The primary goal of twin studies is estimation of heritability, or the proportion of the total variance of the trait that can be explained by genetic variation. In other words, heritability is a quantitative measure of the extent to which observed individual differences in a trait of interest (e.g. ERP component amplitude) emerge as a result of genetic differences among individuals. Heritability can be expressed in percentage units and varies from 0 to 100%. By definition, the remainder of the variance in the trait is caused by non-genetic (environmental) factors that can be further subdivided into two categories: shared environmental factors representing those aspects of the environment that are common to co-twins (e.g. ethnicity, culture, family, neighborhood) and therefore tend to increase their similarity, and non-shared, or individual, environment that includes environmental factors and experiences that are unique to each of the co-twins and therefore tends to decrease twins’ similarity. In addition, genetic factors can be subdivided into additive (reflecting additive effects of genes contributing to the trait) of and non-additive (reflecting non-additive allelic interaction including within-locus dominance and between-loci
