In Genesis 21, God's promise to Abraham is finally fulfilled, as Sarah gives birth to Isaac. But she frets over whether Ishmael will also be Abraham's heir. God tells Abraham to do as Sarah wishes, so he sends Hagar and Ishmael into the desert with only meager food and water. As Hagar begins to despair, God speaks to her, promising that Ishmael will become "a great nation" and showing her a well that saves both their lives.
Archaeologists have found no vidence attesting to the existence of Abraham, Sarah, and Hagar. Yet the story of Sarah and Hagar does reflect real historical concerns, including the paramount importance of inheritance in Middle Eastern societies of the second millennium B.C.
These ancient societies were "patrilineal" and "patrilocal," says Steinberg: Inheritance passed through the male line, and the bride was expected to live with the groom's family. These societies also practiced "endogamy," or marrying within certain kinship limits. "That explains why ultimately Isaac, the son of Sarah, was chosen as Abraham's heir rather than Ishmael, the son of Hagar," Steinberg says. In the biblical account, Abraham at one point identifies Sarah as his half sister, although Steinberg believes a better translation would be "kinswoman."
Alice Bach, Archbishop Hallinan Chair of Catholic Studies at Case Western Reserve University in Cleveland and author of Women, Seduction, and Betrayal in Biblical Narrative (Cambridge University Press, 1997), says that the taking of secondary wives was a common practice in ancient Israel. "What Abraham did to [Hagar] was completely legal," she says. "If your wife cannot have children, you can have children by your wife's slaves, and the child becomes yours." And polygamy is certainly one way to ensure that a patriarch can reproduce. Abraham converts his "power and resources into women and children," notes Laura Betzig, an anthropologist and historian who has studied sex in the Bible.
The story can also be seen as a reaffirmation of both divine and male supremacy. "One thing women can do that men cannot do is conceive and bear a child," Bach says. But Sarah's conception of Isaac makes clear that "God—who in those days was [considered] male—was more powerful and could open or close a womb."