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Jensen's most controversial work, published in February 1969 in the Harvard Educational Review, was titled "How Much Can We Boost I.Q. and Scholastic Achievement?" It concluded, among other things, that "head start" programs designed to boost African-American IQ scores had failed, and that this was likely never to be remedied, largely because, in Jensen's estimation, 80% of intelligence was based on heredity, and much of the 20% based on environment was related to pre-natal conditions.

When the work was initially published, students and faculty staged large, loud protests outside his University of California, Berkeley office, and he received multiple death threats. He was even denied reprints of his work by his publisher and was not permitted to reply in response to letters of critism -- both extremely unusual and exceptional policies for their day. Many colleagues at the time felt that even if Jensen's work contained no scientific merit, his treatment was itself against the spirit of science and the free exchange of ideas.

In a later article, Jensen argued that his claims had been misunderstood:

...nowhere have I "claimed" an "innate deficiency" of intelligence in blacks. My position on this question is clearly spelled out in my most recent book: "The plain fact is that at present there exists no scientifically satisfactory explanation for the differences between the IQ distributions in the black and white populations. The only genuine consensus among well-informed scientists on this topic is that the cause of the difference remains an open question" (Jensen, 1981a, p. 213).

Nevertheless, eugenicists and others point to passages such as the following (from his book The g Factor: The Science of Mental Ability) to support their claim that Jensen has proven that differences in IQ scores between races are mostly genetic:

In Chapter 12: Population Differences in g: Causal Hypotheses, Jensen writes: "The relationship of the g factor to a number of biological variables and its relationship to the size of the white-black differences on various cognitive tests (i.e., Spearman's hypothesis ) suggests that the average white-black difference in g has a biological component. Human races are viewed not as discrete, or Platonic, categories, but rather as breeding populations that, as a result of natural selection, have come to differ statistically in the relative frequencies of many polymorphic genes. The genetic distances between various populations form a continuous variable that can be measured in terms of differences in gene frequencies. Racial populations differ in many genetic characteristics, some of which, such as brain size, have behavioral and psychometric correlates, particularly g."

Paleontologist Stephen Jay Gould, known for his popularizations of science in mass market books and magazines, attacked Jensen's work in his 1981 book The Mismeasure of Man.

Gould makes three criticisms. The first is the criticism most commonly leveled against Jensen by other anthropologists and biologists: that Jensen misunderstands the concept of "heritability." Heritability measures the percentage of variation of a trait due to inheritance, within a population. Jensen, however, has used the concept of heritability to measure differences in inheritance between populations (Gould 1981: 127; 156-156).

The second criticism is relatively minor: Gould disagrees with Jensen's support of the attempts of others to calculate the IQ of dead people (such as the famous Polish astronomer and Prussian monetary theorist Copernicus) (1981: 153-154).

The third criticism is significant: Gould disagrees with Jensen's belief that IQ tests measure a real variable, g, or "the general factor common to a large number of cognitive abilities" which can be measured along a unilinear scale. This is a claim most closely identified with Cyril BurtSir Cyril Lodowic Burt ( March 3, 1883 October 10, 1971) was a British educational psychologist, one of the few to ever be knighted for his work. He was also a member of the London School of Differential Psychology Brief Biography Burt was born on March 3 and Charles Spearman . According to Gould, Jensen misunderstood the research of L.L. Thurstone to ultimately support this claim; Gould however argues that Thurstone's factoral analysis of intelligence revealed g to be an illusion (1981: 159; 13-314).

In a 1982 review of Gould's book Jensen gives point by point rebuttals to Gould's characterizations of his work, including Gould's treatment of heritability, the "reification" of g and the use of Thurstone's analysis (see [1] or [2]). Gould's responses can be found in the latest edition of The Mismeasure of Man (1996).

See also: the discussion of race and intelligenceDuring the 17th century, Western philosophers began arguing over the relative role of "nature" or "nurture" in shaping human capacities and tendencies. By the 19th century, most of the Western world had come to the view that this was a question that would.