The same genes predicting educational attainment also predict a woman's age at first birth, body mass index, and household wealth. This suggests selection acts not on "studiousness" but an underlying trait like executive function or propensity to defer gratification, which manifests differently across environments.

While hunter-gatherer life seems cognitively demanding, their genetic profile predicts dramatically lower scores on modern intelligence tests. The subsequent rise in Europe's average score was driven primarily by the migration of farming populations with a different genetic setpoint, not gradual evolution within the hunter-gatherer lineage.

To test if the "years of schooling" genetic signal was an artifact, researchers applied it to a separate dataset of Chinese individuals. The fact it still predicted educational outcomes strongly suggests the genes are linked to a fundamental, cross-cultural biological trait, not just a quirk of European society.

Once a population reaches millions, every possible mutation occurs regularly. Therefore, the rapid selection seen in the Bronze Age wasn't enabled by larger populations creating more variants. Rather, it reflects sufficient time (thousands of years) for strong selective pressures to act on existing genetic variation.

Genetic data shows natural selection on immune and metabolic traits intensified dramatically 5,000 to 2,000 years ago. This suggests that high-density living and close contact with animals during the Bronze Age created a more powerful evolutionary pressure than the initial shift to farming.

Counterintuitively, the heritability of traits like cognition and personality increases from childhood into adulthood. This occurs because individuals increasingly select and shape their own environments based on their genetic predispositions, a process that amplifies the influence of their genes over time.

The TYK2 gene variant, a risk factor for tuberculosis, increased in frequency for thousands of years before plummeting in the last 3,000. This suggests it protected against an earlier threat but became a liability with the rise of endemic tuberculosis in denser populations, showing how selection can reverse direction.

Behavioral traits are genetically complex, shaped by thousands of genes with tiny effects (highly polygenic). Current methods can detect strong selection on simpler immune traits but lack the statistical power to pick up the weak, distributed signals acting on complex behaviors. Absence of evidence is not evidence of absence.

In restrictive environments where choices are limited, genetics play a smaller role in life outcomes. As society provides more opportunity and information—for example, in education for women or food availability—individual genetic predispositions become more significant differentiators, leading to genetically-driven inequality.