biological siblings is greater than one-half the correlation between MZ twins. Nonshared environmental influences refer to environmental influences that contribute to differences among relatives, including measurement error. MZ twin correlations less than 1.0 are evidence of nonshared environmental influences. Biometric models were fit using the computer program Mx33 with a full information maximum likelihood estimator that accommodates missing data. Symptom counts were log(x+1) transformed to reduce skew and kurtosis.
