Abstract
Twin method is a traditional way to investigate genetic and environmental (shared and nonshared) influences on
human behavior. The number of publicity using twin method is continuously growing recently which robustly shows
substantial genetic contribution to almost any aspects of psychological individual differences. Molecular biological
techniques and multivariate statistical method called structural equation modeling (SEM) provide several generalizable
findings such as very small effect of each single gene, significant genetic mediation in phenotypic correlation,
heritability increase in IQ from infancy to young adulthood, and socially important gene x environment interaction.
Discordant identical twin method has a powerful possibility to investigate epigenetics.
