Heritability is expressed in percentage, e.g. insulin-dependent diabetes mellitus has a heritability of 70% and asthma – 60%, indicating a strong genetic predisposition (Book Rags Com 2006. Janson-Smith 2003).
Scholars have been interested in twins since ancient times. So Hippocrates, the famous Greek physician living in the 5th c BCE, connected similar diseases in twins with material circumstances. The stoic philosopher Posidonius (1st c BCE) believed that similar diseases were caused by astrological circumstances. In the 4th c CE St Augustine of Hippo used the difference in fraternal twins Jacob and Esau in the Old Testament as an argument against the claims of astrologists that time of birth determined human personality and fate (Book Rags Com 2006).
The modern history of twin studies began in the nineteenth century. In 1870s Darwin’s cousin, Sir Francis Galton published several articles arguing that in determining the traits of twins heredity (nature) played a greater role than environment (nurture). Galton noted that twins provided a ‘naturally occurring experimental design’, a ‘living laboratory’ in which to study the effects of genes and environment. He pointed to the similarities in twins’ constitution and the tendency to develop similar diseases. Galton also suggested that identical twins might develop from a single egg and non-identical twins might develop from two separate eggs, which are fertilized and implanted simultaneously. This idea, published in Inquiries into Human Faculty and its Development (1883), was proved in the twentieth century. It was found out that those identical twins are monozygotic, while non-identical or fraternal twins are dizygotic. Galton had no necessary equipment to prove his guess (Book Rags Com 2006. Janson-Smith 2003).
Since then, twin studies gained and lost its popularity among researcher at different times. Today the power of twin studies, providing a natural control.