A firstborn receives a double portion of his father's estate, as Deuteronomy 21:17 states: "To give him twice the portion."
What is implied? If a father left five sons, one the firstborn, the firstborn receives a third of the estate and each of the other four receives a sixth. If he left nine sons, the firstborn receives a fifth and each of the other eight receive a tenth. We follow this pattern in dividing the estate in all instances.
When a firstborn is born after his father's death, he does not receive a double portion. This is derived from ibid.: 16-17: "On the day when he transfers his inheritance to his sons... he shall recognize the firstborn, the son of the hated one." If his forehead emerged during the lifetime of his father, even though his entire head did not emerge until after his father's death, he receives a double portion.
When a firstborn was born with his genitals covered by flesh and afterwards, an operation was performed and it was discovered that he was male, he does not receive a double portion. Conversely, when an ordinary son was born with a similar condition and after the operation was performed, it was discovered that he was male, he does not reduce the firstborn's share. These concepts are derived from ibid.:15 "And she will bear him sons." Implied is that the sons must be sons from the moment of birth.
What is meant by saying that such a son does not reduce the firstborn's share? A person had a firstborn, two ordinary sons, and this son whose genitals were covered by flesh and afterwards were revealed through an operation. The firstborn receives one fourth of the estate as his extra share as the firstborn, as if there were only two other sons. The remaining three fourths of the estate are divided equally among the two ordinary sons, the son who underwent the operation, and the firstborn.
A child who lived for only one day reduces the portion of the firstborn, but a fetus does not. Similarly, a son born after his father's death, does not reduce the portion of the firstborn.
When there is a question if a son is a firstborn or an ordinary son - e.g., the firstborn became mixed together with another - he does not receive a double portion.
What is done? If at first, the babies were distinct and then they became mixed together," they may compose a document granting power of attorney to each other, and on that basis take the portion of the firstborn with their brothers. If the identity of the firstborn was never known - e.g., the two wives gave birth in one hiding place, - they should not compose a document granting power of attorney to each other, for there is no extra portion for the firstborn.
The following laws apply when a person had two sons - a firstborn and an ordinary son - and they both died in his lifetime, after fathering children. The firstborn left a daughter and the ordinary son left a son. The son of the ordinary son inherits one third of the estate of his grandfather - i.e., his father's portion. And the daughter of the firstborn inherits two thirds of that estate, her father's portion.
The same laws apply with regard to the sons of the deceased's brothers, or the sons of his uncles, or any other set of heirs. If the father of any of the heirs was a firstborn, the person who inherits his share of the estate also receives the firstborn's share.
A firstborn does not receive a double portion of his mother's estate. What is implied? When a firstborn and an ordinary son inherit their mother's estate, they divide it equally. This applies with regard to a son who was the firstborn with regard to the laws of inheritance, and to one who "open his mother's womb."
The firstborn with regard to the laws of inheritance is the first child born to the father, as ibid.:17 states: "Because he is the first manifestation of his strength." We do not pay attention to the child's status vis-a-vis his mother. o Even if she gave birth to several sons previously, since this was the first son born to the father, he receives a double portion of the inheritance.
A son who is born after stillborn babies, even if the stillborn baby was alive when its head emerged from the womb, is considered the firstborn with regard to the laws of inheritance. Similarly, when a fetus was born after a full-term pregnancy, but was not alive when its head emerged, the son who follows is considered the firstborn with regard to the laws of inheritance.
The term "the first of his strength," Deuteronomy 21:17, used with regard to the firstborn implies that no child before him emerged alive into the world. Hence, when a fetus was alive after its head emerged after a full-term pregnancy, a son born afterwards in not a firstborn even the first baby died immediately thereafter.
Neither a son born by Cesarean section, nor the son born after him, is considered "the firstborn." The first son was never "born," and ibid.: 15 states "and she bore sons to him." And the second son is not given this privilege, for he was preceded by another.
When a person had sons as a gentile and then converted, he does not have a firstborn with regard to the rights of inheritance. If, however, a Jewish man fathered sons from a maid-servant or from a gentile woman, since they are not considered his sons, a son he fathers afterwards from a Jewish woman is considered his firstborn with regard to the laws of inheritance, and he receives a double portion of his father's estate.
Even if the firstborn is a mamzer, he receives a double portion. This is reflected by Deuteronomy 21:16: "But rather he will recognize the firstborn, the son of the hated one." This refers to a woman whose marriage is "hated." Needless to say, this applies if the firstborn is the son of a divorcee or a woman who performed chalitzah.
There are three individuals whose word is accepted with regard to the designation of a firstborn: the midwife, the mother and the father.
The midwife's word is accepted only at the moment of birth. For example, a woman gave birth to twins; if the midwife said: "This one emerged first," her word is accepted.
His mother's word is accepted for the first seven days after birth, when she says: "This one is the firstborn."
His father's word is always accepted. Even if the father said that a person who was not known to be his son was his firstborn son, his word is accepted. Similarly, his word is accepted if he says that the person whom we consider to be his firstborn is not his firstborn.
When a father loses his ability to speak, we check the soundness of his intellect in the same way as is done with regard to a bill of divorce. If through his motions he indicates - or he writes - that this is his firstborn son, that son receives a double portion.
If witnesses testify that they heard a father make certain statements that clearly indicate that a child is his firstborn son, the son receives a double portion even though the father did not explicitly say: "This is my firstborn son."
If the father was heard saying: "This son of mine is a firstborn," the son does not necessarily receive a double portion of the estate because of this testimony. Perhaps the son was the mother's firstborn, and this was his father's intent. For the son to receive a double portion, the father must call him: "My son, my firstborn."
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