This heritable variation is estimated from heritability studies based on monozygotic twins. Inheritance patterns in families, heritability and genetic correlations, molecular markers, and associations have been studied. This could be due to the genetic heritability of genes related to obesity. This allows researchers to estimate the heritability of certain traits. The mechanisms of reproductive heritability and the origin of new traits remained a mystery. In classical quantitative genetics, there were two schools of thought regarding estimation of heritability. The Environmental variance will appear in other sections, such as "Heritability" and "Correlated attributes". The controversy over heritability estimates is largely via their basis in twin studies. This can lead to underestimation of heritability, however. It starts by expanding the heritability into its variance components. The standard error for heritability estimates is improved with large sample sizes. A 2004 twin study suggests a heritability of 0.81 for developing dependent personality disorder. The overall heritability of the bipolar spectrum has been estimated at 0.71. Twin studies of intelligence have reported high heritability values. The heritability of sarcoidosis varies according to ethnicity.