The Torah does not forbid a man from having multiple wives. Abraham, Jacob, David and Solomon are notable examples of biblical figures who wedded more than one wife.

A close reading, however, reveals that in virtually all cases where our forefathers took multiple wives, it was for a specific reason. Abraham married Hagar only after Sarah suggested that he do so because she and Abraham had no children together. Another classic example is Jacob. He married Leah only because he was tricked into it by Laban. Later he married her sister Rachel, then he took Bilhah and Zilpah at the advice of his first two wives, who wished to bear children through them.

Yet the Torah does not outlaw polygamy.

Approximately one thousand years ago, the noted German scholar Gershom ben Judah, (c. 960 -1040) best known as Rabbeinu Gershom and also commonly known to scholars of Judaism by the title Rabbi Gershom “the Light of the Diaspora” banned polygamy. This ban was accepted as law by all Ashkenazic Jews, but was not recognized by Sephardic, Mizrahi and Yemenite communities.

Practically speaking, polygamy is almost nonexistent today even amongst Sephardic Jews, due to the fact that the overwhelming majority of them live in societies where polygamy is not legally and/or socially acceptable.
A number of reasons are given for Rabbi Gershom’s ban:

• It was instituted to prevent people from taking advantage of their wives.
• It was intended to avoid potential infighting between rival wives, which may also lead to the transgression of a number of biblical violations.
• Rabbi Gershom was concerned lest the husband be unable to provide properly for all his wives (especially during the difficult times of exile).
• The ban is intended to avoid the inherent rivalry and hatred between rival wives
• There is a concern that a man may marry two wives in different locations, which may lead to forbidden relationships between offspring.
• While it has been suggested that it was adopted from Christian practice and laws, to avoid Christian attacks against Jews who act otherwise, this argument has been assailed by many other halachic authorities.

The ban is intended to avoid the inherent rivalry and hatred between rival wives

As far as Jewish thought is concerned, it would seem that polygamy is not, and never was, an ideal state. The mystical works are replete with references to husband and wife being two halves of one whole. Interestingly, I’ve never encountered an episode in the Talmud or Midrash—which predate Rabbi Gershom’s ban on polygamy—which involves a polygamous family. While it is certainly possible that such stories do exist, it is quite apparent that polygamy was never the norm.

Practically speaking as well, polygamy is a big financial strain, as the husband is required to provide for all the needs of, as well as separate housing for, two households.

In all probability, polygamy was always considered a last-resort option for men who were married to barren women and who wished to have children without divorcing the wives they loved. Monarchs also routinely used polygamy to cement relationships with different tribal factions and families.