Of course, it would be a simplification to say that assessment of heritability is the only goal of twin studies. Twin data provide rich information that can be used to address many additional questions. Multivariate analysis of twin data can be used to estimate genetic correlations between different traits, i.e. the extent to which genetic influences on two or more different psychophysiological characteristics are common or specific. For example, one may ask the question whether individual differences in novelty or oddball P3, or resting EEG and ERPs are influenced by same or different genetic factors. The application of this multivariate approach is illustrated below (section 4.2.8) by showing that substantial proportion of genetic variance in P3 amplitude is shared with resting-state EEG power. Another extension of the classical twin method is longitudinal genetic analysis that permits us to address such questions as whether the strength of genetic influences (heritability) changes with age, whether same or different genes influence the trait at different stages of development, and whether the rate of developmental changes is influenced by genetic factors (McArdle, 2006; Posthuma
