On a biblical level, a man is allowed to marry more than one woman. While it has never been a common Jewish practice, at least not in biblical or talmudic times nor in Ashkenazic countries, it was technically permitted. Tradition has it that around the year 1000 CE, Rabbenu Gershom (Me'or ha-Golah) promulgated a ban or a decree prohibiting, among other things, polygamy. Also included in that ban is a prohibition against divorcing a woman without her consent, reading someone else's mail and a number of other acts.
However, I have seen and heard it claimed that the Herem of Rabbenu Gershom was only intended to last for one thousand years. Since that time has past, the prohibitions no longer apply. This argument is incorrect for a number of reasons that we will discuss.
I. Time of the Herem
If there was a single time in which the decree took place, and we will discuss whether that is historically true, we do not know when it was. Rabbenu Gershom passed away in either the year 1028 or 1040. The first record we have of a decree is from the year 1056. Therefore, there is no way to be certain that the thousand years has already passed. It could be that the decree was in the last year of Rabbenu Gershom's life and the thousandth anniversary of the decree will be in either 2028 or 2040.
II. Incorrect Calculation
Regardless, the tradition that certainly inspired this claim is that of the Rashba, made famous by the Maharik, that Rabbenu Gershom's decree only applied until the end of the millennium in which he was living, i.e. until the year 1240. If that is true, then there is nothing new in our situation. It is the same situation as that which faced the Maharam, the Rosh, the Rema, the Hasam Sofer, etc. We will soon see how they dealt with this issue.
III. Who Made the Herem?
There is an historical question as to whether Rabbenu Gershom actually made a Herem. It seems quite unusual that he would arrogate to himself the position of authority over not only Germany but also all of Northern France. And even if he did, it would be entirely out of character for the French rabbis to unquestioningly accept the authority of a German rabbi, no matter how great he may have been. There is also the point that we have very different reports of which prohibitions were included in the Herem. Avraham Grossman, in his seminal work Hakhmei Ashkenaz Ha-Rishonim, throughly covers all of the scholarly theories on this issue until the time of his writing. As he describes it, theories range from the decrees actually preceding Rabbenu Gershom and he only giving his added approval, to his leading a gathering of united communities in agreeing to these decrees (possibly in stages), to some being from his time and others being of later origin, etc. Grossman gives substantial evidence that the decrees were from Rabbenu Gershom's time or before (that he renewed) and that shortly afterwards even leading French rabbis were agreeing with the decrees. But none of this matters.
The real question is what gave Rabbenu Gershom the ability to bind everyone in his region beyond to his decrees. After all, he did not have a Sanhedrin with which to religiously bind others with his interpretations. The answer is given much later by the Rosh in one of his responsa (43:8):
There was a scholar in our land and his name was Rabbenu Gershom. He lived during the time of the Ge'onim and instituted good decrees in the matter of divorce. His decrees are established as if they were given from Sinai because [the people] accepted them upon themselves and transmitted them generation after generation.The force of Rabbenu Gershom's (or whomever's) decrees is not the personality behind them but their acceptance by the community. Because all "Ashkenazim" accepted these decrees in surprising unison, that itself is the reason that they are binding.
Therefore, returning to our original question, the Rosh was writing after the year 1240 but still claimed that the decrees were binding because people accepted them! The same can be said for a number of scholars. Therefore, the decrees must still be in force. It could, however, be suggested that the decrees are no longer in force as decrees but only as custom.
This issue is discussed at length in the commentaries to the Tur, Even Ha-Ezer, 1. The Beis Yosef (in a responsum - Responsa Beis Yosef, dinei kesuvos no. 14) claims that the scholars who stated after 1240 that the decrees were still in effect were only doing so because they were unaware of the condition that they only lasted until the year 1240. Therefore, the Shulhan Arukh (Even Ha-Ezer 1:10) states that the decrees are no longer in effect. The Rema, however, states that the custom is to still adhere to the decrees. The Maharshal (Yam Shel Shlomo, Yevamos 6:41; Responsa Maharshal, no. 14), another contemporary of the Beis Yosef and the Rema, argues forcefully that the decrees are still in effect.
The aharonim wrote about this issue at length and all followed either the Rema or the Maharshal, as would be expected of Ashkenazic scholars, that the decrees are still in effect. See the Pis-hei Teshuvah, Even Ha-Ezer 1:19.
IV. Conclusion
In summary, the prohibitions that are traditionally subsumed under the category of Herem of Rabbenu Gershom are still in effect. There is no time limit to the prohibitions, regardless of whether Rabbenu Gershom inserted one while issuing the decrees (or even made any actual decrees in the traditional sense).
Bibliography:
Avraham Grossman, Hakhmei Ashkenaz Ha-Rishonim (Jerusalem: 1981 ,2001), pp. 132-149
Encyclopedia Talmudis, vol. 17 cols. 384-386