How did eugenicists use intelligence tests to classify people?

Eugenicists utilized intelligence tests to reinforce existing prejudices and promote the pseudoscientific belief in biological determinism. While Alfred Binet originally developed mental ability tests to assist students struggling in school, American psychologists, most notably Lewis Terman at Stanford University, modified these tools into IQ tests to measure what they claimed was innate, permanent intelligence.12

Eugenicists used these tests to classify individuals and groups into hierarchies, operating under the false assumption that intelligence was strictly inherited and determined one's success or failure in life. Because the tests were culturally biased, they consistently produced lower scores for marginalized groups and the disadvantaged, which eugenicists then cited as "scientific" evidence of inherent inferiority.12

This practice was ethically indefensible, as it weaponized flawed metrics to justify discriminatory social policies and the marginalization of specific populations. Modern science recognizes that intelligence is not a fixed, purely hereditary trait and that performance on such tests is significantly impacted by environment, education, and cultural experience.12