Intelligenza e razza

THE BELL CURVE DI CHARLES MURRAY

Come collasserà lo stato sociale?

Per molti non sarà sostenibile finanziariamente.

Ma altri privilegiano una seconda ipotesi: la scienza dimostrerà che il tentativo di ridurre le diseguaglianze sociali mirando alle pari opportunità è vano poiché tali diseguaglianze si fondano su talenti con una solida base genetica, come l’intelligenza.

Un esempio? Il gap tre neri e bianchi.

Un esempio del genere – anche se presentato in forma di ipotesi –  ha attirato critiche abbondanti quanto superficiali…

… cherry picked studies, ignores copious data, and dismisses the impressive explanatory power of hereditarianism…

Le critiche si articolano intorno a quattro evidenze che ora passeremo al vaglio.

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1.  Il gap in termini di IQ tra bianchi e neri va chiudendosi.

L’affermazione si fonda su un unico studio, per altro molto problematico…

… Although there is some evidence that the Black-White IQ gap did narrow in the 20th century, the claim that the IQ gap is now 10 points is based largely on the results of a single, heavily criticized paper, and is not consistent with the vast majority of the evidence…

Per altri studi il recupero sul gap è trascurabile e da tempo svanito…

… Murray conducted analyses of the Woodcock-Johnson cognitive test batteries and found no decrease in the Black-White gap for those born after the 1960s (and, in fact, found a slight increase in this gap since the 1960s). He found similar results using data from the National Longitudinal Survey of Youth. Other scholars, examining the available evidence up to 2005 in their response to claims of a narrowing Black-White gap, concluded, “… the best evidence of the Black-White convergence over the past 100 years is between 0 and 3.44 IQ points…”…

Diciamo così: nel XX secolo c’é stato un leggero recupero che si è poi stabilizzato…

… On achievement, the Black-White gap narrowed during the 20th century, but multiple indicators of educational achievement provide convergent evidence that the gap has stopped narrowing and, in some cases, is widening…

Ecco su questo punto un giudizio riassuntivo equilibrato

… Summarizing research on cognitive ability across the 20th century in employment and educational settings, Roth and colleagues concluded, “As a whole, these studies suggest that there are observed gains for both groups [Blacks and Whites], but the reduction in the between-group difference is either small, potentially a function of sampling error, or nonexistent for highly g loaded instruments.”… the IQ gap has stabilized near 12-15 points…

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2. Esiste un effetto Flynn. Ovvero, l’ IQ misurato nei test incrementa molto di generazione in generazione.

Sottointeso: se l’ IQ è dinamico il recupero dei neri diventa possibile…

… Turkheimer and colleagues asserted that IQ scores have increased 18 points in the US from 1948 to 2002. They noted that this increase is larger than the Black-White IQ gap. They then accused Murray of ignoring the importance of this Flynn effect..

Tuttavia, questa speranza è malriposta e si fonda su una incomprensione: IQ e intelligenza non sono la stessa cosa, il dinamismo del primo non implica il dinamismo della seconda…

… secular gains in IQ are inversely correlated with g loadings whereas the Black-White gap is positively correlated with g loadings…

Analogia…

… Consider the following example: Men are taller than women (there is a height gap between men and women). Both men and women have increased in height since the 1700s. So, there is a secular increase in height (“ Flynn effect” for height). But the gap between men and women in height has remained relatively stable (in the US, it has increased slightly in the past 100 years). The cause of the secular increase (probably nutrition) is different from the cause of the disparity between men and women (almost certainly genes)…

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3. I neri adottati in famiglie di bianchi registrano notevoli vantaggi in termini di IQ.

L’unico studio su cui si fonda questa affermazione ha almeno tre vizi, almeno per chi lo vuole utilizzare contro Murray: 1) considera solo casi di povertà estrema, 2) registra il dinamismo del gap in età infantile quando sappiamo che i gap si stabilizzano successivamente (effetto Wilson) e 3) è comunque compatibile con un’ereditarietà del 60%…

… adoption “from a poor family into a better-off one is associated with gains of 12 to 18 points.” This claim stems from an analysis of six studies with 253 total subjects, and should be interpreted cautiously for several reasons. First, in one of the studies, the “poor families” from which the children were adopted were so poor that the children were suffering from primary undernutrition at the time of adoption. Second, most of the studies examined IQ at young ages, which is important because the genetic effect (heritability) of intelligence increases with age, a phenomenon labelled the “Wilson effect” by behavior geneticist Thomas Bouchard; therefore, the gains might be smaller if the subjects had been tested later in life. Third, even if one accepted the large IQ gains uncritically, those gains are still consistent with heritability estimates of 60%, as was clearly noted by Herrnstein and Murray in The Bell Curve (see page 771, note 86). And, fourth, as with the “Flynn effect” the adoption gains are primarily on subtests that are not strongly associated with g, which means that the cause( s) of the Black-White gap probably are different from the cause( s) of adoption IQ gains….

