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Spearman’s hypothesis and the Jensen Effect

Group differences and g. There are numerous psychometric tests of intelligence. It has been found that there is a central common factor. This central factor is called general intelligence or g. G is of interest because: 1) it is psychometrically structurally similar across populations, sexes, ages, and cultures, (and several species), 2) it represents a behavior-psychometric manifold with numerous educational, psychological, and sociological correlates, 3) it correlates with the cognitive complexity of activities, 4) it is highly heritable (within populations) and correlates with inbreeding depression scores, and 5) it has numerous neurophysiological correlates such as brain neural conduction velocity, cerebral glucose metabolic rate, the latency and amplitude of evoked electrical brain potentials, the speed and efficiency of brain functioning inferred from reaction time, neural organization, the volume of white and grey matter, the mass of the prefrontal lobe, total brain size, and cranial capacity.

In the history of the debate, the truth of 1-5 were vigorously fought by environmentalists for a reason. If it turned out that g was just a “test artifact” or if it was shown that there was no such physiological manifold — or that groups did not differ in that regards — the hereditarian hypothesis, and its implications, would have been weakened (see: Brand, 2001; Jensen, 2000). Flynn (2007), in fact, recently made the later environmental case, holding that psychometric intelligence is a conglomerate of physiological functions which can “swim free of g,” and, years before (1987), he made the former case, arguing that IQ was only “a correlate with weak causal links to intelligence” (i.e. there’s no central factor binding the various manifestations of intelligence). Steve Rose (1995) articulated one of the environmentalist reasons for opposing the reality of g, worrying that “if intelligence is one thing, it becomes appropriate to seek a single causative agent.” Indeed. As such, Flynn’s (2010) concession, in wake of the mounting evidence, that general intelligence “does have some root in brain physiology” is significant. For a nice summary of the g affair refer to Brand, Constales, and Kane (2003).

The reality of g does two things: it makes the implications of the hereditarian hypothesis, if true, unavoidable, and it substantially strengthens the hereditarian argument. With regards to the former, as Jensen (2000) pointed out, g “lies at the heart of the whole problematic nexus involving the nature of group differences, the merits of meritocratic selection in a diverse society, the legitimacy of using tests, their adverse impact on certain groups, and its redress by group preferences in college admissions and employment.” With regards to the latter, the reality of g loaded differences makes implausible a number of environmental arguments (including virtually all purely sociological ones, such as test bias, motivation, stereotype effect, etc.). Murray (2005) articulates this point well:

A concrete example illustrates how Spearman’s hypothesis works. Two items in the Wechsler and Stanford-Binet IQ tests are known as “forward digit span” and “backward digit span.” In the forward version, the subject repeats a random sequence of one-digit numbers given by the examiner, starting with two digits and adding another with each iteration. The subject’s score is the number of digits that he can repeat without error on two consecutive trials. Digits-backward works exactly the same way except that the digits must be repeated in the opposite order.

Digits-backward is much more g-loaded than digits-forward. Try it yourself and you will see why. Digits-forward is a straightforward matter of short-term memory. Digits-backward makes your brain work much harder.

The black-white difference in digits-backward is about twice as large as the difference in digits-forward.[60] It is a clean example of an effect that resists cultural explanation. It cannot be explained by differential educational attainment, income, or any other socioeconomic factor. Parenting style is irrelevant. Reluctance to “act white” is irrelevant. Motivation is irrelevant. There is no way that any of these variables could systematically encourage black performance in digits-forward while depressing it in digits-backward in the same test at the same time with the same examiner in the same setting.

The g-loadedness of group differences also allows for the argument from Spearman’s hypothesis.

Jensen and Rushton used the method of correlated vectors to show that the B-W gap correlates with more heritable IQ subtests and that the gap is gloaded. They argue that the correlation with heritability implies that there is a genetic etiology to these differences. Additionally, the g-loadedness of the gap supports the hereditarian position. First, given that the environmental hypothesis predicts a relationship between environmentality and group differences and no relationship between heritability and group differences, the correlation with heritability supports the hereditarian hypothesis. Second, environmental explanations or combinations thereof are found wanting when it comes to explaining the said average differences in general intelligence, given that g stands at the nexus of a whole web of psychological, social, and neurophysiological factors; this leaves genetic explanations as the default.

