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JPS Torah Commentary: Deuteronomy: "Filial insubordination is a grave offense because respect and obedience toward parents is regarded as the cornerstone of all order and authority"
From the The JPS Torah Commentary: Deuteronomy:
Verses 18–21 describe the procedure to be followed if a son is repeatedly insubordinate and his parents conclude that there is no hope of reforming him: they are to bring him before the town elders who will hear the case and, if they agree, order his execution. The law seeks to deter filial insubordination, but, by requiring that the case be judged by the elders, it also places limits on parental authority, as does the preceding law. Earlier, in the patriarchal period, it appears that the father’s authority over his children was absolute, like the patria potestas of early Roman law, even to the point of his being able to have them executed for wrongdoing; this is implied by Judah’s ability to order the execution of his daughter-in-law for adultery, with no trial (Gen. 38:24). The present law respects the parents’ right to discipline their son, but it prevents them from having him executed on their own authority. This may only be done by the community at large on the authority of the elders.
Ancient Near Eastern laws and documents also mention legal action by parents against misbehaving children. The grounds include such offenses against parents as disobedience, flight, repudiation, lawsuits against them, failure to respect and provide for them in their old age, and striking them. The punishments range from disinheritance to enslavement and mutilation.
Filial insubordination is a grave offense because respect and obedience toward parents is regarded as the cornerstone of all order and authority, especially in a tribal, patriarchal society like ancient Israel. If the death penalty specified by the present law is meant literally, it implies that biblical law regards insubordination and the danger it poses to the stability of society more severely than do other known ancient Near Eastern laws. The fact that Exodus 21:15 requires the death penalty for striking one’s parents, whereas the Laws of Hammurabi require only that the son’s hand be cut off, supports this inference. Nevertheless, some scholars, modern and ancient, believe that the death penalty stipulated in the present law is meant only rhetorically, in terrorem, to strengthen parental authority and deter the young from disobedience. As in the case of the apostate city (13:13–19), halakhic exegesis subjected the law to an exceedingly narrow reading, according to which it could hardly ever be carried out. Several rabbis held that it was never actually applied, but was stated in the Torah only for educational purposes. 
- JPS Torah Commentary: Deuteronomy