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4. L’ereditarietà è minore tra le classi meno abbienti. Sottointeso: dove i neri sono sovrarappresentati…

… heritability of intelligence is “markedly lower among American children raised in poverty” than among middle- or upper-class children… Turkheimer et al believe this undermines the hereditarian position because more Black than White children are raised in poverty in the US…

Tuttavia, altri studi smentiscono, e smentisce anche una rassegna esaustiva della letteratura scientifica disponibile sull’argomento…

… many other studies that have examined the heritability of cognitive ability have found no or little evidence that heritability is lower in poor children. In a meta-analysis of available studies, Tucker-Drob and Bates found that there was no overall effect of socioeconomic status on the heritability of intelligence…

Ma perché una gaffe del genere? Il solito effetto Wilson viene trascurato…

… Second, the heritability of intelligence increases with age (the “Wilson effect”); therefore, even if the heritability of intelligence in some groups was low in childhood (say, 10% or so), it is not clear that it would remain low into adulthood….

Sul tema gli studi più aggiornati sembrano chiarire una volta per tutte…

… Third, researchers recently found evidence that the heritability of intelligence is the same in Blacks and Whites. In fact, this was found in the same sample of youths (but also including non-twin siblings and half-siblings) that Turkheimer used for his original analysis…

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Fin qui abbiamo visto come gli argomenti critici contro l’ipotesi ereditaria non tengano, ora vediamo come quelli a supporto siano ben più solidi.

1. La correlazione tra dimensioni del cranio, intelligenza e razze è robusta e coerente…

… Best evidence suggests a small to moderate correlation between brain size and… intelligence; there is a pattern of brain size differences that matches IQ scores. East Asians have larger brains than Caucasians, who have larger brains than African Americans….

2. È possibile un rendiconto evoluzionistico coerente sui differenziali intellettivi tra le varie razze…

… Humans migrated out of Africa at least 50,000 years ago, settling in diverse… environments. Those furthest from the equator likely confronted novel cognitive challenges. Populations likely evolved higher… intelligence in response to these novel selection pressures. IQ scores correlate with distance from equator and with skin color (which… indicates ancestral distance from equator)…

3. Si registrano costantemente differenti IQ medi tra le differenti etnie

… Many ethnic groups score differently on IQ tests. Sub Saharan Africans = 75; African… Americans = 85; Caucasians = 100; East Asians = 105; Ashkenazi Jews = 110….

4. C’è una robusta ereditarietà infra-razza  che non fatichiamo ad accettare. Tuttavia, questo fatto rende più probabile la presenza di una ereditarietà anche tra razze…

… Although not dispositive, the high within-race heritability of… IQ (50%-90%) makes it plausible that at least some of the Black-White IQ gap is genetically caused. This is especially true because researchers… have yet to find some unique environmental force that suppresses Black intelligence while not affecting White… intelligence…

5.  Ci sono parecchi tratti etnici chiaramente ereditati, molti motivi portano a pensare che anche le facoltà cognitive possano rientrare nel novero delle caratteristiche sottoposte a pressione evolutiva

… IQ patterns among races also match a suite of life history traits (traits associated with growth, survival, and reproduction)…

6. Normali andamenti statistici (regressione alla media) rendono conto molto bene del dinamismo dell’ IQ alla base di molte critiche superficiali all’ipotesi ereditaria…

… Population traits tend to move toward the population (racial) mean from extreme cases. Two high-IQ people tend to have children with lower IQs. The… opposite is also true. Two low-IQ people tend to have children with higher IQs. Multiple studies support hereditarian hypotheses…. When children are matched at 120 IQ, Black siblings average about 100, whereas White siblings average about 110. When children are matched at 70… IQ, Black siblings average about 78, whereas White siblings average about 85….

7. Gli studi sulle adozioni interazziali, ieri una freccia all’arco dei critici, oggi costituiscono un argomento a supporto dell’ipotesi Murray…

… Although adoption studies provide mixed… evidence, the single best study, the Minnesota Transracial adoption study (MTRAS) clearly conforms with a priori hereditarian… predictions. IQ W-W biological parents = 106; W-B biological parents 99; B-B biological parents = 89….

8. La mobilità nel gap intellettivo tra bianchi e neri si registra invariabilmente  in quei subtests dove l’intelligenza pura è meno coinvolta…

…  Race differences are greatest on… tests that are the most g loaded, which are also the most heritable. The Flynn effect manifests on tests that are less g loaded and less… heritable. The Black-White gap on heavily g loaded tests has shrunk less than on more weakly g loaded tests….

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Come concludere?

Insistere su nobili bugie sembra deleterio. L’ipotesi dell’ereditarietà dell’intelligenza è solida, anche se non conclusiva. Diciamo che, vista la mole di prove a supporto, non crederla vera implica come minimo il rigetto immediato di altre ipotesi su cui di solito si ostenta sicumera, come per esempio quella relativa al riscaldamento  globale della terra causato dall’uomo.

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