Why were claims that intelligence and criminal behavior were inherited difficult to prove?

Claims that intelligence and criminal behavior are strictly inherited have been difficult to prove due to the complex interaction between genetics and the environment, as well as significant methodological and conceptual limitations.

• Complexity of Traits: Intelligence is a polygenic trait, meaning it is influenced by multiple genes rather than a single locus. Because these traits are highly complex, they are susceptible to significant modification by environmental variables, including education, culture, and childhood experiences.123
• Measurement Challenges: Intelligence is often defined by standardized IQ tests, which frequently fail to account for diverse cultural and environmental backgrounds. Similarly, behavioral traits like criminality are difficult to quantify objectively compared to physical traits like height.12
• Nature-Nurture Confounding: It is difficult to isolate genetic influence from environmental factors in humans because genes and environments cannot be manipulated in laboratory settings. While twin and adoption studies attempt to separate these influences, they are subject to bias; for example, identical twins are often treated differently than other siblings, complicating the assumption that their environments are equivalent.21
• Misuse of Heritability: Heritability estimates measure variation within a population, not the cause of differences between populations. Applying these estimates to argue for genetic superiority or inherent criminality across different groups is scientifically invalid and often reflects social or class prejudice rather than biological fact.415