Table 1.

Mechanisms of potential bias in heritability estimates due to chorionicity

Mechanism of chorionicity effects	MC and DC twin similarity	Bias in heritability estimate	Rationale
Vascular differences: placental sharing inequalities	MC twins less similar than DC twins	Underestimated	MZ twins would have lower correlation, closer to DZ twins (reducing contrast)
Similar placental function: diffusion, osmosis, endocrine	MC twins more similar than DC twins	Overestimated	MZ twins would have higher correlations than DZ twins, chorionicity effect would be included in heritability estimate
Mis-classification of MZ and DZ twins	MC twins that are less similar may be called DZ instead of MZ twins	Most likely underestimated	Including MZ twins in DZ group would mean more genetic similarity in DZ group, reducing contrast in twin correlations
MC twins have poorer outcomes than DC twins	MC twins less or more similar to DZ twins	Underestimated or Overestimated	MC twinning is indicative of a prenatal environmental risk factor(s). If the MC twinning environmental factor(s) is shared, MZ twins would have a higher correlation than DZ twins; if the MC twinning factor was unshared, the MZ twins would have a lower correlation, closer to DZ twins.