Commentary: The birth of the twin study--a commentary on Francis Galton's 'The History of Twins'

Abstract

In this passage, the primary stirp is the stirp of a zygote immediately after it has been fertilized and the secondary stirps of monozygotic twins are their stirps immediately after the zygote has divided into two. Galton explains the similarities of three classes of twins as follows. (i) Monozygotic twins who divided shortly after conception are very similar because they have very similar secondary stirps derived from the division of the same primary stirp, and these secondary stirps develop in very similar conditions. (ii) Dizygotic twins and siblings are moderately similar because they are derived from very similar primary stirps, which develop under somewhat similar conditions. No prediction is made about the degree of similarity. (iii) Monozygotic twins who divided a considerable time after conception are very dissimilar because the primary stirp, which divides into two, has been organized into contrasting halves. Galton could not in 1875 have invented the classical twin method based on this non-Mendelian theory of heredity. What he did was to track the life-history changes of twins to see whether twins who were very similar at birth diverged in later life or whether twins who were very dissimilar at birth converged later, as a test of the relative importance of nature and nurture. The classical twin method of comparing the similarities of monozygotic and dizygotic twins on Mendelian assumptions was first used in the 1920s.