Twins also share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) because they are born into the same family.
The presence of a given genetic trait in only one member of a pair of identical twins (called discordance) provides a powerful window into environmental effects.
Thorndike incorrectly reasoned that his data supported for there being one, not two, twin types.
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.
It is also possible to examine non-additive genetics effects (often denoted D for dominance (ADE model); see below for more complex twin designs).
The ACE model indicates what proportion of variance in a trait is heritable, versus the proportion due to shared environment or un-shared environment.
Modern twin studies have shown that almost all traits are in part influenced by genetic differences, with some characteristics showing a stronger influence (e.g. personality traits) and some more complex heritabilities, with evidence for different genes affecting different aspects of the trait — as in the case of autism.
Twins have been of interest to scholars since early civilization, including the early physician Hippocrates (5th century BCE), who attributed similar diseases in twins to shared material circumstances, More recent study is from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior.
C is simply the MZ correlation minus this estimate of A.
This allowed him to account for the oversight that had stumped Fisher, and was a staple in twin research prior to the advent of molecular markers.
Wilhelm Weinberg and colleagues in 1910 used the identical-DZ distinction to calculate respective rates from the ratios of same- and opposite-sex twins in a maternity population.
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.
Chief among Siemens' innovations was the polysymptomatic similarity diagnosis.