Balak, King of Moav, called on Bilam to curse the Nation of Israel. Bilam responded that he must wait for G‑d’s instructions. After Bilam was told by G‑d not to curse Israel, “For it is blessed”, Bilam says (Bamidbar 23:8) “How can I curse whom G‑d has not cursed”, and thereby got Balak very angry. It seems that Bilam realized that cursing has no real physical effect, while Balak believed that curses could be effective and enable him to defeat Israel.

The Torah (Shmot 22) prohibits us to curse judges and leaders and prohibits cursing parents, which is punishable by death (Shmot 21:17). Another prohibition forbids cursing a deaf person (Vayikra 19). Commenting on this prohibition, Rashi there sites the Midrash that this verse also forbids cursing any living person.

Ramban explains that the Torah found it necessary to specify that we may not curse even a deaf person, who does not hear the curse and is therefore not angry or upset by the curse. Therefore, it is certainly prohibited to curse a person who can hear and may suffer humiliation, anger and distress on hearing someone curse him.

The Sefer Hachinuch (231) acknowledges that he does not understand how curses affect the victim, but does state clearly that a curse can harm a person. He therefore writes, “The root of the Mitzva not to curse is that we should not verbally harm others, just as we may not harm them physically.”

The Sefer Hachinuch then sites Rambam’s opinion that the reason for this Mitzva not to curse, is to eliminate anger and vengeance from people’s hearts. The Chinuch deduces from this explanation that Rambam does not agree with his view, and maintains that curses have no effect at all, it’s all psychological. Therefore, this prohibition is merely an educational device to improve our moral character.

Rambam (Guide for the Perplexed 111) also writes that most people consider verbal curses more severe than physical damage. He implies that although the masses indeed entertain such a notion, it is not true, and only do to this mistaken impression, therefore is this prohibition of cursing is so severe. Cursing is one of only three transgressions that involve no action but is nevertheless punished by lashes.

Thus, Bilam could not curse the Jewish People at all according to Rambam, since no one can actually really curse anyone. According to the Chinuch, the explanation would be that even if a person’s curse is effective, G‑d’s blessing overcomes and cancels every curse.

However, it gets even better. The Torah informs us (Devarim 23:6), “G‑d refused to listen to Bilam, and G‑d overturned the curse to a blessing, because your G‑d really loves you.”

According to the Talmud, when someone curses another person, the curse boomerangs back to the curser. Therefore, King David, who cursed Yoav, his IDF Chief of Staff, the curse came back to King David’s descendants.