I've heard that Rabeinu Gershom (a 10th-11th century scholar) banned 3 things, and I know one of them is polygamy. Does anyone know what the other two are?
Be'er Hagolah at the end of Yoreh Deah 334 lists the following:
- Not to marry more than 1 wife.
- Someone who is in Cherem should not be a Shaliach Tzibur.
- A person should not be away from his wife more than 18 months.
- Not to rent a house from a non Jew if a Jew is living there.
- Not to cut off the margin of a page of a Sefer, even to write on it.
- Not to embarrass a Baal Teshuva about his previous sins.
- Not to look at his friends letters without permission.
There are many more, see the link above.
I have heard that he wrote all of sha"s and said that that is the authoritative girsa and was gozer not the change it.
(from sefer פירושי רבינו גרשום printed by אור החיים, in the הקדמה)