A research design used to determine the extent to which genetic and environmental factors contribute to variation in a population of twins.
Twin studies determine the extent to which individual differences (variation) on a particular phenotype are attributable to genetic differences between individuals (heritability). The basic tenets of twin studies originate from Ronald Fisher’s 1918 paper which demonstrated that by assuming a normally distributed liability for any given phenotype, genetic differences across multiple loci, acting additively, contribute to the variation in that trait. Consequently, the similarity or dissimilarity between pairs of relatives is attributable to genes and environments shared or not shared between them, respectively. Numerous twin studies have indicated that psychiatric and behavioral disorders are genetically influenced (Boomsma et al. 2002). For example, in a recent twin study by Young et al. (2006) the results...