Twin Studies: Software and Algorithms

In 1865, Sir Francis Galton (1822–1911) formally described the differences between identical (monozygotic, MZ) and fraternal (dizygotic, DZ) twins, and suggested that MZ twins were genetically identical (Galton, 1865). This distinction yields a natural experiment, because comparison of the similarity of these two types yields information about the relative importance of heredity versus environment to individual differences. To quantify these sources of variation, it is necessary to have (1) a way to assess similarity and (2) an algorithm to use this information to estimate the parameters of interest. The optimal choices for (1) and (2) vary according to the type of measurement that is taken from the twins.