The twin method is an approach within behavioral genetics that reveals both genetic and environmental effects on traits by comparing the behavioral similarities of monozygotic twins, who share the heredity and family environment, with those of dizygotic twins, who share half their genes but have the same family environment similarity as monozygotic twins. In classical twin studies, genetic factors are treated as latent variables rather than at the molecular level, and the focus is on variance rather than mean values. There is a vast accumulation of twin studies in the various fields of psychology that show significant and substantial genetic effects on all behaviors, but no trait has been found to be 100% genetic. It has been universally shown that most environmental factors are not shared within families. From a developmental psychology viewpoint, evidence for dynamic changes in genetic effects, such as the expression of new genetic factors (genetic innovation) and the increase in the heritability of intelligence with age, is of particular interest. Stability between ages in many traits, moreover, has generally been found to be mainly attributable to heredity. The specific examples of these findings will be introduced through a meta-analysis of large-scale cross-sectional twin studies and the results of our longitudinal twin projects, using the Cholesky decomposition model, latent growth model, cross-lagged model, and the analysis of differences in monozygotic twins.
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