The environmentalists’ reply. Flynn (2006) makes the case that the GQ (general intelligence) gap is closing, which implies that environmental factors are effecting a change; as such, the current lack of coherent environmental explanations does not imply genetic origin to the gap. With regards to the latter argument, Flynn (2010) replies that since g is a proxy for cognitive complexity and since environmental deficits increasing impose disadvantage with complexity, the environmental hypothesis can offer a plausible explanation to the gloaded specificity. As such, a genetic etiology to the differences is not implied. To quote Flynn:

(1) g would be of no interest were it not correlated with cognitive complexity. (2) Given hierarchy of tasks, a worse performing group (whatever the cause of its deficit) will tend to hit a “complexity ceiling” — fall further behind a better group the more complex the task. (3) Heritability of relevant traits will increase the more complex the task. (4) Thus, the fact that group performance gaps correlate with heritability gives no clue to the origin of group differences. (5) When a lower performing group gains on a higher performing one, their gains will tend to diminish the more complex the task. Thus, blacks have gained 5.50 IQ points on whites since 1972 but only 5.13 GQ points. (6) Recent achievement test data confirm these IQ gains but the data as a whole pose problems for the external validity of black IQ. (7) The FE is irrelevant to showing that the racial IQ gap is environmental but it was historically valuable in clarifying the debate.

Both of Flynn’s counters are flawed. With regards to the closing of the GQ gap, the best explanation to date for this was put forth by Murray (2006) and Chay et al. (2009). Accordingly, the g gap was partially closed by health improvements (i.e . environmental influences that had immediate biological impact). This explains the change in the substantially biological g and leaves environmental explanations wanting to explain current g-loaded differences, at least between mid to upper SES members of the said populations. With regards to the g-loadedness of group differences, Flynn’s cognitive explanation does not hold in wake of the vast social, psychological, and neurophysiological manifold that general intelligence represents.

We could use a basketball analogy to capture both positions on this matter. Flynn argues that g is analogous to general basketball ability; it’s important because it correlates with the ability to do complex moves, say like making reverse two-handed dunks. Flynn’s point is that to do a reverse two-handed dunk, one needs to learn all the basic moves. Since environmental disadvantages (poor coaches, limited practicing space, etc.) handicap one when it comes to basic moves, they necessarily handicap one more when it comes to complex basketball moves. Rushton and Jensen argue the g is analogous to a highly heritable athletic quotient; it’s important because it correlates with basic physiology, generalized sports ability, and basic eye-motor coordination. Their point is that it’s implausible that disadvantages in basketball training would lead to across the board disadvantages in all athletic endeavors and, moreover, lead to a larger handicap in general athleticism than to a handicap in basic basketball ability. Rather than disadvantages in basketball training leading to disadvantages in general athletic ability, it’s much more plausible that disadvantages in general athletic ability would lead to a reduced effectiveness of basketball training.

Flynn and other environmentalists can only circumnavigate g by insisting that a web of g affecting environmental circumstances, in effect, constructs g from the outside in. Given that g is psychometrically structurally similar across populations, sexes, ages, and cultures this seems implausible as it would necessitate that either everyone happened to encounter the same patter of g formative environmental circumstances just at different levels of intensity or that environmental circumstances were themselves intercorrelated.

Given the weakness of environmental accounts of GQ differences, the hereditarian hypothesis is a more plausible explanation than the environmental (0-genetic) hypothesis.


Brand, 2001. The g Factor – General Intelligence and its Implications (This is a free, downloadable book)

Brand, Constales, and Kane, 2003. WHY IGNORE THE G FACTOR? — Historical considerations.

Chay, et al., 2009. Birth cohort and the black-white achievement gap: The roles of access and health soon after birth

Dickens and Flynn, 2006. Black Americans reduce the racial IQ gap

Flynn, 2010. The spectacles through which I see the race and IQ debate

Murray, 2005. Inequality taboo.

Murray, 2006. Changes over time in the black-white difference on mental tests: Evidence from the children of the 1979 cohort of the National Longitudinal Survey of Youth

Rushton and Jensen, 2010. Race and IQ: A Theory-Based Review of the Research in Richard Nisbett’s

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