Recognition of Doctrine
The Double Portion
The manner of acknowledging the firstborn incidentally referred to in De is "by giving him a double portion of all that he hath" (De 21:17), that is to say, double the share of each of the other brothers. Jewish tradition (Bekho. 46a, 47b, 51a, 51b; Babha’ Bathra’ 122a, 122b, 123a, 124a, 142b) accepts and elaborates on this right of the firstborn son. Thus, it applies only to the firstborn and not the eldest surviving son; it does not apply to daughters; it has reference only to the paternal estate, and not to the inheritance left by a mother or other relative, nor to improvements or accessions made to an estate after the death of the father.
Reasons for the Custom
The Firstborn in Ancient Society; Sacrifice and Redemption
Light is thrown on the attitude of the ancient world toward the firstborn, and hence, on the history of primogeniture, by the language used in connection with the plague of the firstborn: "from the first-born of Pharaoh that sitteth upon his throne, even unto the first-born of the maidservant that is behind the mill" or "the captive that was in the dungeon." Apparently no more dreadful catastrophe for all classes of society could be thought of than this slaying of the firstborn (Ex 11:5; 12:29). The misguided fervor of the ancient Semites who offered their firstborn as the thing most dearly beloved as a sacrifice to their gods must be considered in this light, whether it appears among the Moabites, the Phoenicians or the Hebrews themselves (Jer 32:35; Eze 20:26,31; 2Ch 28:3). It is difficult to predicate a connection between the basis of the doctrine of primogeniture and that of the Redemption of the First-born, other than that both are ultimately based on the importance of a firstborn son and the fondness of his parents for him. It is interesting to note, however, that the tradition of redemption and the law of primogeniture are kept so distinct that, while the latter has reference only to the firstborn of a father, the former has reference only to the firstborn of a mother (Bekho, viii. l, 46a; compare peTer rechem, "whatsoever openeth the womb," Ex 13:2). In a polygamous society such as that presupposed in De 21 it is natural to suppose that the distinction between paternal and maternal primogeniture would be clearly before the minds of the people.
See Birthright; Firstborn.