Are differences in performance on standardized tests indicative of differences in intelligence? Are differences in intelligence, in turn, rooted in genetic difference? Are the genetic determinants of intelligence distributed in predictable patterns along racial lines? Finally, are race-based discrepancies in mean intelligence brute biological facts, impervious to social or technological efforts to alleviate them?

These are hardly new questions. It wasn't very long ago when Richard J. Herrnstein and Charles Murray answered all the foregoing questions affirmatively in The Bell Curve, thereby inciting fresh brawls in the public policy world and in academia over political correctness, institutional and societal racism, academic freedom, the limits of social policy, meritocracy, the ethics of biological and anthropological research, the validity of intelligence testing—and on and on.

Perhaps the most remarkable fact about The Bell Curve, in light of the controversy surrounding it, is that, as Stephen Jay Gould put it in his New Yorker review, the book "contains no new arguments and presents no compelling data." Or to be somewhat more charitable, Herrnstein and Murray's work is a straightforward retread of studies by psychometricians, psychologists, and anthropologists stretching back at least to the late nineteenth century, from Charles Spearman's purported discovery of the famous "general factor of intelligence" (on which more in a little while), to Lewis Terman's invention of the Stanford-Binet IQ test and hereditary interpretation of its results, to Carleton Coon's lifelong pursuit of a scientifically valid hierarchical taxonomy of races. Neither Herrnstein and Murray's arguments nor, for that matter, the arguments of The Bell Curve's critics, are anything new under the sun.

Which brings us to William Saletan's "Created Equal" series, which has been running in Slate since Sunday. Taking James Watson's ignominious career-suicide as his point of departure, Saletan argues that the longitudinal data concerning racial disparities in IQ are recalcitrant in the face of well-meaning beliefs in fundamental genetic equality —a view Saletan caricatures as "liberal creationism"—and that if we are unwilling to acknowledge this stark reality, we can't begin to address racial gaps in academic or professional success. In other words, liberal fantasies of a purely environmental explanation of differential achievement stand squarely in the way of the actual amelioration of such differences.

There are several tangled logical threads upon which Saletan's argument precariously hangs, including a barely questioned assumption that IQ is a reasonable approximation of intelligence, a related and similarly barely questioned assumption that there is a general intelligence factor (g) that standardized tests can measure, several rather hopelessly contorted interpretations of contradictory studies, and a seeming utter ignorance of the significance of intra-group changes in IQ scores over the last hundred years. I'll come back to all these points and more momentarily; but I feel it's my journalistic duty not to bury the lede, which so far none of Saletan's blogospheric critics seem to have noticed. Namely, the principal study on which Saletan rests his case is a two-year old paper by J. Phillippe Rushton and Arthur Jensen.

To put this as fairly as it can be put: Rushton and Jensen are anything but a new wave of scholars come to shed light on a heretofore intractable problem, as Saletan presents them. On the contrary, they have spent nearly a century combined harping on the same theme again and again, in paper after paper, and that theme is black racial inferiority. (Care for a taste of just how old-fashioned they are? They group human beings into a tripartite classificatory scheme of "Caucasoids," "Mongoloids," and "Negroids." It's in the 2005 paper, and it's roughly as credible as the Shem/Ham/Japheth theory of race.)

Jensen, as Melvin Konner noted in The Tangled Wing, has been tossing up one-sided hypotheses about the relationship between race and IQ since the 1960s that have consistently been swatted away by Gould, Howard Gardner, and others, but the fact that Jensen's findings have since been debunked did not prevent them from seeping into The Bell Curve. (Thus Saletan's articles come full circle, to say nothing of the geometric configuration of the chain of research supporting the hereditarian position.)

As for Jensen's co-author, in the excellent book Human Biodiversity: Genes, Race, and History, Jonathan Marks described the nature and integrity of Rushton's scholarly puruits as follows:

J. Phillipe Rushton calculated, on the basis of crude skull measurements of army inductees, that the average brain size of Asian males was 1403, of whites 1361, and of blacks 1346 cubic centimeters...

Have we thus discovered the biological basis for the differences in intelligence that previous generations have always assumed were there?...[T]he scientific issues and assumptions are as false as they have always been. First, we must admire the apparent cranial expansion of Asians over the last half-century, when researchers consistently reported their having smaller brains than whites. Obviously this implies the possibility of a comparable expansion in blacks. More likely, it implies the possibility of scientists finding just what they expect when the social and political stakes are high.

Meanwhile, in his review of Rushton's Race, Evolution, and Behavior in the Canadian Journal of Sociology Online, neuroscientist Douglas Wahlstein maintains an air of scholarly understatement, writing "I believe that great harm could be done to both the social and natural sciences if the standards for evidence and proof advocated in this book were to gain wider acceptance." Wahlstein further quotes Rushton replying to his critics to the effect that they "have failed to show an opposite predicted ordering in brain size, intelligence, sexual restraint.'' Apropos of that last point, Marks is delicate enough not to mention Rushton's companion studies of average penis size by race—and there, as the saying goes, you have it.

