The Distance Between Genotype and Human Intelligence Is Long, Indirect, and Densely Mediated by Experience

Jensen (Jensen, 1998) stated that the “holy grail” of intelligence research was the elucidation of the biological basis for its’ variations. Gargus and Haier (2025) expand on this notion, suggesting that a more complete understanding of the molecular biology of general intelligence will provide actionable plans to increase the intelligence of individuals. While the approach described by Gargus and Haier is hypothesis-driven, the distance between genotype and phenotype is long, indirect, and densely mediated by experience. Common strategies do not capture nonlinear biological interactions, feedback loops, or developmental and environmental influences, i.e., they provide no insight into how or why a phenotype emerges. The main difficulty with the molecular account of intelligence advanced by Gargus and Haier (2025) is not that the biological pathways they discuss are implausible, rather, it is that the phenotypes themselves are so complex that even well-replicated genetic associations give only limited traction on questions of causation. Based on the hidden influence of gene-environment interplay, small effect sizes of identified gene variants, the low generalizability of GWAS signals across time and place, and the fact that parental genotypes can shape offspring outcomes through environmentally-mediated pathways rather than direct biological transmission, we think the quest to find global (and timeless) genes for intelligence is not simply difficult, but may be futile from the start. A focus on the role of educational opportunity, family environment, and socio-economic circumstance might yield more immediately tangible strategies to promote human intelligence.