Interest in twins has its origin in mythology. Thus, Romulus and Remus, sons of Mars, founded Rome—no mean accomplishment for any two men, even sucklings of a she-wolf. Although they may have been dizygotic and male, they were anything but fraternal. Witness the story that Romulus quarreled with Remus and killed him. Another mythical pair, Castor and Pollux, remained friendly and fared better. As Gemini, they reached the heavens whence they could appear as St Elmo's fire to reassure storm-ridden sailors of benign friendship.
Medical interest in twinship is more practical and has enhanced understanding of genetic influences in the etiology of disease. This has been especially true for studies of large collections of twins that permit long-term follow-up, as in the US Veterans Administration Twin Registry and the Swedish Twin Registry. For instance, the latter collection has demonstrated a significantly higher concordance rate for ischemic heart disease among male