[rat87]

The Torah allows polygamy.

Ashkenazi Jews(descendant of those living in Germany around 900 or so AD many of whom later move east to Poland, Ukraine, and Russia) banned polygamy around ~1000 AD.

I think it mostly died down in Sephardic and Yemeni after most of them moved to the Israel but I'm not sure if it was that common before that time.

[ars]

Even when permitted it was very rare.

For one thing only the very wealthy could afford it - each wife was required to have her own house (separate rooms in one house was not enough).

For the most part it was practiced mainly by traveling businessmen/salesmen who would have a wife in each city where they worked. (Don't forget how slow and long travel used to be.)

The main reason for permitting it is that historically males died more often than females, so there is a small surplus of females, and permitting a small amount of polygamy helps with that.

It's no longer relevant these days.

[xerophyte12932]

As an insurance student, let me just point out male mortality is still considered higher than female mortality (hence higher premium rates for men). Though i have no numbers if the world has more females than males right now

[eru]

There's female infanticide and gender-biased abortions to more than balance higher male mortality in lots of the world.

[PhantomGremlin]

The Torah allows polygamy.

It may have been a practical necessity after husbands died or were killed. No welfare or social security survivor benefits or similar programs existed.

But IMO it's something that only single men would truly find appealing. Married men quickly realize that additional wives and accompanying additional children are the last thing they want or need. :)