Be very careful. The GSS survey is very addictive.

"I'm not certain what the expected correlation would be between IQ and skin color among Blacks for a given measure of "between-group heritability" (BGH) as described by Jensen (1998). "

Light skin vs dark skin IQ gap is probably more a result of successful high IQ black men taking light skin women as wives rather than due to white admixture (the actual effect of which is reduced as a result of assortive mating).

1: Nat Genet. 2004 Nov;36(11 Suppl):S54-60.

Implications of correlations between skin color and genetic ancestry for
biomedical research.

Parra EJ, Kittles RA, Shriver MD.

Department of Anthropology, University of Toronto at Mississauga, Mississauga,
Ontario L5L 1C6, Canada.

Skin pigmentation is a central element of most discussions on 'race' and
genetics. Research on the genetic basis of population variation in this
phenotype, which is important in mediating both social experiences and
environmental exposures, is sparse. We studied the relationship between
pigmentation and ancestry in five populations of mixed ancestry with a wide
range of pigmentation and ancestral proportions (African Americans from
Washington, DC; African Caribbeans living in England; Puerto Ricans from New
York; Mexicans from Guerrero; and Hispanics from San Luis Valley). The strength
of the relationship between skin color and ancestry was quite variable, with the
correlations ranging in intensity from moderately strong (Puerto Rico, rho =
0.633) to weak (Mexico, rho = 0.212). These results demonstrate the utility of
ancestry-informative genetic markers and admixture methods and emphasize the
need to be cautious when using pigmentation as a proxy of ancestry or when
extrapolating the results from one admixed population to another.

PMID: 15508005 [PubMed - indexed for MEDLINE]

Table 1 Relationship of melanin content and individual ancestry
Samplea Spearman's rho (95% c.i.)b
Relationship of melanin index and ancestry
African Americans (W-AF) 0.440 (0.330 to 0.538)
African Caribbeans (W-AF) 0.375 (0.239 to 0.496)
Mexicans (I-AM) 0.212 (0.057 to 0.357)
Puerto Ricans (W-AF) 0.633 (0.457 to 0.761)
Relationship of skin reflectance and ancestry
Hispanics (I-AM) -0.259 (-0.141 to -0.369)
aAncestral proportion axes are indicated by the abbreviations W-AF (West African) and I-AM (Indigenous American).
bNinety-five percent confidence intervals for the correlation estimates are shown in parentheses.

I saw the evidence. Now can we need to know why.

Two major hypotheses:
1. white admixture causes (some fraction of) skin color and IQ differences each thru hereditary mechanisms
2. skin color differences leads to different social/cultural environments which causes IQ differences

Hypothesis #1 predicts that direct measurement of individual ancestry thru genotyping would produce stronger correlations than those seen here; and that if MALD were used, individual loci would be found that contribute to the association. I'm not sure what Hypothesis #2 predicts.

One additional data point to note: the skin color effect is stronger in the North and West than in the South, and that for all groups/races WORDSUM scores are higher in the North and West than in the South.