---
title: "Race and intelligence"
chunk: 8/9
source: "https://en.wikipedia.org/wiki/Race_and_intelligence"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:18:16.114269+00:00"
instance: "kb-cron"
---

Spearman's hypothesis states that the magnitude of the black–white difference in tests of cognitive ability depends entirely or mainly on the extent to which a test measures general mental ability, or g. The hypothesis was first formalized by Arthur Jensen, who devised the statistical "method of correlated vectors" to test it. If Spearman's hypothesis holds true, then the cognitive tasks that have the highest g-load are the tasks in which the gap between black and white test takers are greatest. Jensen and Rushton took this to show that the cause of g and the cause of the gap are the same—in their view, genetic differences.
Mackintosh (2011, pp. 338–39) acknowledges that Jensen and Rushton showed a modest correlation between g-loading, heritability, and the test score gap, but does not agree that this demonstrates a genetic origin of the gap. Mackintosh argues that it is exactly those tests that Rushton and Jensen consider to have the highest g-loading and heritability, such as the Wechsler test, that have seen the greatest increases in black performance due to the Flynn effect. This likely suggests that they are also the most sensitive to environmental changes, which undermines Jensen's argument that the black–white gap is most likely caused by genetic factors. Nisbett et al. (2012a, p. 146) make the same point, noting also that the increase in the IQ scores of black test takers necessarily indicates an increase in g.
James Flynn argued that his findings undermine Spearman's hypothesis. In a 2006 study, he and William Dickens found that between 1972 and 2002 "The standard measure of the g gap between Blacks and Whites declined virtually in tandem with the IQ gap." Flynn also criticized Jensen's basic assumption that a correlation between g-loading and test score gap implies a genetic cause for the gap. In a 2014 suite of meta-analyses, along with co-authors Jan te Nijenhuis and Daniel Metzen, he showed that the same negative correlation between IQ gains and g-loading obtains for cognitive deficits of known environmental cause: iodine deficiency, prenatal cocaine exposure, fetal alcohol syndrome, and traumatic brain injury.

=== Adoption studies ===
A number of IQ studies have been done on the effect of similar rearing conditions on children from different races. The hypothesis is that this can be determined by investigating whether black children adopted into white families demonstrated gains in IQ test scores relative to black children reared in black families. Depending on whether their test scores are more similar to their biological or adoptive families, that could be interpreted as supporting either a genetic or an environmental hypothesis. Critiques of such studies question whether the environment of black children—even when raised in white families—is truly comparable to the environment of white children. Several reviews of the adoption study literature have suggested that it is probably impossible to avoid confounding biological and environmental factors in this type of study. Another criticism by Nisbett et al. (2012a, pp. 134) is that adoption studies on the whole tend to be carried out in a restricted set of environments, mostly in the medium-high SES range, where heritability is higher than in the low-SES range.
The Minnesota Transracial Adoption Study (1976) examined the IQ test scores of 122 adopted children and 143 nonadopted children reared by advantaged white families. The children were restudied ten years later. The study found higher IQ for white people compared to black people, both at age 7 and age 17. Acknowledging the existence of confounding factors, Scarr and Weinberg, the authors of the original study, did not consider that it provided support for either the hereditarian or environmentalist view.
Three other studies lend support to environmental explanations of group IQ differences: 

Eyferth (1961) studied the out-of-wedlock children of black and white soldiers stationed in Germany after World War II who were then raised by white German mothers in what has become known as the Eyferth study. He found no significant differences in average IQ between groups.
Tizard et al. (1972) studied black (West Indian), white, and mixed-race children raised in British long-stay residential nurseries. Two out of three tests found no significant differences. One test found higher scores for non-white people.
Moore (1986) compared black and mixed-race children adopted by either black or white middle-class families in the United States. Moore observed that 23 black and interracial children raised by white parents had a significantly higher mean score than 23 age-matched children raised by black parents (117 vs 104), and argued that differences in early socialization explained these differences.
Frydman and Lynn (1989) showed a mean IQ of 119 for Korean infants adopted by Belgian families. After correcting for the Flynn effect, the IQ of the adopted Korean children was still 10 points higher than that of the Belgian children.
Reviewing the evidence from adoption studies, Mackintosh finds that environmental and genetic variables remain confounded and considers evidence from adoption studies inconclusive, and fully compatible with a 100% environmental explanation. Similarly, Drew Thomas argues that race differences in IQ that appear in adoption studies are in fact an artifact of methodology, and that East Asian IQ advantages and black IQ disadvantages disappear when this is controlled for.