The foundation of Saletan's hereditarian argument is the pseudo-science of a man obsessed not only with his preconceptions of the inferior intelligence of blacks, but their lack of "sexual restraint" as well. Pity Saletan could not contact D.W. Griffith for comment.

In any case, Rushton's enquiries into differential skull and brain-sizes among the races, and the problems attendant upon them, are particularly relevant for present purposes, because Saletan himself places some significant weight on the implications of brain size, writing that magnetic imaging studies show " at least a 40 percent correlation of brain size with IQ. One analysis calculates that brain size could easily account for five points of the black-white IQ gap." To outline just one of Saletan's numerous misunderstandings of what correlations do, and do not, imply, let's turn back to Marks:

[B]asic scientific protocol requires that all relevant variables be controlled before drawing conclusions about the cause of an observed difference between samples. But in this case we do not even know what those variables are, or what the appropriate statistical corrections (for example, for body size) may be. Brain size correlates, for example, with age and with nutritional state in early life...

[T[hough there was agreement that women have smaller average brains than men (assuming their brains don't grow in subsequent studies!), they apparently do not have lower average IQs. This obviously would undermine the strict determination of intelligence by brain size, which should already be common sense...

By now, this approach to the determination of the average intellectual abilities of group members has degenerated into sophistry. The populations within each "race" vary widely in measured cranial capacity, with the four largest sets of skulls deriving from the aboriginal males of Hawaii, Tierra del Fuego, France, and South Africa, respectively.

The upshot is twofold: First, there is no good reason to suppose that differentials in brain size are not attributable to dietary and other non-genetic variables, and second, whatever correlation between brain size and IQ exists must be weak enough to allow for women to have the same average IQ as men despite having smaller brains. Furthermore, Saletan's reliance on differential brain size stands in direct contradiction to his take on another of his data points, a study of children born in post-war Germany showing that children of white GIs had the same average IQ as children of black GIs.

Saletan explains this finding away by noting that military recruits are already pre-screened for intelligence. The trouble is that the very same brain-size data Saletan uses to bolster his hereditary interpretation of the IQ gap is originally based on measurements of soldiers, hence the fact that the offspring of black and white soldiers have the same average IQ implies that brain size differentials are either not genetically-determined, or else can be positively ruled out as explaining the black-white IQ gap.

Similarly, Saletan concedes that the black-white IQ gap is narrower than it used to be, points out the hereditarian explanation that "the gap closed fractionally in the middle decades of the 20^th century, but...scores in the last two to three decades show no improvement," and dismisses the import of this narrowing of the gap in extraordinarily blithe terms:

On the one hand, the IQ surge is hugely exciting. If it closes the gap to zero, it moots all the putative evidence of genetic barriers to equality. On the other hand, the case for it is as fragile as the case for the Iraq surge. You hope it pans out, but you can't see why it would, given that none of the complicating factors implied by previous data has been adequately explained or taken into account.

Here is where Saletan's argument really comes off the rails, even leaving aside the hopelessness of his analogy. The question before us is whether IQ scores are (strongly or weakly) genetically determined in patterns that roughly correspond to racial designations. The evidence for the affirmative is that there has always been and remains a gap between blacks and whites. (Recall that Asians currently score better than whites, but that, as Marks notes, early intelligence tests found higher scores among whites than Asians. One might conclude, as a hereditarian would no doubt be tempted to, that those early generations of intelligence tests were tests of social fit rather than intelligence. On what grounds, in that case, would one base one's confidence that current supposed intelligence tests do not in fact test various other qualities?) Never mind the fact that the black-white gap used to be greater than it is now; suppose instead, for the sake of argument, that the difference in average IQ scores of blacks and whites had remained the same throughout the entire history of IQ tests. Would that finding, at least, lend credence to the hereditarian argument?

It would not, absent relevant information about absolute changes in the average scores of blacks and whites in the same period. Only if blacks and whites not only maintained a nearly constant difference in scores relative to each other, but a nearly constant average score as well, would data about black and white IQ scores begin to support the case that IQ is in part racially determined. Alternatively, if, for example, the average IQ of whites improved from 50 to 100, while the IQ of blacks improved from 36 to 86, observers would note precisely the same absolute difference in average scores between the two groups at each end point, but a substantially different percent difference (half, in fact), not to mention intragroup changes in IQ that are completely inconsistent with the suggestion that racial genetics determine IQ.

And on this score, the data are unequivocal. According to Brad Delong's summary of recent studies:

The average IQ score of America's "white" population today is 100. According to Ulric Neisser, America's "white" children in 1932 had an average today's-test IQ score of 80. Dutch army conscripts in 1952 scored 30 IQ points lower than conscripts in 1982... [T]he African-American IQ test average rose by 6 points relative to the "white" average between 1972 and 2002. According to Brierley (1970), in the 1960s African-Americans from Ohio had an average IQ score greater than that of whites from Arkansas by 10 points.

