History
While twins have been of interest to scholars since early civilization, such as the early physician Hippocrates (5th c. BCE), who attributed similar diseases in twins to shared material circumstances, and the stoic philosopher Posidonius (1st c. BCE), who attributed such similarities to shared astrological circumstances, the modern history of the twin study derives from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior. Galton, however, was unaware of the critical genetic difference between MZ and DZ twins.
This factor was still not understood when the first study using psychological tests was conducted by Edward Thorndike (1905) using 50-pairs of twin. Notably this paper was perhaps the first statement of the idea (formulated as a testable hypothesis) that C (family effects) decline with age: comparing 9-10 and 13-14 year old twin-pairs, and normal siblings born within a few years of one another.
Fatefully, however, Thorndike incorrectly reasoned that his data gave support for there being one, not two types of twins: Missing the critical distinction that makes within-family twin studies such a powerful resource in psychology and medicine. This mistake was repeated by Ronald Fisher (1919), who argued
- "The preponderance of twins of like sex, does indeed become a new problem, because it has been formerly believed to be due to the proportion of identical twins. So far as I am aware, however, no attempt has been made to show that twins are sufficiently alike to be regarded as identical really exist in sufficient numbers to explain the proportion of twins of like sex.".
The first published twin study utilizing the distinction between MZ and DZ twins is sometimes cited as that of the German geneticist Hermann Werner Siemens in 1924. Chief among Siemens' innovations was the "polysymptomatic similarity diagnosis". This allowed him to overcome the barrier that had stumped Fisher and was a staple in twin research prior to the advent of molecular markers. Wilhelm Weinberg, however, had already by 1910 used the MZ-DZ distinction to calculate their respective rates from the ratios of same- and opposite-sex twins in a maternity population, worked out partitioning of covariation amongst relatives into genetic and environmental elements (anticipating Fisher and Wright) including the effect of dominance on relative's similarity, and begun the first classic-twin studies.
Read more about this topic: Twin Study
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