What kind of heritability estimates (broad sense or narrow sense) are obtained from human twin studies?

Broad sense heritability (H^2) is the proportion of phenotypic variation in a population that is attributable to genetic variation. It includes all genetic effects (additive, dominance, and interaction). Narrow sense heritability (h^2) refers to the proportion of phenotypic variation that is due to additive genetic effects only. Additive genetic effects are those that can be passed from parents to offspring and can be used to predict the response to selection.

Twin studies are used to estimate heritability by comparing the phenotypic similarities between monozygotic (identical) twins and dizygotic (fraternal) twins. Monozygotic twins share 100% of their genes, while dizygotic twins share, on average, 50% of their genes. By comparing the phenotypic similarities of these two types of twins, researchers can estimate the heritability of a particular trait.

Twin studies provide estimates of narrow sense heritability (h^2). This is because the additive genetic effects are directly measured by comparing the phenotypic similarities between monozygotic and dizygotic twins, while non-additive effects (such as dominance and epistasis) are not precisely captured. By comparing the phenotypic correlations between monozygotic and dizygotic twins, researchers can estimate the proportion of phenotypic variation due to additive genetic effects, which is the narrow sense heritability. To sum up, human twin studies are suited for obtaining narrow sense heritability estimates since they focus on the portion of phenotypic variation that is attributable to additive genetic effects.