いとこ婚
Cousin marriage were legal in ancient Rome from the Second Punic War (218–201 BC), until it was banned by the Christian emperor Theodosius I in 381 in the West, and until after the death of Justinian (565) in the East, but the proportion of such marriages is not clear. Anthropologist Jack Goody said that cousin marriage was a typical pattern in Rome, based on the marriage of four children of Emperor Constantine to their first cousins and on writings by Plutarch and Livy indicating the proscription of cousin marriage in the early Republic. Professors Brent Shaw and Richard Saller, however, counter in their more comprehensive treatment that cousin marriages were never habitual or preferred in the western empire: for example, in one set of six stemmata (genealogies) of Roman aristocrats in the two centuries after Octavian, out of 33 marriages, none was between first or second cousins. Such marriages carried no social stigma in the late Republic and early Empire. They cite the example of Cicero attacking Mark Antony not on the grounds of cousin marriage, but instead on grounds of Antony's divorce.