In Jewish thought and texts, curses exemplify the belief that speech can have tremendous power.
Reprinted with permission from the Encyclopedia of Magic, Myth, and Mysticism (Llewellyn Worldwide).
A curse is a verbal invocation pronounced to bring harm, evil, or detriment to another. More than a threat or a wish, a curse is assumed to have the power to make the desired harm a reality.
Two elements make up the logic of cursing: a magical/symbolic view of causality, and "formalism"--the belief that a speech-act has power, regardless of intention, justification, or authority. While some assume that the "power" of the speaker underpins the efficacy of the curse (Numbers 22:3), because of formalist assumptions in rabbinic thinking, even curses uttered unintentionally by ordinary people were believed to have the potential to be detrimental (Megillah 15a-b, 28a).
In the Bible
"The skies above your head shall be copper,
and the earth under you iron." Deuteronomy 28:23
In the Bible, God has the power to both bless and curse Creation. Both powers are demonstrated in the first three chapters of Genesis. Humans also have the power to curse individuals and whole classes of people. Some biblical authors simultaneously try to limit the use of curses and undermine their formalist assumptions by claiming unjustified curses will have no effect (Proverbs 26:2).
Biblical curses can be absolute or conditional. An absolute curse is meant to be immediate (Genesis 4:11; II Samuel 16). A conditional curse only becomes efficacious when certain conditions are met or violated (Deuteronomy 27-28).
A notable form of conditional curse that appears in the Bible is the conditional self-curse (I Kings 19:2, 20:10). Often included in an oath, this curse was placed on oneself accompanied with a symbolic act of destruction--shattering a pot, chopping up an animal, or some other deed that signified what would happen to the one making the vow if he or she should fail. Even God uses a form of this when making a covenant with Abraham (Genesis 15:7-21).
In Rabbinic Literature
The Sages elaborate upon these biblical beliefs (Makkot 11a; Eruvin 18b-19a; Terumah 3b-4a; Makkot 16a). Demons as well as human beings can utter curses. Using a curse can actually invite unwanted demonic attention on the person uttering the curse. The talmudic sage Rav reportedly had the power to curse others with sterility (Shabbat 108a).
At least one sage, Joshua ben Levi, had the power to curse crops. In the Jerusalem Talmud, Tractate Hagigah, we read that the curse of Simeon ben Shetah's son was considered so potent that eighty witnesses recanted their perjury rather than see his curse realized.