Sunday, March 25, 2007

Charles Murray's latest paper on changes in the black-white IQ gap over time   posted by Darth Quixote @ 3/25/2007 12:28:00 PM
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Charles Murray has an important new paper in press at Intelligence. Here is the abstract:

The black-white difference in test scores for the three standardizations of the Woodcock-Johnson battery of cognitive tests is analyzed in terms of birth cohorts covering the years from 1920 through 1991. Among persons tested at ages 6-65, a narrowing of the difference occurred in overall IQ and in the two most highly g-loaded clusters in the Woodcock-Johnson, Gc and Gf. After controlling for standardization and interaction effects, the magnitude of these reductions is on the order of half a standard deviation from the high point among those born in the 1920s to the low point among those born in the last half of the 1960s and early 1970s. These reductions do not appear for IQ or Gc if the results are restricted to persons born from the mid-1940s onward. The results consistently point to a B-W difference that has increased slightly on all three measures for persons born after the 1960s. The evidence for a high B-W IQ difference among those born in the early part of the 20th century and a subsequent reduction is at odds with other evidence that the B-W IQ difference has remained unchanged. The end to the narrowing of the B-W IQ difference for persons born after the 1960s is consistent with almost all other data that have been analyzed by birth cohort.

What I find rather interesting is the evidence adduced for (1) a much larger IQ gap between blacks and whites (~1.5 standard deviations) in the early part of the twentieth century than has usually been claimed and (2) a subsequent narrowing of this gap to the familiar one standard deviation. However, for various reasons Murray admits that the evidence for this trend is "inconclusive." A graphical display of the data analyzed by Murray is reproduced below.

The Flynn Effect has done much to convince differential psychologists that there are aspects of the relationship between latent constructs and their indicators that remain poorly understood, including perhaps the partition of within- and between-group phenotypic variance into genetic and environmental sources. However, hereditarians have long granted that certain environmental influences, not operating for the most part within the great bulk of white populations living in modern Western societies, may contribute to observed differences in test scores among racial groups. For example, Arthur Jensen (1998, pp. 495-498) found extremely convincing evidence for potent environmental effects on black IQs in a rural Georgia county where black SES was exceedingly low even relative to other blacks in the US. Older black sibs systematically scored worse on an IQ test than their younger sibs, indicating some environmental insult that accumulated over time. The overall black-white IQ difference of ~30 points in this county would have been reduced to the typical ~15 points if the decrement attributable to this cumulative environmental effect had been removed. Suppose that a much greater proportion of the American black population born earlier in the twentieth century suffered from whatever environmental deficits were responsible for the effects observed in this particular Georgia county. This is perhaps not an unreasonable assumption, given the concentration of the blacks in the rural South at the time. This would account for the trend displayed in the figure above.

I anticipate that more sophisticated psychometric analyses of these data will be forthcoming. In the meantime I have a question that perhaps Dr. Murray might be able to answer for us. In the first two samples, blacks show a larger variance in IQ. This goes against the grain of previous findings. Is this an artifact of the heterogeneity of scores across birth years within the black samples? What happens to the white:black variance ratio if age is held constant?

The paper is available in GNXP Forum.

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