1930s, most psychologists had conceded that culture and environment played a major role in causing group differences in intelligence test scores. Brigham even admitted that he had overstated the case for genetic differences. He acknowledged that the tests of the day assessed not only pure intelligence, but also knowledge of language and culture. In Brigham's own words (as quoted by Gould): "Comparative studies of various national and racial groups may not be made with existing tests. . .One of the most pretentious of these comparative racial studies—the writer's own— was without foundation."
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For as much as we believe we train our brains and give them a good workout, we seldom actually do it on a regular basis. In most cases, our brains are not used in a balanced way. We're creatures of habit. We find a way to do things that we consider comfortable and we seldom change our ways.