IQ variance splits into additive genetics (heritability 60-80% in adults), shared environment (near zero in adults), and unshared environment. Heritability under 50% in early childhood but increases with age as shared environment fades. Scarr-Rowe hypothesis predicts lower heritability in poorer environments and lower-scoring social classes or racial groups. Lower classes and racial minorities expected to show reduced IQ heritability if differences are environmental. Jensen and Rushton find IQ heritability similar across ethnic groups. Plomin states IQ heritability similar across cultures. Turkheimer claims otherwise for race. European-derived polygenic scores predict IQ worse in African Americans. Lower heritability in non-Whites proposed to explain polygenic score failure beyond linkage disequilibrium decay. Meta-analysis tests Scarr-Rowe effects for US racial/ethnic groups. Only US studies found for racial heritability comparisons. Adult IQ architecture surprisingly simple per ACE model. ACE fits IQ data without complex kinship designs. Polygenic score accuracy depends on within-group heritability.