The most common approach used to examine the extent to which genetic and environmental influences are important for a trait of interest is the twin methodology.17 Twin studies involve comparing the similarity between monozygotic twins, who share all of their genetic variation, and dizygotic twins, who share on average half of their genetic variation, to estimate the relative importance of genetic and environmental influences. In traditional twin studies, genetic material (ie, DNA) is not collected, and no specific genotypes are measured; rather, the relative importance of genetic and environmental effects is estimated statistically based on the twin correlations. This means that the heritability estimate, which represents the proportion of the variance in the trait due to genetic influences, along with the estimates for the relative importance of environmental influences, are specific to the population being studied.
