In the 11th and 12th centuries a struggle unfolded in Rus' between two types of matrimonial norm: the exogamous (forbidding marriage within a family, clan, tribe or other group) and the endogamous (permitting it)

In the 11th and 12th centuries a struggle unfolded in Rus' between two types of matrimonial norm: the exogamous (forbidding marriage within a family, clan, tribe or other group) and the endogamous (permitting it). Strange as it may seem, this confrontation is reflected in the bylina of Ilya Muromets, who defeated Nightingale the Robber.

It is no accident that the villain's name contains a bird. Love affairs between close relatives used to be called the "bird's sin". The home of Nightingale the Robber is a nest in which such a sin flourishes. This comes out when the hero asks the vanquished man: why do your children all have one and the same face? And Nightingale answers: I shall raise a daughter and give her in marriage to my son, I shall raise a son and wed him to my daughter, so that the Nightingale line may never come to an end.

A splendid programme for an endogamous family. It is easy to see why the hero, a peasant's son and a representative of the "new order", a champion of the exogamous family, takes this tale for a boast of sin, and in his wrath kills Nightingale.