Meta-analysis covers 17,804 traits from 2,748 twin studies and 14 million twin pairs. Average heritability across all traits is 49%. Heritability estimates cluster strongly within functional domains. For 69% of traits, twin correlations match additive genetics only. Data reject substantial shared environment influence. Data reject non-additive genetic variation. MZ twin correlations consistently higher than DZ. No evidence for epistasis or dominance in most traits. Results support simple additive model for complex traits. Findings guide gene-mapping efforts.