Rome and the Northern Catch-Up: Direct Genetic Effects, Harder Answers

In Rome and the Northern Catch Up Reanalysed, I used EA4, a polygenic score for educational attainment, based on genetic associations with years of schooling in modern people, to test an Iron Age Italian lead and later northern convergence in a much larger European sample. The northern side no longer meant mostly Denmark: it included six continental countries. EA4’s weights came from a conventional genome-wide association study comparing people from different families. With that score, I found a higher early Italian score and a relative northern rise over the full time span. What happens if the genetic weights are estimated within families instead?

For education, the choice of weights matters unusually much. A conventional genome-wide association study compares unrelated people, so its associations can reflect a person’s own genetic effects alongside parental genes that shape the home and educational opportunities, patterns of mate choice, and residual ancestry differences. Kong and colleagues found that even parental alleles a child did not inherit predicted the child’s education. In Okbay and colleagues’ study, accounting for parental scores roughly halved the education score’s association with education. Tan and colleagues instead estimated genetic effects from comparisons within families, making the resulting weights less vulnerable to those indirect and population-level influences.

That gain comes with a loss of predictive accuracy. In a modern benchmark discussed by Tan and colleagues, a direct-effect education score explained 1.8% of the variation in education, compared with 5.3% for a population-effect score.

Below, I ask how closely the family-based score tracks EA4 in the same ancient people; whether Republican-era Rome still stands apart; how Italy and Northern Europe compare across five periods; and whether their later convergence reflects a northern rise, an Italian decline, or both. I also test how much those answers depend on ancestry adjustment and on the studies that supplied the ancient remains.

Upgrade to read which parts of Rome’s early lead and the northern catch-up survive when the genetic weights come from within families.