The largest study ever of American ethnic IQ differences in a representative sample is the Coleman study. Here we saw 5 different ethno-racial groups compared at multiple ages on 5 different mental tests. The study involved 645,000 students from 4000 public schools. All test scores were expressed using a scale where the Americans on the whole average 50 with a standard deviation of 10.

Because American Indians and Oriental Americans come from non-English homes I decided to use the non-verbal test as the most valid measure of general intelligence. And because the environmental inequality between these races compounds every year like compound interest, I decided to compare them in the first grade before environment has had much time to cause differences.

The other reason for using the non-verbal test is it consists of conceptual problems like figure series which are neither verbal nor spatial but purely abstract and thus a pure measure of general intelligence that is not significantly biased towards verbal or spatial thinkers.

I wanted the scores to be expressed as IQs using white norms so the white mean of 54.1 was equated to IQ 100 and the white SD was equated to 15.

I don’t in fact know the white SD on these tests but historically the white SD is about 97% as large as the American SD so I assume it was 9.7.

Using these parameters, we get the following IQs:

Oriental Americans: 104

White Americans: 100 (by definition)

American Indians: 98

Mexican Americans: 94

Puerto Ricans: 87

The American Negro: 83

Estimated score of the American pure Negro: 78*

What these numbers suggest is that when anatomically modern humans evolved in Sub-Saharan Africa about 200,000 years ago, we had an average IQ of about 78, however leaving the tropics to survive the cold ice age caused massive natural selection for IQ. Indeed by the end of the Last Glacial Maximum, commenter Melo discovered that brain size in Europe had reached 1512 cc (60 cc higher than it was before humans left Africa).

Assuming a within-sex, within ethnic brain size standard deviation of 91 cc, and assuming a brain size IQ correlation of 0.4, this 0.65 SD increase in brain size implies a 0.65/0.4 = 1.63 SD increase in IQ (24 points). Not all that different from the 22 point IQ gap between white Americans and the estimated IQ of full-blooded African Americans.

The fact that there’s virtually no IQ gap between white Americans and American Indians suggests human intelligence (like human brain size) peaked at the end of the Last Glacial Maximum.

Then as Native Americans headed South, the warm hospitable climate required less intelligence and we see a rare example of backwards evolution, thus explaining why despite white admixture, the Mexican Americans score lower than their indigenous cousins in the U.S..

Puerto-Ricans, like Africans Americans, are hybridized blacks but with slightly more white admixture, thus explaining why they score a bit higher too.

Meanwhile the incredible scores of Oriental Americans is not at all explained by selective immigration. Indeed in the 1960s, most Orientals were descended from railroad indentured laborers and thus, if anything, were below the average of their home countries. Rather, the high East Asian IQ can be explained by Northern Siberia being the coldest (and thus most cognitively demanding) place modern humans ever lived in large numbers

*estimated by assuming American blacks are 75% whites and thus dividing their deviation from whites by 0.75.