In other words, in periods of time so short as to beggar any suggestion that they contained evolutionary augmentations of innate intelligence, average IQ scores of numerous groups improved by well more than the current 15-point gap between whites and blacks. Which is as much as to say that a genetic explanation of this phenomenon, the Flynn Effect, is about the least compelling explanation conceivable. At this point, to buy the line that, at least absent genetic engineering, racial discrepancies in average intelligence are immutable, one would have to deny the possibility that equal exposure to whatever environmental and educational factors produce the Flynn effect could close the IQ gap, an absurdly premature position to put it mildly. (In fact, a more recent study than the Rushton-Jensen paper shows no IQ gap once you control for acquired knowledge going into the test.) Alternatively, one could disclaim the notion that IQ is a reliable proxy for intelligence, but in that case there is no evidentiary basis for hereditarianism. Either way, the case that average intelligence varies with race is looking moribund.

So far, we have left alone an initial assumption implicit in the case for insuperable genetic differences in intelligence between races, and that is the existence of g, the general intelligence factor, which in its initial formulation by Spearman is the quality that all tests of mental ability measure in addition to any specific qualities, and is still supposed today to be responsible for a significant proportion of observable correlations in performance among standardized tests, and between tests and academic performance. If g does not exist, then there is no one quality that even a perfectly designed intelligence test would measure, and therefore, naturally, there could be no sense in which a racial gap in IQ scores would indicate a racial gap in intelligence.

Saletan comes recklessly close to acknowledging this point in the midst of his musings about brain size, when he ingenuously concedes, "[y]ou can debate the reality of g, but you can't debate the reality of head size." To be sure, no one doubts the reality of head size, but if g is not real, then the quantities purporting to correlate with g, head size among them if Saletan insists, are vacuous markers of differential intelligence.

Perhaps, at this point, it will not come as a surprise that, despite the ubiquity of g in some (but only some) currents of psychometrics, there is ample reason to doubt that g is anything besides a mathematical manipulation, representing no real cognitive quality. And indeed, the case that g is a fabrication is nearly as old as Spearman's original proclamation of the discovery of g. As Gould recounted in his piece on The Bell Curve, as long ago as the 1930s, L.L. Thurstone "showed that g could be made to disappear by simply rotating the dimensions [on which correlated test results are plotted] to different positions. In one rotation Thurstone placed the dimensions near the most widely separated attributes among the tests, thus giving rise to the theory of multiple intelligences (verbal, mathematical, spatial, etc., with no overarching g)." The problem with g goes even deeper (and here I beg the reader's patience), since for any set of correlated variables, a descriptive factor analysis—the statistical method that led Spearman to believe he'd discovered g—can produce the appearance of a single factor underlying all the correlations to a statistically significant degree, even if the variables in question plot invented abilities that are stipulated not to have any interdependence.

Cosma Shalizi, a statistician at Carnegie Mellon, offers an exhaustive exploration of the mythical existence of g, the bottom line of which is that factor analysis, without which we would have no concept of g, while useful for descriptive statistics, is virtually useless for doing explanatory statistics. That is, if you already know that some given abilities are interdependent, a factor-analytic examination of them could greatly simplify the explanatory picture. If, on the other hand, you are seeking to explain correlations among variables, all that factor analysis can do is confirm your pre-theoretical biases. In other words, when Spearman, or Rushton and Jensen, or Herrnstein and Murray, or Saletan look at IQ test data and infer a general factor behind the results, they are doing nothing but gazing in the mirror. None of which is to say that there might not be a genetic determinant or determinants of cognitive ability, but rather that the people seeking to make a case for one are going to need to do better. As Shalizi puts it:

If, after looking at your watch, you say that it's 12 o'clock, and I point out that your watch has stopped at 12, I am not saying that it's not 12 o'clock, just that your watch doesn't actually give you any evidence about the time. Similarly, pointing out that factor analysis and related techniques are unreliable guides to causal structure does not establish the non-existence of a one-dimensional latent variable driving the success of almost all human mental performance. It's possible that there is such a thing. But the major supposed evidence for it is irrelevant, and it accords very badly with what we actually know about the functioning of the brain and the mind.

We are all familiar (or at least we should be) with the Michelson-Morley experiment, which signaled the death-knell of the theory of a "luminiferous aether" as the medium through which light travels. Considerably less well-known is the fact that Michelson and Morley did not set out to disprove the existence of the aether, but instead to confirm it. They genuinely believed it would be there, as the best physics of the day required a medium for any wave to travel in, so they looked and looked, but in the end were unable to detect any statistically significant displacement. The history of science is littered with similar failed experiments and obsolete concepts, from Ptolemaic astronomy to phlogiston and caloric fluid. What such concepts have in common is that they fit a substantial enough subset of data to merit belief from the best scientific minds of their day; nevertheless, they weren't real, and eventually reality demands that the scientists who held them jettison their false beliefs.

Likewise, the theory of g and of race-based differences in intelligence has a prima facie plausibility based on differential IQ scores, and a sufficiently clever, if shallow, statistical manipulation can make it appear to be explanatorily powerful. Nonetheless, in more than a century of searching, we have yet to find any actual substance to it. The hereditarians are free to re-run the Michelson-Morley experiment as many times as they like, but at a certain point, if they continue to fail to produce any results, it might be time just to give up.