In reality, individual differences in complex phenotypes result from a combination of these genetic and environmental influences. Using the observed MZ and DZ twin pair correlations, it is possible to estimate standardised A, C and E variance components, which represent the proportion of total variance accounted for by additive genetic, and shared and unshared environmental influences. All A, C, and E estimates reported in this paper refer to standardised variance components. These estimates are obtained by employing maximum-likelihood modelling procedures, which determine the combination of genetic and environmental parameters that best fits the covariance structure of the observed data. In addition, confidence intervals around these estimates can also be calculated. Most reports used in our analyses employed maximum-likelihood modelling procedures using the statistical package MX (38), and others used LISREL (39, 40).
