Why did eugenicists place more importance on heredity than on environment?

Eugenicists prioritized heredity over environment because they operated under the framework of biological determinism, the belief that human characteristics—including complex social and psychological traits—are fixed at conception by genes.12

Following the rediscovery of Gregor Mendel’s laws of inheritance, eugenicists argued that social issues such as alcoholism, criminality, and poverty were the result of an accumulation of "defective" genetic traits within the population. By framing these conditions as strictly hereditary, they concluded that society was deteriorating due to the reproduction of "degenerate" individuals.12

This perspective led eugenicists to view nature as far more influential than nurture in shaping humanity. Consequently, they advocated for selective breeding and compulsory sterilization as the primary tools to "improve" the human race, as they believed that if a trait was genetic, it was immutable and could only be managed by controlling reproduction rather than altering environmental conditions. This focus on genetic inheritance was used to justify policies that restricted human potential rather than addressing social or economic disadvantages.12