Layered ancestry testing reduces overfitting

Run ancestry models in discrete temporal strata (deep hunter‑gatherer/Neolithic, prehistoric steppe/Bell Beaker, historical Iron Age–medieval) and stop adding sources when model fit doesn’t improve; this prevents later sources from soaking up variance that really belongs to earlier layers. The approach yields different retained components across nearby regions (here, northern vs central Italy), showing regional heterogeneity that single‑pass admixture lists obscure. — If adopted as a standard, layered testing would curb misleading precision in consumer ancestry reports and improve public and scholarly interpretation of ancient‑DNA claims about identity and migration.