Do Unique Influences Operate on One Group’s Intelligence Test Scores?

The previous chapter outlined arguments suggesting between-group intelligence test score differences may be partly genetic—a mainstream view among psychologists studying group differences, though the exact between-group heritability (h2b) remains unknown. That discussion assumed environmental influences on IQ are identical across racial and ethnic groups. However, it's theoretically possible that environmental variables could affect only one racial group while leaving others unaffected. Such a variable could explain average group IQ differences non-genetically (h2b = 0) while maintaining high within-group heritability (h2w).

Psychologists term this hypothetical influence the "X-factor"—a label reflecting its unique nature. For an environmental variable to qualify as an X-factor, it must satisfy four criteria:

These criteria eliminate common explanations like poverty, malnutrition, lead exposure, inadequate education, or poor parenting. None are exclusive to any racial or ethnic group, nor do they affect all group members with equal force. Thus, these variables cannot systematically depress one group's IQ scores.

Discrimination and Racism: While all scholars acknowledge racism's existence and harmful effects, whether it functions as an X-factor remains questionable. Anti-minority racism likely meets the first requirement, affecting only minorities. Regarding the second requirement, polling data shows 82% of African Americans view racism as widespread, and 76% report experiencing discrimination regularly or occasionally. Using broader racism definitions might encompass all minority experiences, though such definitions are contested.

The third requirement poses greater challenges. Poll data reveals varying frequencies of discrimination experiences among African Americans, suggesting unequal impacts across individuals. The fourth requirement—demonstrating causal effects on IQ—presents the most substantial obstacle. Most scholars proposing racism as an X-factor don't explain the causal mechanism. Moreover, any pathway (like attending lower-quality schools) must itself meet all X-factor requirements, which seems implausible.

Involuntary Minority Status: Anthropologist John Ogbu theorized that involuntary minorities (descended from enslaved, conquered, or colonized populations) underperform because their cultures are devalued and opportunities restricted across generations. He proposed that negative experiences lower motivation, create defeatist attitudes, and cause internalized stereotypes, thereby depressing test performance.

While Ogbu's theory meets the first two requirements, it lacks evidence that involuntary minority status equally affects all group members. The proposed causal mechanisms remain empirically undemonstrated. Additionally, the theory cannot explain Spearman's hypothesis pattern or why voluntary immigrant groups (like South Asian indentured laborers in Africa) sometimes outperform majority populations.

The Flynn Effect: Some speculate that the Flynn effect (IQ score increases over decades) might explain racial group differences, with higher-scoring groups simply receiving more environmental stimulation earlier. However, this explanation also fails to meet X-factor criteria.

Tests of measurement invariance provide strong evidence against X-factors. Intelligence tests demonstrate measurement invariance across racial groups, meaning factor analyses produce identical results for different groups. An X-factor's presence would mathematically prevent measurement invariance by altering score relationships within the affected group.