In the well-known passage from Deuteronomy, the 25th chapter, the faithful are commanded that "if brethren dwell together, and one of them die, and have no son, the wife of the dead shall not be married without unto a stranger: her husband's brother shall ...

take her to him to wife, and perform the duty of a husband's brother unto her.

And the first-born ...

shall succeed in the name of his brother that is dead, that his name be not blotted out of Israel." The same passage shows that while it was doubtless at first an imperative social law, there came a time when the living brother had a choice as to whether or not he should take to wife the widow of one who had died.

Perhaps there might have been an economic pressure that made it difficult to perform this ancient duty.